Copyright

William Collins

An imprint of HarperCollinsPublishers 1 London Bridge Street London SE1 9GF WilliamCollinsBooks.com This eBook edition published by William Collins in 2019 Copyright © John C. Coulson, 2019 Photographs © Individual copyright holders John C. Coulson asserts his moral right to be identified as the author of this work Cover design linocut by Robert Gillmor A catalogue record for this book is available from the British Library. All rights reserved under International and Pan-American Copyright Conventions. By payment of the required fees, you have been granted the non-exclusive, non-transferable right to access and read the text of this eBook on-screen. No part of this text may be reproduced, transmitted, downloaded, decompiled, reverse engineered, or stored in or introduced into any information storage and retrieval system, in any form or by any means, whether electronic or mechanical, now known or hereinafter invented, without the express written permission of HarperCollins Publishers. Source ISBN: 9780008201425 Ebook Edition © April 2019 ISBN: 9780008201449 Version: 2019-02-11

EDITORS
SARAH A. CORBET, SCD
DAVID STREETER, MBE, FRSB
JIM FLEGG, OBE, FIHORT
PROF. JONATHAN SILVERTOWN
PROF. BRIAN SHORT

*

The aim of this series is to interest the general

reader in the wildlife of Britain by recapturing

the enquiring spirit of the old naturalists.

The editors believe that the natural pride of

the British public in the native flora and fauna,

to which must be added concern for their

conservation, is best fostered by maintaining

a high standard of accuracy combined with

clarity of exposition in presenting the results

of modern scientific research.

Contents

Cover

Title Page

Copyright

About the Editors

Editors’ Preface

Author’s Foreword and Acknowledgements

1 An Overview of Gulls

2 Black-headed Gull

3 Mediterranean Gull

4 Common Gull (Mew Gull)

5 European Herring Gull

6 Lesser Black-backed Gull

7 Great Black-backed Gull

8 Black-legged Kittiwake

9 Yellow-legged Gull

10 Little Gull

11 Rare Gulls in Britain and Ireland

12 Methods Used to Study Gulls

13 Urban Gulls

14 Conservation, Management and Exploitation of Gulls

Appendices

Select Bibliography and Further Reading

Species Index

General Index

About the Author

About the Publisher

Editors’ Preface

GULLS ARE A FAMILY OF DISTINCTIVE and easily recognised birds that are familiar to us all, whether we live close to the sea or many miles inland, and whether we live in urban or rural surroundings. Long gone are the days when the sight of flocks of gulls overhead or in nearby fields was taken as a sure and ominous sign of storms at sea. For most of us today, birdwatchers or not, gulls are genuinely everyday birds, as common a sight as Woodpigeons (Columba palumbus) or the various members of the crow family. Other than in immature plumages (which may cause problems), the handful of commoner species are readily identifiable to those with any interest.

Seeking an expert author for a new book on gulls was a relatively easy task for the editors. Dr John Coulson was, until his retirement, Reader in Animal Ecology at the University of Durham, and has a lifetime’s experience of research into many aspects of gull (and other seabird) biology and ecology, with a particular focus on the Black-legged Kittiwake (Rissa tridactyla). He is an expert of world renown, and recipient in 1992 of the British Ornithologists’ Union’s highest accolade, the Godman-Salvin Medal, and in 1993 of the Waterbird Society’s Robert Cushman Murphy Prize.

A fulsome initial chapter introduces the gull family as a whole, including a succinct assessment of the rather complex current (worldwide) status of gull taxonomy. Things have changed dramatically since the eighteenth century, when Carl Linnaeus first established the single genus Larus, containing all the then known species! After this, the reader is treated to nine chapters – each in effect a treatise – on the regular British and Irish gulls, including the latest newcomer, the rapidly increasing and very elegant Mediterranean Gull (Ichthyaetus melanocephalus). These are followed by chapters on the rarer species; methods used in studying gulls; urban problems; and conservation, management and exploitation.

In common with many components of our fauna and flora, times have changed for gulls, and not always for the better – as the author describes. They may not universally be seen as slender-winged, elegant seabirds, and to some – especially those living in or visiting towns or cities with rooftop gull colonies – they are raucously noisy neighbours. Similarly, many seaside promenades are blighted by the menacing presence of gulls with unblinking eyes seeking scraps, often boldly. Since the late twentieth century, even conservation bodies seeking to encourage nesting terns or other seabirds have resorted to culling in an attempt to limit predatory gull activity. In the last few decades, however, Herring Gull numbers, boosted in the second half of the twentieth century by the ready feeding opportunities offered by poorly covered landfill sites, have fallen to the extent that they themselves have become of conservation concern.

Such problems, as well as the many fascinating but less nefarious aspects of our British and Irish gull populations, are dealt with in substantial detail in John Coulson’s admirably comprehensive text. This is a most noteworthy addition to the New Naturalist Library.

Author’s Foreword and Acknowledgements

MY FIRST RECOLLECTION OF GULLS was when I was about eight years old, when a maritime pilot on the Tyne told me that the gulls that frequently followed commercial ships entering the busy river were the reincarnations of past pilots, and were keeping a supportive eye on the steering of current pilots. An individual gull the pilots recognised because it called frequently was said to be the reincarnation of pilot ‘Clagger’ Purvis, who apparently had had as much to say for himself in life. Even at that age I found this story hard to believe, but the pilot was right about the frequency of gulls following both large and small vessels into the river.

My interest in birds, and particularly in gulls, developed during my school days. As a member of a small group mentored by Fred Grey, a master at South Shields High School for Boys and a person with a lifelong passion for birds, I learned much about the identification of birds during mid-morning breaks and on field trips. Through his guidance, my questioning eventually expanded from ‘What is it?’ to ‘What is it doing?’ I was also very much influenced by the Reverend Edward Armstrong’s book Bird Display and Behaviour (1942), David Lack’s The Life of the Robin (1943) and, in particular, Sir Arthur Landsborough Thomson’s Bird Migration (1936), which highlighted some of the information that could be gained by ringing birds.

While still in the sixth form at school, I wrote to Elsie Leach, who ran the national bird-ringing scheme in an honorary capacity for the British Trust for Ornithology (BTO) from a small office and a cupboard in the Natural History Museum in London, asking her how to become a ringer. I received a prompt response requesting a letter of support from an acknowledged ornithologist that I could identify birds. I obtained this from a local doctor; he had an excellent knowledge of the birds of northern Norway and I had previously been able to draw his attention to uncommon birds visiting the vicinity, which he would stop off to see while making slight detours en route to his house calls.

Having sent the recommendation to Miss Leach, I received a reply within a few days simply asking what numbers and sizes of rings I wanted. At that time, 1948, there was no training whatsoever for potential ringers, nor an age limit. Every ring used had to be recorded on a thin cardboard sheet, which took only six ring entries and was guillotined into separate strips if the ringed bird was recovered. Ringing was on a very much smaller scale in those days! I remained an active ringer for 62 years, and have seen many changes to the British ringing scheme, including the intensive training now required for those who wish to ring. I also received one of the first mist nests in the country, and had to work out for myself how to use it – all I was told was to hang it between two poles!

One day in early June 1950, Edward White (a school and university friend) and I walked along the coast to where Black-legged Kittiwakes (Rissa tridactyla, here referred to simply as Kittiwakes) were nesting on a stack called Marsden Rock and on the nearby mainland cliffs. As we walked on the beach below the colonies, we noticed that about a third of the nests on the stack contained chicks, while only one out of a hundred nests on the mainland cliffs had chicks. Two weeks later, we excluded predation as the cause of this difference, as almost all nests now contained chicks, even though those in nests on the mainland cliff tended to be smaller than those on the stack.

This brief observation started a chain of interest and speculation. Why was there this difference, which was obviously not due to predation? Our attention was drawn to Frank Fraser Darling’s book Bird Flocks and the Breeding Cycle (1938). The local public library managed to borrow a copy from another library and we were able to read about the ecologist’s idea of social stimulation, which he believed affected the timing and success of breeding in colonial birds. Some authors suggested that the differences Fraser Darling had observed in small colonies of large gulls were due to young birds breeding later than old individuals, and that more young birds were breeding in some colonies than in others. However, we failed to find any information on the effect of age on the breeding biology of gulls and very little on birds of any species.

Edward and I were both undergraduates at Durham University, and we planned to make twice-weekly visits the following year to the Kittiwake colonies at Marsden to count the numbers of nests containing chicks (counting when eggs were laid was not possible, as most nests could not be viewed from above). The difference we had observed the previous year was repeated, and we presented our first results at one of the delightfully informal annual student bird conferences held at St Hugh’s College in Oxford each January, organised by David Lack. This resulted in the opportunity to meet and engage with many knowledgeable researchers, including Niko Tinbergen, Charles Elton, Arthur Cain, Mike and Esther Cullen, ‘Mick’ Southern, Reg Moreau, David Snow, Lance Richdale and, of course, David Lack and his students.

The problem of whether the differences we found between colonies were an effect of age or social conditions required knowledge of the breeding biology of Kittiwakes of different ages and, in turn, this needed marked individuals whose ages were known. The nests on the limestone cliffs at Marsden were inaccessible, but by chance, while crossing the river Tyne on a ferry two years previously, I had noticed a few pairs of Kittiwakes nesting on the window ledges of a riverside warehouse at North Shields. At Durham University, the head of zoology, Professor J. B. Cragg, was kind enough to take an interest in our study and in 1953 he obtained access for us to the building from the owners, Smith’s Dock Co. The company actively protected the Kittiwakes and the birds were the delight of the two employees working there. We were able to reach the nests from inside the warehouse, allowing us to capture and individually mark the breeding adults and chicks. Fate was very kind to us, because the windows on the ledges used by the Kittiwakes opened inwards, and we were able to snare adults without pushing off or even disturbing the nests and their contents. Our main tool was a 60 cm length of wire, which had been used to bind a crate of oranges. We shaped one end into a crook that could be passed out of a narrow gap when the window was ajar and slipped around the leg of the adult. The Kittiwakes were not alarmed by the wire, and indeed were often curious and pecked at it. Pairs on neighbouring windowsills did not react when an individual suddenly disappeared into the ‘cliff’ to be colour-ringed. Within a few years we were able to measure the timing of laying in individuals of different ages.

The study eventually spanned more than 35 years, although there was never any guarantee that we would be able to continue it beyond a year or two at a time. At that time, long-term studies on birds were virtually unknown, the exception being that undertaken by Huijbert Kluijver on Great Tits (Parus major) in the Netherlands (which began in 1912), and it was many years before the research councils in the UK appreciated their considerable value and supported some of them.

The opportunity arose for both Edward and me to study for a PhD, but independently we came to the same opinion that a doctorate in ornithology was unlikely to lead to employment. There were only three people at that time professionally employed as ornithologists in Britain, and one of these held a post of reader in entomology! Only two universities employed ornithologists (Oxford and Aberdeen), and there was one ornithologist who worked for the Ministry of Agriculture and Fisheries. An eminent Scottish professor of zoology at that time openly stated that the study of birds and butterflies was not part of zoology, and ecology was only just starting to develop in Oxford, under the influence of Charles Elton, David Lack and Alister Hardy.

This situation was in marked contrast to the present day, when many of the now much more numerous universities have one or usually more staff researching in ornithology and ecology, while currently Natural England, the BTO, the Game and Wildlife Conservation Trust, Scottish Natural Heritage and the Royal Society for the Protection of Birds (RSPB) all employ professional ornithologists. Because of the situation in 1953, Edward and I both decided to work on entomological topics for our higher degrees, and studying Kittiwakes remained a happy hobby.

Our cooperative studies of Kittiwakes had to cease after we both completed our PhD studies. Edward went to work at a college in Freetown, Sierra Leone, and then moved to New Zealand. I went into school teaching but continued the Kittiwake studies in my spare time. Eventually, I obtained a post-doctoral fellowship from the Royal Commission for the Exhibition of 1851, and this allowed me to develop a full-time research programme of my own choice – the study of both Kittiwakes and moorland insects.

Two years later, I was offered a lectureship in zoology at Durham University, and in the following years I supervised a series of students working on seabirds and also on moorland invertebrates. Those studying seabirds included Jeff Brazendale, Brian Springett, Dick Potts, Andy Hodges, Ron Wooler, Jean Horobin, Callum Thomas, Jennifer Butterfield, Julie Porter, John Chardine, Fiona Dixon, David Jackson, Nicholas Aebischer and Jackie Fairweather, many of whom worked with me using the individually marked Kittiwakes nesting on the warehouse at North Shields from 1961 to 1990. Étienne Danchin visited from France and contributed much to Kittiwake studies during his post-doctoral year at Durham. In addition, Tom Pearson made a comprehensive study of the feeding biology of seabirds, including Kittiwakes, European Herring Gulls (Larus argentatus, here referred to simply as Herring Gulls) and Lesser Black-backed Gulls (L. fuscus) breeding on the Farne Islands, Northumberland.

Dick Potts started a PhD study on Shags (Phalocrocorax aristotelis) in 1962, and this took us to the Isle of May in Scotland’s Firth of Forth. There, we met those managing the Isle of May Bird Observatory, who were concerned about the lack of use their accommodation and facilities received during the summer. As a result, they encouraged me to start studies on the tens of thousands of Herring Gulls nesting on the island. First, Jasper Parsons, and then Neil Duncan and George Chabrzyk, developed studies on the large gull colony, while the contribution started by Margaret Emmerson was sadly curtailed by her untimely death. We continued studies on Herring Gulls for 11 years, until the major and ‘experimental’ reduction of the large gulls on the island that was carried out by the Nature Conservancy Council. Elsewhere in Britain, Niko Tinbergen had researchers working on gulls at Ravenglass and the Isle of Walney in Cumbria, and more recently several others have studied large gulls nesting on islands in south Wales.

Herring Gulls started to invade and breed in coastal towns in north-east England in the 1960s, and Pat Monaghan was the first to study these and others feeding at landfill sites in detail. We mapped the distribution and rapidly increasing numbers of large gull species nesting in urban areas throughout Britain and Ireland in 1977, building on Stanley Cramp’s initial study. Twenty years later, Susan Raven and I repeated the national census and found both a dramatic spread and increases in numbers.

Investigations on Lesser Black-backed Gulls breeding on Tarnbrook Fell were started by Neil Duncan, and later developed further by Mark O’Connor and Nick Royle, with the aim of establishing non-lethal methods to reduce the extent of the large gull colony there. From 1999 to date, my wife, Becky, and I have studied and counted Herring Gulls nesting in South Shields, Sunderland, Newcastle upon Tyne and Berwick-upon-Tweed in north-east England, and Lesser Black-backed Gulls in Dumfries in south-west Scotland.

In 1992, a new Kittiwake colony was established on the low cliffs of Coquet Island, Northumberland. With the goodwill of the RSPB, Becky and I were able to ring every chick reared there for the first 15 years of the colony’s existence, and so were able to identify the huge immigration of adults reared in other colonies as the Coquet colony rapidly grew in size.

The Natural Environmental Research Council supported my research on gulls in collaboration with their Unit of Virology, which was concerned with the possibility of viruses being carried and distributed by the birds. This involved post-doctoral studies by Neil Duncan, Callum Thomas and Jennifer Butterfield, aided by Nicholas Aebischer, David Jackson and Sarah Wanless. The range of the study was expanded from the north of England to include Scotland, with Pat Monaghan working from Glasgow University, where she had recently been appointed as a lecturer. This work subsequently led to a joint study by Susan Greig, Pat Monaghan and myself on gull feeding methods at landfill sites, which relied on video recordings and then frame-by-frame play-backs of the feeding activity of individually marked gulls of known sex and age during their mass feeding frenzies. Collaborative work was developed with the Public Health Laboratory Service over several years on the microorganisms carried by gulls (particularly Salmonella, Campylobacter and Cryptosporidium). Later, Gabriella MacKinnon and David Baines carried out studies on wintering Black-headed Gulls (Chroicocephalus ridibundus) and Common Gulls (Larus canus).

I have also had considerable exchange of ideas with gull researchers from other countries, in particular Rudi Drent and Arie Spaans in the Netherlands, and Ian Nisbet, Michael Gochfeld and Joanna Burger in the United States. In addition, I have had the pleasure of meeting very many others at international ornithological congresses.

My studies on gulls developed rapidly at Durham with the cooperation of a large series of dedicated and able researchers, well supported by a number of technicians, particularly Eric Henderson, Michael Bone and John Richardson. In addition, we received welcome financial support at different times from the Department of Scientific and Industrial Research, the Science Research Council and the Natural Environment Research Council, supporting facilities supplied by Durham University itself.

Ornithology, including the study of seabirds, has expanded considerably since 1950, and the worldwide output of publications on aspects of the biology of gulls has consequently increased at a remarkable rate. Many, but by no means all, ornithological publications are listed annually in Zoological Record, and it is remarkable how the number of scientific papers included there has increased 15-fold since 1950. Journals such as Ibis, Bird Study, Ardea, Auk and British Birds, along with several journals published in North America, have greatly increased the annual number of pages dedicated to ornithology, while many new journals have also been started. Having been an editor of three ornithological journals over a period of 40 years, I have been particularly aware of this change and, incidentally, the increasing number of papers with multiple authors. In part, this results from more cooperative studies, but perhaps also from the increasing demands (and needs) of students to publish with tenured members of staff so their work can be included in research assessments. In the 1950s and 1960s, most scientific papers in ornithology had single authorship, with a minority having two authors. Recently, however, there has been a change – for example, in a recently published issue of 24 papers in one symposium, the contributions averaged more than four authors per paper, two having 12 authors and one 13, while only two were written by a single author.

The numbers of ornithological papers published in ecological and behavioural journals has also increased dramatically. These, together with information on gulls published elsewhere, such as annual county reports and those of conservation bodies, theses, records, online reports and books, have increased almost beyond belief, to such an extent that a complete bibliography for this book would have been taken up more than fifty pages and the main text would have been littered with multiple references to data sources. In 2011, my monograph on the Kittiwake required 16 pages of bibliography for this one species alone. In dealing currently with the Herring Gull, the references consulted exceeded a thousand items, and since this book covers the information on all gulls on the British list, I found that including a complete bibliography was a major problem. Providing references in books is not new problem, however, and was one faced by both James Fisher when writing The Fulmar (New Naturalist Monograph 6) in 1952 and Niko Tinbergen in his Herring Gull’s World in 1953.

This book has had the main objective of bringing together and digesting information on the gull species that occur in Britain and Ireland, and is primarily aimed at readers who are interested in gulls but not necessarily in researching their biology in depth. In discussing this problem with the editors and others, I have been encouraged to reduce the number of references reported in the text appreciably, and instead present a Select Bibliography and Further Reading section at the end of the book (here), and listing select key studies mentioned in the text, along with other informative publications. Readers wishing for more information should be able to obtain further details from the extensive sources available on the Internet. The extent of published (and unpublished) information on gulls is now so extensive that I have had to be selective about the information presented for several species, and I have been able to include information and analyses not previously published or available to most readers.

This book is not meant as an identification guide, although some of the salient features of each species and their geographical distribution are briefly mentioned. Those wishing to identify gulls should use one of the excellent field guides available, as listed in the Select Bibliography. The world distributions of all gull species considered here have been fully described in standard identification texts, including the gull sections in volume 3 of the Birds of the Western Palaearctic and volume 3 of the Birds of the World.

It has not been possible to write this book without encountering areas of controversy, which include taxonomy, conservation, and the ability to identify some closely related species and subspecies in the field. Ornithology has thrived on controversy in the past and it will continue to do so in the future. In many cases, disagreement has led to the development of new methods of study and more in-depth investigations. In writing this book, debate has reared its head about where a species is placed in red or yellow categories of conservation concern based on a series of possible criteria, the existence of any one of which is enough to indicate that it is threatened. Some ornithologists feel that the system should be improved, and that a more critical scientific approach is needed to interpret data used to estimate the current risk of extinction of individual gull species.

For the same reason, there is no comment on climate change in this book. While there have been several claims that this has affected gulls, sound evidence in favour of such effects is currently poor and is not supported by evidence that adequately allows other possible causes to be excluded. Increasing studies have been made to estimate the potential risks to gulls from offshore wind farms and the rotating arms of the turbines. At present, most of these are based on informed speculation and such factors as the flight height of individual species and their numbers in the key areas. In the future, this problem will be investigated in more detail and the level of the perceived risk to gulls and other seabirds will be based on actual information, not just models of the situation, but the research is not yet at this stage.

In recent years, the common names of some gulls have lengthened, allegedly to avoid international confusion. Hence, the Black-legged Kittiwake, European Herring Gull, American Herring Gull and Yellow-legged Gull join the Lesser Black-backed Gull, Great Black-backed Gull, Black-headed Gull and Slender-billed Gull, which already have long names. Proposals have been made to change the Great Black-headed Gull to the shorter Pallas’s Gull, while it has been suggested that the Common Gull is changed to the Mew Gull. In this book, as mentioned earlier, Herring Gull is used for the European Herring Gull and the American Herring Gull is referred to in full. Similarly, and as mentioned above, the Black-legged Kittiwake is referred to as the Kittiwake, while its sibling species, the Red-legged Kittiwake (which is infrequently mentioned) is written out in full. In most cases this follows the vernacular English names listed in the ninth edition of ‘The Simple British List Based on a Checklist of Birds of Britain’ (2018). In general, this edition retained names already familiar to most readers.

Statistical tests are important in evaluating differences in quantitative data, but to many the presentation of these and their outcomes are but an irritation. In general, I have commented on quantitative differences only when they have been shown to be statistically significant and so are likely to be real and meaningful, although I have not given details of the tests used.

ACKNOWLEDGEMENTS

Much of the information in this book derives from two sources. Some comes from professional research, but much is the result of the activities of amateur birdwatchers who spend their spare time visiting sites where they are likely to encounter unusual bird species or birds in exceptionally large numbers, and who then send the details to local recorders or contribute to national surveys and national censuses. Over the years, the number of observers has increased markedly and systems of notifying others of the presence of unusual birds have developed. Both categories of people studying birds have swelled progressively over the past 60 years, and the information and numbers of records have increased to a remarkable extent. In addition, the methods of identifying and confirming rare species have been increasingly supported by good-quality photographs. Accompanying these trends has been a dramatic increase in the numbers of gulls ringed both in Britain and Ireland, and also elsewhere in Europe. Capturing and ringing adult gulls has increased dramatically in recent years with the development of cannon nets and more frequent visits to landfill sites by teams of ringers. All of these additional efforts are appreciated, for without them, much of the information in this book would not have been available.

I would like to acknowledge the detailed contribution of Alan Dean, both for his detailed analysis of the records of gulls in the West Midlands in England over many years and for the photographs he (and others) have so willingly contributed.

The data used in writing this book has been collected over many years and it is possible here to identify only a small number of the hundreds of contributors. The many research students I supervised and advised have all made appreciable contributions over the years, and many have since made further contributions in ornithology and science in general while holding permanent posts both in the UK and North America. I was fortunate in having such an able set of students who contributed wholeheartedly and consistently to studies often made under difficult conditions. They all willingly volunteered to take part in teamwork as required, often at the most unsocial hours of the day and in adverse weather conditions, all while advancing their own studies.

I owe a great deal to the BTO, not only for the ringing scheme they manage so efficiently, but also for the extensive data they have collected. When I first had contact with the BTO in the early 1950s, it had a small office at the top of a set of outside stairs in a side street in Oxford, with a single salaried member of staff, Dr Bruce Campbell. Here, I acknowledge the trust’s willingness to allow me to reproduce maps of ringing recoveries of gulls and to use the most recently available maps of the breeding season distribution of gulls in Britain and Ireland in 2007–11.

Many people have offered photographs for inclusion in the book, and these have all been credited in the captions. In particular, I thank Nicholas Aebischer, Pep Arcos, Rob Barrett, Colin Carter, Becky Coulson, Anthony Davison, Alan Dean, Andrew Easton, Phil Jones, John Kemp, Mark Leitch, Fred van Olphen, Daniel Oro, Mike Osborne, Viola Ross-Smith, Steven Seal, Charles Sharp, Michael Southcott, Brett Spencer, Norman Deans van Swelm, Thermos (fi.wikipedia), Dan Turner and an anonymous photographer for their photographic help and willing permission to allow their excellent images to be included in this book.

The extensive data set on British seabirds now archived and maintained by the Joint Nature Conservancy Council and readily accessible online is a valuable asset and source of information. My thanks also go to Natural England for granting a freedom of information request concerning culls and licences issued to collect gull eggs.

Help and information have been supplied by many, in particular Nicholas Aebischer, David Baines, Robert Barrett, Peter Bell, Richard Bevan, Tim Birkhead, Bill Bourne, Joanna Burger, David Cabot, Kees Camphuysen, Colin Carter, Keith Clarkson, Ian Court, J. B. Cragg, Francis Daunt, Ian Deans, Greg Douglas, Andy Douse, Steve Dudley, Tim Dunn, George Dunnet, Andrew Easton, Julie Ellis, Mike Erwin, Sheila Frazer, Bob Furness, Michael Gochfeld, Thalassa Hamilton, Gill Hartley, Scott Hatch, Martin Heubeck, Grace Hickling, Keith Houston, Jon Jonsson, Heather Kyle, Susan Lindsay, Roddy Mavor, Jim Mills, Ian Nisbet, Daniel Oro, Ian Patterson, Ray Pierotti, Jean-Marc Pons, Julie Porter, Dick Potts, Richard Procter, Chris Redfern, Jim Reid, Sam Rider, Peter Rock, Robin Sellars, Peter Shield, Robert Swann, Mike Swindells, Martin Taylor, Mike Toms, Andrew Tongue, Mike Trees, Daniel Turner, Sarah Wanless, Matt Wood, Vero Wynne-Edwards, Bernard Zonfrillo and Jan Zorgdrager. Over the years, many others have helped in studies and investigations, and I appreciate the assistance of them all.

I owe a major debt of gratitude to my wife, Becky, for the many ways she has supported and helped my gull studies, solved many computing problems, and given help in preparing and checking the text of this book.

CHAPTER 1
An Overview of Gulls

GULLS ARE CONSPICUOUS WEB-FOOTED, long-winged, medium or large seabirds that are readily recognised by the public. Adults are mainly white with shades of grey or black on the mantle and wings. Most species have black wing-tips, some have white ‘mirrors’ within the black areas, but a few species – mainly those restricted to an Arctic breeding distribution – have entirely white wing-tips. In the breeding season, adults of different species either have entirely pure white or very dark (black or brown) heads, and all revert to white heads in the autumn and winter, often with small grey marks behind the eye or grey streaking on the neck.

Gulls are widely known to the public because of their size and the habit of many species to frequent harbours, follow ships, visit landfill sites and visit outdoor areas also frequented by humans, such as seaside resorts, sports fields, beaches, rivers, picnic areas and large car parks at shopping complexes. In recent years, they have become even more familiar in parts of Europe and North America because their numbers have increased and several species have taken to nesting on buildings in urban areas. This habit of urban breeding has developed independently several times in different countries during the twentieth century and has spread rapidly. Urban nesting is now occurring in several species, and has almost certainly arisen through the marked increases in the size of gull populations, coupled with the increased protection given to them over the last century. The presence of gulls in urban areas has been given considerable adverse publicity, including reported cases of adults protecting their unfledged young by diving close to people’s heads, or of gulls snatching food from unsuspecting members of the public. Such reports have resulted in gulls, and particularly the large species, acquiring pest status in certain areas.

EVOLUTION OF GULLS

The Charadriiformes constitutes a single large and distinctive lineage of modern birds, and includes waders, skuas, auks, terns, gulls and a few apparently aberrant species such as jacanas. Although the skuas appear to be similar to gulls, current evidence – including DNA studies – suggests that their ancestry may be nearer to the auks than to gulls.

The lightly built bones of birds associated with flight are fragile and therefore do not often produce good fossil remains. As a result, the evolution of present-day birds is poorly known, and much less so than that of reptiles, fish or mammals. Fossil remains attributable to gulls are particularly scarce. Many of those that have been found have not or cannot be attributed to the presently recognised genera, and certainly not to present-day species. More recently, several fossil bones initially attributed to gulls have been found to belong to other avian groups.

Fossil bones attributed to gulls and possibly members of the genus Larus have been reported both in Europe and the USA from deposits from the Middle Miocene, 20–15 million years ago. The relationship of these fossils to modern-day gulls is unclear; fossilised bird bones are often, and perhaps uncritically, given different specific names to those of currently existing species, ignoring the fact that the bones of a present-day species vary considerably in size according to sex and locality.

TAXONOMY OF GULLS

Initially, gull species were separated and identified on the basis of plumage, skeletal structure and size. In many species, specimens from different geographical areas held in museums and private collections were often named and given subspecies status based on minor differences in size and plumage, but all too frequently this relied on small numbers collected from only a few localities. Many of these named subspecies are still used today and, while the majority are probably justified, others that were described and named many years ago should be re-evaluated using modern techniques and larger samples. Some subspecies have already been rejected on this basis, and it is likely that others will not stand critical re-examination and will also be rejected.

In other cases, some existing subspecies have been promoted to the status of a full species, as has occurred recently within the Herring Gull complex in Europe and Asia. Still others may show only gradual changes in size, structure or plumage shades over their geographical range, a concept not recognised by early taxonomists until Julian Huxley applied the term clines to these groups in 1942. Such clines have already been demonstrated for the Black-legged Kittiwake (Rissa tridactyla), the Puffin (Fratercula arctica) and the Common Guillemot (Uria aalge) breeding in the North Atlantic. Questionable subspecies of gulls still exist, and some are discussed in more detail later in this chapter.

Initially, the eighteenth-century taxonomist Carl Linnaeus placed all gulls in the genus Larus, and most species remained there in what became a very large taxon. Eventually, the two species of kittiwake were removed from Larus and placed in the genus Rissa, while the Ivory Gull was moved to the genus Pagophila (P. eburnea), Sabine’s Gull was transferred to the genus Xema (X. sabini), and Ross’s Gull was placed in the genus Rhodostethia and then, more recently, to Hydrocoloeus (H. rosea), alongside the Little Gull (H. minutus). The Swallow-tailed Gull became the sole species in the genus Creagrus (C. furcatus). These separations were not unreservedly accepted, however, and as late as 1998, Philip Chu proposed returning all gulls to a single genus, Larus. Seven years later, the intensive study of the mitochondrial DNA of many gull species made by Jean-Marc Pons, Alexandre Hassanin and Pierre-Andre Crochet (2005) moved in the opposite direction and separated gulls into nine genera, and in so doing created the new genera Chroicocephalus (with 10 species worldwide), Hydrocoloeus (with two species) and Saundersilarus (comprising only Saunders’ Gull, S. saundersi, in China). Worldwide, at least 24 gull species, especially those with white heads in the breeding season, are still retained in the large genus Larus. Aside from Saundersilarus, three other genera are composed of only one species: Creagrus, containing the Galapagos Islands’ Swallow-tailed Gull; Xema, containing the High Arctic Sabine’s Gull; and Pagophila, including the Ivory Gull, also breeding in the High Arctic. Like Hydrocoloeus, the genus Rissa also contains two species.

One of the major findings made during the in-depth investigation by Pons et al. (2005) was that the gulls that had dark heads as adults did not form a single taxonomic group, as had been suggested by studies made in the second half of the twentieth century, but were composed of three distinct groups of species. These groups were called the ‘black-headed gulls’ and placed in the genus Ichthyaetus, while ‘hooded gulls’ were separated into another new genus, Leucophaeus. The third group, including the Black-headed Gull, were called ‘masked gulls’ and were placed in the genus Chroicocephalus. To an extent, this separation of gulls with dark heads in the breeding season is supported by similar courtship behaviour within each group, as originally suggested by Niko Tinbergen and his co-workers in the 1950s and supported by more extensive recent studies.

GULL SPECIES WORLDWIDE

Currently, there are about 50 species of gulls in the world. This total has increased in recent years and will probably be increased further as improved molecular techniques are used to revise their status; even the definition of a species may be modified or revised. The uncertainty about the precise number of species reflects the fact that the gulls as a group present a taxonomic nightmare, and this has resulted in years of confusion and disagreement. For example, the American Ornithologists’ Union (AOU) considers the Herring Gull breeding in North America to be a subspecies of the European Herring Gull (Larus argentatus smithsonianus), while the British Ornithologists’ Union (BOU) regards it as a separate species, Larus smithsonianus. There is still much confusion within the extensive Herring Gull and Lesser Black-backed Gull complex of species and subspecies, particularly those occurring in Asia (see box).

Speciation concepts

The decision as to whether and under what circumstances two populations of animals that occur in different geographical areas can be considered distinct species remains a taxonomic problem, because the level of genetic difference between the two that justifies specific status is often an arbitrary decision and one that is not always universally accepted.

One major taxonomic problem relates to the Herring Gull and Lesser Black-backed Gull complex of subspecies. In 1942, the evolutionary biologist Ernst Mayr suggested that the subspecies formed a chain around the northern hemisphere, starting with the Lesser Black-backed Gull in Europe, and then further subspecies occurring eastwards through Asia, each having progressively lighter-coloured wings and leading to the American Herring Gull in North America, and finally completing the ring with the Herring Gull of western Europe. The theory is that, by the time this chain of subspecies has spread eastwards around the northern hemisphere and the ends meet up again in Europe, the Lesser Black-backed Gull and the Herring Gull are obviously separate species and interbreed only very rarely. This beautiful explanation of a series of subspecies first spreading, then part of each becoming isolated, allowing the formation of further subspecies around the northern hemisphere and eventually producing two distinct species, was widely acclaimed and has been frequently quoted in books, scientific papers and lectures on genetics and speciation.

However, the recent development and application of mitochondrial DNA techniques has shown that the American Herring Gull is not the closest relative of the European Herring Gull as was previously thought, nor did it spread eastwards historically from North America to Europe to evolve into the European Herring Gull. That said, while the fascinating concept of a ring of gull subspecies spreading around the northern hemisphere and ending with two distinct species has been discredited, it may soon, with some minor modifications, become viable again. This is because the Lesser Black-backed Gull is currently spreading from Europe to North America via Iceland and Greenland, and is beginning to breed in Canada. As such, it is establishing new end points of the chain of subspecies, this time in North America and involving the American Herring Gull.

The Iceland Gull (Larus glaucoides), which breeds in Greenland and parts of arctic Canada, has entirely white primaries and is a well-established species, but there is conflict over the status of two similar gulls, Thayer’s and Kumlien’s gulls, both of which show some black on the tips of the primaries. The AOU recognises Thayer’s Gull as a distinct species (L. thayeri), but regards Kumlien’s Gull as a subspecies of Thayer’s Gull (L. thayeri kumlieni). Within Europe, there is considerable disagreement about the status of the three forms, with some national bodies (such as those in Ireland) agreeing with the AOU classification, and others (including the BOU) considering both Thayer’s Gull and Kumlien’s gull as subspecies of the Iceland Gull (L. glaucoides thayeri and L. g. kumlieni). Yet other bodies believe that they represent three distinct species. In this book and without strong conviction on the matter, I have treated Thayer’s Gull as a distinct species but have followed the BOU and regarded Kumlien’s Gull as a subspecies of the Iceland Gull. Fortunately, most individuals that visit Britain are typical Iceland Gulls and lack any black or brown on the wing-tips.

Elsewhere in the world, birds in the Kelp Gull and Dominican Gull complex (currently all known as Larus dominicanus) are similar and obviously related to the Great Black-backed Gull (L. marinus) of the North Atlantic. They are also a taxonomic problem and currently are regarded as consisting of five geographically separated subspecies. Just as some of the former subspecies of the Herring Gull have been recognised as distinct species, some of the L. dominicanus subspecies may also be elevated to species status when more intensive genetic and ecological investigations have been completed.

CURRENT GEOGRAPHICAL RANGES

Gulls breed on all continents, with Kelp Gulls extending their southern range into Antarctica and several gull species breeding in the High Arctic. The number of gull species breeding in each 10-degree zone of latitude varies considerably, with two peaks of abundance, one in each hemisphere (Fig. 1). In the northern hemisphere, the number of species peaks between 40°N and 60°N, and in the southern hemisphere, a smaller peak occurs between 20°S and 40°S. Few gull species breed in the tropics, Antarctica or the High Arctic regions. This variation in species abundance, particularly between the two hemispheres, correlates reasonably closely with the amount of land within each latitude zone. This may offer a partial explanation as to why appreciably fewer species of gulls are found and breed in the southern hemisphere.

Few individual gull species breed over a wide range, with about 80 per cent spread over less than 20 degrees of latitude, and very few breed in both hemispheres. These patterns differ markedly from the terns, where many species breed in both the northern and southern hemispheres. A comparison of the ratio of gull and tern species breeding in different latitude zones throughout the world is shown in Fig. 2. There are more gull than tern species in only the zones north of 40°N, which begs the question as to why fewer gull species occur in the other zones. Could this be the result of competition between gulls on the one hand, and with petrels and shearwaters in the southern hemisphere?

FIG 1. The number of gull species (upper graph) and tern species (lower graph) breeding throughout the world in each zone of 10-degrees latitude. The distribution of gull species is clearly bimodal, while that for tern species peaks in the tropical zone between 30°N and 30°S. Tern data from Cabot & Nisbet (2013).

FIG 2. The ratio of the number of breeding gull species to the number of tern species in relation to zones of latitude. Gull species are more numerous than tern species only north of 40°N. The dashed line indicates equality.

The dominance of gulls over terns in temperate and arctic regions of the northern hemisphere is even greater when numbers of individuals are considered. For example, the average numbers of gulls per species breeding in Britain are very much greater than for terns. Using the figures from the national census in Britain and Ireland in 2000, there was a total of 1,810,000 breeding gulls (of seven species), but only 176,000 breeding terns (of five species), indicating a ratio of 10 gulls for every tern. Each gull species was represented by an average of seven times the average numbers for each tern species. Similar large differences are evident elsewhere in Europe and in North America.

The numerical dominance of gull species in the northern hemisphere suggests that much of their speciation occurred there. However, species of the genera Larus, Leucophaeus and Chroicocephalus breed in both the northern and southern hemispheres, indicating that in the past at least one species belonging to each genus must have spread, as breeding birds, across the equator on at least one occasion.

GULL HABITATS

The majority of gull species frequent coastal areas, marshes, rivers, estuaries and large inland lakes. Many occupy the same habitat zones used by marsh and sea terns, and in this respect they contrast markedly with shearwaters and petrels, which are pelagic. The smaller species often feed and breed inland, while the larger gulls breed mainly at coastal sites. Within the last hundred years, several species of large gulls have bred inland more frequently, a change in behaviour that has coincided with their overall increase in abundance.

Gulls breeding on the coast move only moderate distances from the shore. They are tied by relatively short incubation shifts and the need to feed their young frequently and regularly. In general, the density of gulls at sea tends to decline rapidly as the distance from shore increases, although the Kittiwake does not show this tendency in winter. Outside the breeding season, most gulls remain within daily flying distance of the shore, preferring to roost overnight on land or on sheltered coastal waters. The exception to this is when they are migrating. Only the two kittiwake species, Sabine’s Gull and Ross’s Gull, occur regularly in oceanic waters far from land throughout the long non-breeding season.

GULL SPECIES RECORDED IN BRITAIN AND IRELAND

The box is the current list of 26 species recorded in Britain and Ireland as breeding species, regular visitors or occasional vagrants. The list represents about half of all gull species in the world. Kumlien’s Gull is listed, but is retained as a subspecies of the Iceland Gull.

An approximate phylogenetic tree of the evolution of gull species recorded in Britain and Ireland (mainly based on the research by Pons and colleagues) is shown in Fig. 3 and involves eight genera. Such a representation can be only approximate, as their evolution has most likely been multi-dimensional and so cannot be presented accurately in two dimensions. There is still considerable uncertainty about the relationships between the species in the genus Larus, and no attempt is made in the order shown in Fig. 3 to indicate these, other than to suggest that the species with totally white wing-tips probably represent a distinct group.

Audouin’s Gull, which lacks a black head at any time, is placed in the same genus (Ichthyaetus) as two black-headed species on the British list (Mediterranean Gull and Great Black-headed Gull), along with three other black-headed species that occur elsewhere in the world, so its inclusion is surprising. Similarly, the Slender-billed Gull, which has a white head in all seasons, is included with the dark-headed Black-headed and Bonaparte’s gulls. However, Jean-Marc Pons in response to my query believes that ‘the dark hood is not a good character to construct evolutionary relationships because it has repeatedly been lost during the evolution of gulls’. In addition, he confirms that there is additional evidence indicating that the Black-headed and Bonaparte’s gulls should be included in the genera Ichthyaetus and Chroicocephalus, respectively.

The national censuses of gulls and other seabirds have been incredibly important and informative, and at last we have a sound knowledge of both the distribution and numbers of adults. However, we do not have a census value for numbers of immature individuals that have never bred for any of the gull species. Since immature birds may include up to five year classes (varying according to species), the numbers involved are appreciable and can be estimated only from a life table formed from survival rates obtained from detailed marking studies. Table 1 gives rough estimates of the proportion of immature gulls of the six commonest species occurring in Britain and Ireland. The figures are approximations but indicate that, by autumn, there is a large proportion (probably about 40 per cent) of individuals of every species listed that have not yet matured and bred. The proportions of immature individuals will have decreased by early spring because the mortality rate of young birds in their first year of life is usually markedly higher than that of adults, but they will still form an appreciable minority of the numbers of each species.

Gulls recorded in Britain and Ireland

Genus Hydrocoloeus |
Little Gull (H. minutus) | Regular visitor, very occasional breeder
Ross’s Gull (H. roseus) | Vagrant
Genus Xema |
Sabine’s Gull (X. sabini) | Regular visitor, usually in small numbers
Genus Pagophila |
Ivory Gull (P. eburnea) | Vagrant
Genus Chroicocephalus |
Slender-billed Gull (C. genei) | Rare vagrant
Bonaparte’s Gull (C. philadelphia) | Vagrant
Black-headed Gull (C. ridibundus) | Abundant breeder and winter visitor
Genus Larus |
Common Gull (L. canus) | Common breeder and winter visitor
Ring-billed Gull (L. delawarensis) | Vagrant
Great Black-backed Gull (L. marinus) | Common breeder and winter visitor
Glaucous-winged Gull (L. glaucescens) | Vagrant
Glaucous Gull (L. hyperboreus) | Regular winter visitor
Iceland Gull (L. glaucoides) | Regular winter visitor
Kumlien’s Gull (L. glaucoides kumlieni) | Vagrant subspecies
Thayer’s Gull (L. thayeri) | Vagrant
European Herring Gull (L. argentatus) | Abundant breeder and winter visitor
American Herring Gull (L. smithsonianus) | Vagrant
Caspian Gull (L. cachinnans) | Currently vagrant but increasingly recorded
Yellow-legged Gull (L. michahellis) | Visitor and now an occasional breeder
Lesser Black-backed Gull (L. fuscus) | Abundant breeder
Slaty-backed Gull (L. schistisagus) | Vagrant
Genus Ichthyaetus |
Great Black-headed Gull or Pallas’s Gull (I. ichthyaetus) | Vagrant
Mediterranean Gull (I. melanocephalus) | Rapidly increasing breeder
Audouin’s Gull (I. audouinii) | Vagrant
Genus Leucophaeus |
Laughing Gull (L. atricilla) | Vagrant
Franklin’s Gull (L. pipixcan) | Vagrant
Genus Rissa |
Black-legged Kittiwake (R. tridactyla) | Abundant breeder

FIG 3. A phylogenetic tree of the gull species that occur in Britain and Ireland. In general, the shorter the line leading from each species, the more recently that species is presumed to have arisen. This does not apply to the large genus Larus, where more work is necessary to elucidate the affinities of the different species. This diagram includes all of the genera in the world, with the exception of the genus Saundersilarus (where Saunders’s Gull, S. saundersi, is the only species) and the genus Creagrus (where the Swallow-tailed Gull, C. furcatus, is the only species), neither of which occur in Europe. The Vega Gull (L. vegae) has not been included, and its presence in Britain has yet to be confirmed, while Kumlein’s Gull is regarded as a subspecies of the Iceland Gull. Based in part on the work of Pons et al. (2005).

Table 1 also shows estimates of numbers of the six most abundant gulls in Britain and Ireland in about 2000. Again, these figures are estimates, but they indicate that for all six species combined there are about 3.2 million gulls of all ages in Britain and Ireland in the early autumn. This total is probably about 3.3 million in winter, when the numbers of departing Kittiwakes are replaced by immigrant Black-headed, Herring and Common gulls. By late spring, the winter visitors have departed and Kittiwakes have returned to their colonies, maintaining numbers at about 2.6 million just before breeding begins.

SUBSPECIES IN WESTERN EUROPE

There are only a few western European gull species that are represented by more than one subspecies. As mentioned earlier in the chapter, the Herring Gull was previously split into several named subspecies, some of which have now been elevated to species (Yellow-legged Gull, Caspian Gull). Two subspecies occur in Britain and Ireland: Larus argentatus argenteus, which breeds here; and the larger, darker L. a. argentatus, which breeds in northern Scandinavia and north-west Russia, and winters in Britain and the North Sea region. The nominate Common Gull subspecies, L. canus canus, occurs widely in western Europe, while the larger, darker subspecies L. c. heinei, which breeds in Russia, probably (based on ringing data) occurs occasionally in Britain, but is difficult to identify. In the Atlantic, the Kittiwake is represented by a single subspecies (Rissa tridactyla tridactyla); individuals are progressively larger towards the north of its range, but this gradual change does not justify separate subspecies status and is described as a cline. There are probably several other clines among gulls that are yet to be recognised, including the Black-headed Gull, Glaucous Gull and Lesser Black-backed Gull.

SURVIVAL AND LONGEVITY

Gulls are medium- to long-lived birds, with an average expectation of adult life and number of breeding years of different species ranging between four and 12 or more years. Annual adult survival rates for different species vary between 80 per cent and 92 per cent, and often vary appreciably from year to year and over longer periods of time. The annual survival rate tends to be higher in the larger gulls, which also have a longer period of immaturity. This delay in reaching breeding age in gulls appears to be associated with the time that is necessary to acquire competence in obtaining food, but why this should be longer in the large species is not evident. Ringed Herring Gulls that are nearly 35 years old have been recorded, but these represent a few extreme individuals comprising less than 1 per cent of those that reached maturity. In several gull species, the peak of mortality occurs during and just after the breeding season, when the adults are at their lowest weights during the year, suggesting that breeding is a significant stress. Data suggest that survival of gulls is usually high in winter, but the Kittiwake may be an exception, with most mortality occurring while the species is in its pelagic wintering range.

Less is known of the survival of immature gulls, but there is usually a lower survival rate in the 12 months following fledging, after which the survival rate approaches that of the adults.

The longevity records based on birds ringed as nestlings and living under natural conditions are given below, although several individuals are known to have lived longer in captivity.

Mediterranean Gull | 22 years 1 month
Little Gull | 20 years 11 months
Black-headed Gull | 30 years 7 months
Common Gull | 33 years 8 months
Lesser Black-backed Gull | 34 years 10 months
European Herring Gull | 34 years 9 months
Great Black-backed Gull | 29 years 2 months
Black-legged Kittiwake | 28 years 6 months
Ivory Gull | 23 years 11 months
Laughing Gull | 22 years 1 month
Ring-billed Gull | 27 years 6 months
Glaucous-winged Gull | 23 years 10 months

The species with the longest recorded lifespans are mainly those that have been ringed in large numbers, and therefore have more chance of an exceptional record. It should be kept in mind that these lifespans are reached by exceptional individuals – perhaps one in several thousand – and so it is very likely that the maximum known age of many gull species in the wild will increase in future years as more recoveries of marked birds accumulate.

SIZE DIFFERENCES BETWEEN SPECIES

Gull species vary considerably in size. The Little Gull is the smallest and weighs about 100 g (the weight of an Arctic Tern, Sterna paradisaea), while the largest is the Great Black-backed Gull, with males averaging 1,800 g and some individuals exceeding 2,000 g. Fig. 4 shows the average weights of adult females of 19 gull species on the British list. The weights of females of nine of these species are less than 400 g on average and overlap with terns, of which the adult females of all except one species on the British breeding list weigh under 400 g. The distribution chart for male weights is similar, but is shifted to the right because of their slightly greater size.

FIG 4. The average weight of the females of 19 gull species on the British list.

INDIVIDUAL VARIATION

Like all animals, individuals of each gull species show variation in many characters, including size, colour and age at first breeding. Males are larger than females and tend to have a more substantial bill, and size within a species can also vary geographically.

Variation in the immature plumage is widespread in gulls of the same age and this is frequently overlooked in the field identification of species, particularly within the genus Larus. The occurrence of hybrid individuals further adds to plumage variation and typical examples of hybrids can often be identified in the field by experienced observers; the characteristics used often overlap with those of other species. Consequently a proportion of immature and even adult birds that are infrequently recorded in Britain and Ireland may fail to be identified because of potential confusion with other species.

Immature plumages

The plumage, leg and bill colours of recently fledged chicks are very different from those of their parents, to such an extent that, many years ago, a first-year Kittiwake was claimed as a species new to science, despite the adult having already been described and named some years earlier. The first plumage of the young of most gull species is made up of feathers of varying shades of brown and grey, producing a cryptic pattern that helps to conceal them in vegetation in a colony and also appears to reduce aggression from adults. In the smaller gull species, the plumage is replaced by one that resembles the adult at the first annual moult; that is, when the bird is 13 months or so old. In the larger species, all feathers are replaced each year, but only some of the new ones resemble those of the adults and the full adult plumage pattern is not achieved until four years after hatching. These progressive changes in plumage at successive annual moults can vary between individuals and produce a series of different plumage patterns that make it a challenge to identify both the species and the age of immature individuals.

The slow and progressive acquisition of the adult plumage through successive moults contrasts with the rapid growth of bones and wing feathers, which reach full size within a few weeks after hatching because they are necessary before flight can be achieved. Why the acquisition of the adult plumage takes longer in the larger species of gull than in the smaller ones is not clear. The mechanism determining plumage patterns is obviously controlled by hormones and is linked with the greater length of immaturity in the larger gull species, but it is not evident why the large species delay reaching maturity for so long.

Differences between the sexes

The sexes of gulls have identical plumage and differ only in that females are usually smaller and tend to have slightly less substantial bills. Fig. 5 shows the relationship between the differences in weight of male and female gulls of seven species, using data from different parts of their geographical ranges where available. The extent of the difference between sexes is not constant from species to species, but increases with the weight of the species, ranging from a 5 per cent increase in the Black-headed Gull to more than 20–25 per cent in large species.

Standard measurements of wing length also tend to be longer in males than in females, but the magnitude of the difference is much smaller, ranging from 1 per cent in some small species to 6 per cent in the largest species (Fig. 6). Even when this difference is converted to wing area, it still results in the wing loading being higher in the large gulls, which explains why these species typically have a more laboured flight, with a slower, more powerful wing-beat. The small gull species, which are of similar weight to many tern species, have a characteristic buoyant flight similar to that of terns.

FIG 5. The percentage by which male gulls of several species are heavier than females, based on data for seven well-studied species. It is evident that there is a much greater difference between the size of males and females in the larger species of gulls.

FIG 6. The relationship between adult weight and the extent to which the male has a longer wing than females. The percentage difference in wing length between the sexes increases in heavier species, but is much less than the difference of body weight shown in Fig. 5.

The reason why there is a greater size difference between the sexes in large gull species is not known, and currently it is possible only to speculate. Perhaps there is a greater need in the large species to reduce competition for food between the sexes, or perhaps the dimorphism is related to the greater need for males of large species to defend nesting territories. The reader might speculate further, bearing in mind that in the skuas, females are invariably larger than males, while male terns are only 1–3 per cent heavier than females.

Despite the average size differences, there is an overlap in the range of sizes of male and female gulls. Niko Tinbergen claimed that, despite the overlap in size between the sexes of Herring Gulls, invariably the male is larger in every pair. Because of the average difference in size between the sexes, by chance the male will be larger than the female in many pairs, and more so in the larger gull species, but I have not found evidence that the male is invariably larger than the female. Size, and particularly the size of the bill, may play a part in individual birds recognising the sex of other gulls, but it is more likely that behaviour – particularly during courtship – plays the major role in sex recognition in gulls, especially in smaller species.

Sexing gulls

As male and female gulls have identical plumage features, distinguishing them in the field can be very difficult. The most reliable way to determine the sex of an individual bird – without killing it and then dissecting it to examine the gonads – is by carrying out a DNA analysis on samples obtained from feathers or blood. While this method is highly efficient, it is time consuming and it is expensive when large numbers of birds are being studied. In the field, biometric measurements taken while a bird is temporarily captured for ringing can also be used for sexing the individual. I found that the best measure was the head and bill length (from the back of the head to the bill tip), which also had the advantage of showing the highest degree of consistency when measured by different people. Further, the proportionate difference in head and bill length between the sexes is almost twice that for wing length (for example, 9.6 per cent compared to 5 per cent in the Great Black-backed Gull). The only disadvantage of this measure is that in some museum specimens part of the back of the skull was removed during preparation, which prevents it being used in these cases. As shown in Table 2, the head and bill measurement is satisfactory in sexing 92–98 per cent of individuals of several gull species. Including two other body measurements (wing length and bill depth) in a discriminant analysis increased the accuracy of sexing only by less than 2 per cent points.

TABLE 2. The head and bill lengths of adults of six species of gulls in Britain, the measure separating the sexes and the proportions sexed correctly by this single measurement. The data are based on samples of at least 80 individuals of each sex breeding in Britain, except for Common Gulls (Larus canus), which were captured in winter and so were from unknown breeding areas. Based on Coulson et al. (1983a) and additional data.

When a group of breeding gulls is being studied, the behaviour of marked individuals can be used as a reliable method of sexing. Copulation is totally reliable in this respect, as is courtship feeding of the female by the male and intensive food begging by the female. More details on the methodology used to sex gulls are given in Chapter 12.

Adult plumages

There is considerable variation in the shade of grey on the wing and mantle in adult gulls of the same species, which is evident in birds nesting in the same colony. This is illustrated in Fig. 7, which shows the extent of such variation in Herring Gulls breeding on the Isle of May in Scotland (subspecies Larus argentatus argenteus) and in northern Norway (subspecies L. a. argentatus). Because of the variation, there is overlap in wing shades between the two subspecies of Herring Gulls and most, but not all, individuals can be identified on this basis alone (see also box). Even using more measurements of body size does not completely separate all argenteus males from argentatus females.

In Lesser Black-backed Gulls breeding in the Netherlands (Fig. 7), there is also considerable variation in wing shade, with the palest approaching the darkest shade of Herring Gulls breeding in northern Norway. The darkest shade reported in Lesser Black-backs in the Netherlands is said to fall within the shade range of the subspecies Larus fuscus fuscus, which breeds in eastern Scandinavia and typically has a black mantle and wings very similar to those of the Great Black-backed Gull. The majority of Lesser Black-backed Gulls breeding in the Netherlands have a range of shades found in both the subspecies L. f. intermedius (breeding in north-west Europe) and L. f. graellsii (breeding in Britain).

Identification of the Lesser Black-backed Gull subspecies intermedius and graellsii in the field is further complicated by whether the individual is seen in bright sunlight or under dull conditions, and also by the direction of the light, all of which affect the apparent shade of grey of the same individual recorded in photographs or observed in the field. Reliable records of shade need to be measured with the bird in the hand, using standard lighting and comparing it against a reliable shade chart, but even this would not identify the subspecies of all individuals.

FIG 7. The shade of the wings of adult (a) Herring Gulls (Larus argentatus) breeding in Britain (n = 1,591) and (b) northern Norway (n = 140), and (c) Lesser Black-backed Gulls (L. fuscus) breeding in the Netherlands (n = 899). The shades increase in darkness from left to right and correspond approximately to shades of grey, which range from 1 (white) to 20 (black). The data for Lesser Black-backed Gulls are taken from Muusse et al. (2011).

Differences in wing pigmentation

A major source of plumage variation within gull species is the pattern of black or brown pigmentation on the wings of immature individuals. These show progressive changes at each annual moult, until adult plumage is eventually achieved. In addition, this patterning varies appreciably between individuals of the same age, even within a single colony. For example, fully adult Herring Gulls in the same colony showed a range in the number of primaries that are tipped with black pigment (Table 3), and they also showed variation in the extent of white on the tip of the longest (10th) primary. Part of this variation is linked to the age of the birds (Table 4), with change continuing for several years after individuals reach breeding age, but the patterning is not related to gender.

TABLE 3. The percentage of fully adult Herring Gulls (Larus argentatus) examined in breeding colonies with different numbers of black-tipped outer primaries. Dutch data from Muusse et al. (2011), Norwegian data mainly from Barth (1975).

TABLE 4. The wing-tip pattern in Herring Gulls (Larus argentatus) of known age breeding in colonies in Britain. The extent of white on the tip of the 10th primary tends to increase with age.

Part of the variation is also geographical, as seen in adult Herring Gulls breeding on the east and west sides of England and Scotland (Table 5). This difference between east and west in the black-and-white patterning on the primaries is maintained in the winter, presumably because the north–south dispersive movements of British Herring Gulls mainly follow either the eastern or the western coastlines, with relatively few individuals crossing between the two coasts. Figs 8 and 9 illustrate the variation in two adult Herring Gulls captured in north-east England, with one showing the thayeri-type pattern on the ninth primary (second from left), where the black does not spread across the whole of the width of the feather.

TABLE 5. Comparison of wing-tip patterns of adult British Herring Gulls (Larus argentatus) breeding on the east and west coasts of England and Scotland. The differences in both characters are significant.

FIG 8. The wing-tip pattern of an adult Herring Gull (Larus argentatus) captured in north-east England, showing what is known as the thayeri-type pattern on the ninth primary. The bird was subsequently found breeding in northern Norway. (John Coulson)

FIG 9. A typical wing-tip pattern of a Herring Gull (Larus argentatus) breeding in Scotland, with the 10th primary still growing. (John Coulson)

Differences caused by hybrids

One definition of a biological species is that the individuals can form a group of interbreeding or potentially interbreeding organisms that produce viable offspring. If hybrids occur between two species, such as in the classical case of a horse and a donkey, the hybrid offspring of the two (a mule or hinny) are usually sterile, probably because the two parent species have different numbers of chromosomes. In the case of gulls, hybrids are not uncommon and are reported far more frequently than in major taxa, such as terns. Studies have revealed that Larus gulls have the same number of chromosomes (72) and as a result, hybrids are usually fertile and they have been reported breeding successfully. There are now numerous records of gulls of different species and even different genera pairing and rearing hybrid offspring, as listed below:

Mediterranean Gull × Black-headed Gull

Herring Gull × Lesser Black-backed Gull

Herring Gull × Yellow-legged Gull

Lesser Black-backed Gull × Yellow-legged Gull

Western Gull (Larus occidentalis) × Glaucous-winged Gull

Great Black-backed Gull × American Herring Gull

Herring Gull × Glaucous Gull

Glaucous-winged Gull × Glaucous Gull

American Herring Gull × Kelp Gull

Iceland Gull × Thayer’s Gull

Common Gull × Ring-billed Gull

Mediterranean Gull × Common Gull

Laughing Gull × Black-headed Gull

Laughing Gull × Ring-billed Gull

Herring Gull × Caspian Gull

In most cases, adults that are believed to be hybrids have been recognised by the intermediate nature of their plumage and the colouring of their legs and bill, but in only a very few instances has the plumage been described for adults that are known to be hybrids and were ringed as such before they fledged. It is usually believed that hybrid gulls show intermediate characters of their parents in terms of plumage, bill colour and leg colour, but this is not always the case, and in several instances they display minor characteristics not evident in either parent.

There is little doubt that some hybrids can share similarities with, and resemble, other gull species. As a result, it is sometimes difficult to accept a new sight record of a species from a geographical area where it has not been previously or convincingly been recorded before, and to confirm that it is not a hybrid between species that breed nearby. Rarity committees have a particularly difficult job with gulls, and ideally need DNA samples obtained from feathers of the presumed rarity to be certain of the record.

Some hybrid gulls, when adult, have been known to pair and mate with an individual of one of their parent species, producing offspring known as back-crosses. Even less is known about the plumage of these offspring, but it is likely that they differ both from the original species and from the hybrid parent. Breeding between pairs of hybrid gulls has not been recorded. However, hybridisation and subsequent breeding is likely to produce at least three different types of individuals, all of which vary in some respect from the original parent species as well as from each other. The immature plumages of hybrid gulls are poorly known and in many cases their origin has been assumed only because of their intermediate characteristics.

Eventually, after several generations of breeding, a particular gene can be transferred via the offspring of a hybrid from one of the parent species to the other. This has been recorded in the American Herring Gull, which appears to have acquired a gene from the Great Black-backed Gull in North America, presumably through hybrids between the two species. To date, this gene has not been recorded in the European Herring Gull.

In Belgium and the Netherlands, mixed pairs of Yellow-legged Gulls and either Lesser Black-backed Gulls or Herring Gulls have occurred particularly frequently. For example, more than 15 mixed pairs were reported in Rotterdam annually from 1986 to 1998 (van Swelm, 1998) and more in more recent years, and others have been frequently identified in at least five other colonies in the Netherlands and Belgium. Hybrid individuals that have reached adulthood and that are presumed to be crosses between Yellow-legged and Lesser Black-backed gulls have also been recorded in Belgium breeding with Lesser Black-backed Gulls, producing back-crosses.

Inter-species breeding is more frequent when one of the gull species is rare and spreading into the main range of the other. For example, when Lesser Black-backed Gulls first started to breed in the Netherlands in the 1930s, a few individuals joined large colonies of Herring Gulls and several mixed breeding pairs were recorded. Despite the fact that both species are now numerous and breed in the same colonies, hybrid pairs still occur, although they are infrequent. Very few pairings between these two species have been reported in Britain, except when experimentally induced (see below).

When Herring Gulls spread to Iceland in the 1920s, individuals formed mixed pairs with Glaucous Gulls, and by 1966 about half of the adults there were considered to be hybrids. These were distinctive in showing small but variable amounts of dark pigment on the tips of the primaries (Ingolfsson, 1970).

When Mediterranean Gulls first started to breed in Britain, early pioneers frequently paired with Black-headed Gulls (as they have done so elsewhere). In fact, this is ongoing, as a few individuals continue to spread north from the south coast of England. The recent arrival of a few adult Yellow-legged Gulls in Britain has seen them join both Herring Gull and Lesser Black-backed Gull colonies. Again, they have formed mixed pairs that on some occasions have managed to fledge hybrid chicks. Perhaps this inter-species breeding occurs because individuals arriving in new areas are mainly of the same sex and fail to find a mate of the same species.

In a study carried out on the island of Skokholm in south-west Wales, Mike Harris (1970) switched large numbers of eggs between Lesser Black-backed Gull and Herring Gull nests. The chicks that subsequently hatched imprinted on their foster parents and apparently considered that they were the same species, so that when they matured they chose a mate of that species, forming a series of mixed-species pairs. The young produced and reared by these mixed pairs were hybrids between the parent species and usually (but not always) showed plumage and leg colour intermediate between the two. At least 40 of these hybrids later returned to breed on Skokholm and on nearby Skomer, and most paired with adult Herring or Lesser Black-backed gulls. The chicks they produced were back-crosses and, when adult, were more similar to one of the parent species than the first generation of hybrids. While some of these hybrids reared chicks, it is not known whether they and their offspring were less viable. However, as each generation was produced, presumably both parent species incorporated small amounts of the genetic material belonging to the other species into their make-up despite appearing to be ‘pure’ Herring or Lesser Black-backed gulls (as discussed above for the American Herring and Great Black-backed gulls in North America).

BREEDING

Gulls are monogamous, although a few cases of male Kittiwakes breeding simultaneously with two females at different nest sites have been recorded. Pairs of gulls produce only one brood each breeding season, but if their eggs are lost, many will lay a replacement clutch. While most gulls breed annually during a well-defined breeding season, some individuals skip breeding for a year. The exception is the Swallow-tailed Gull on the Galapagos Islands, which does not have a clear-cut breeding season and nests throughout the year, with individuals breeding at nine- to 10-month intervals.

Breeding sites

Gulls typically favour bare ground and areas with short vegetation for nesting, or floating vegetation on lakes or marshes. The main exceptions are Bonaparte’s Gull, which regularly nests in trees; Common, Black-headed and American Herring Gulls, which occasionally nest in low trees at a small number of localities; Kittiwakes, which favour narrow ledges on steep sea cliffs; and Herring, Glaucous and Ivory gulls, which sometimes use larger cliff ledges.

Ground nesting makes gulls particularly susceptible to mammalian predators, and most species nest only at sites where these predators are usually unable to reach the colonies, such as small islands or isolated peninsulas. Gulls vary in their ability to deter avian predators. Adults will attack birds of prey and corvids, but in parts of northern Scandinavia White-tailed Eagles (Haliaeetus albicilla) are having an increasing impact on breeding gulls – this is a future risk for Britain, since the species has been reintroduced here and its numbers are increasing. In addition, adult gulls at breeding sites suffer occasional predation from Peregrine Falcons (Falco peregrinus). Ravens and crows are a problem for some smaller gulls, but in general they are attacked and prevented from entering dense gull colonies. Individual Herring, Lesser Black-backed and Great Black-backed gulls, as well as Great Skuas (Stercorarius skua), have developed the ability to reach and prey on eggs and young gulls at otherwise well-protected nesting sites (sometimes even attacking their own species).

Mammalian predators such as Red Foxes (Vulpes vulpes) and Badgers (Meles meles) have now reached some gull colonies in Britain after being absent for many years, and these and the spread of American Mink (Neovison vison) has often resulted in the sudden desertion of sites used by breeding gulls. This desertion may be immediate, while in other cases the decreases in numbers of adults are spread over several years, apparently because new recruits to the colony are deterred by the presence and activity of predators. In particular, Foxes have become very much more abundant in Britain in recent years, and have captured and killed many incubating gulls at night. Six gull species now also nest on buildings in urban areas (here); these have the same characteristics as natural sites, in that mammalian predators cannot normally reach them and they are generally given public protection.

Humans entering gull colonies are not usually attacked by the smaller gulls, which instead tend to fly overhead giving alarm calls. Large gulls do frequently dive at human intruders, however, usually from behind. While they pass closely overhead, they seldom actually strike. I have been struck only once by a large gull, although one of my students was knocked to the ground by a particularly aggressive Lesser Black-backed Gull defending its unfledged chicks.

Colony and nest site fidelity

Adults gulls are highly site faithful, provided that the nesting site remains safe and is not subject to high levels of predation on eggs, young or adults. Males – particularly those that were successful in rearing young in the previous breeding season – tend to return to the same nesting sites. In contrast, young birds are much less likely to return to the place where they were reared as chicks. A proportion – usually dominated by males – does so, and these birds often return to the same part of a large colony in which they were reared. This behaviour is called philopatry, the extent of which is influenced by many factors, including competition for nesting sites and food availability. In the past, the proportion of birds moving elsewhere has often been underestimated because of the much greater difficulty in locating those that have moved. In some species, the majority of the young that survive to breed move to other colonies, with some moving 100 km or more away. In several species, young individuals have been recorded visiting a number of colonies while approaching maturity, including their natal colony, only then to move and breed elsewhere. Such movements are, of course, necessary to form new colonies.

Colonial breeding

Colonial breeding is widespread among gulls, and only a few species regularly breed both as isolated pairs and in colonies. Some gull colonies are composed of mixed species, and the smaller species frequently nest in or alongside colonies of terns. In many gull species, it appears that single pairs cannot breed in isolation and the presence of a group of gulls of the same or even different species is necessary before egg-laying is possible, thus making colonial breeding essential. The main exceptions are Common and Great Black-backed gulls, where a proportion of breeding pairs nest in isolation from others. It is obvious that colonial nesting is not forced upon gull species as a result of shortage of suitable nesting sites, and it is usually regarded that there is an advantage to breeding close together. An obvious reason is that it improves defence of eggs and unfledged young against predation, but while this is evident in deterring Ravens (Corvus corax) and crows, it is less evident that colonial nesting prevents mammalian predators from raiding colonies and consuming eggs, young and even adults.

The reliance upon a group to ensure breeding suggests that stimulation from other individuals is necessary. This was first suggested in 1938 by Frank Fraser Darling, who noted that the display from neighbours within the colony stimulated pairs to breed and in larger groups or colonies to lay eggs earlier, and that it resulted in greater synchrony with neighbouring pairs. He suggested that the effect of this synchrony was that eggs or chicks were available over a shorter time period, and therefore fewer were predated and breeding success was enhanced. At the time, Fraser Darling’s idea appealed to some, but others were critical of the concept. Unfortunately, his original data on Herring and Lesser Black-backed gulls only hinted at this effect, and others later showed that the variations he found between colonies of different sizes could have been produced by chance and were not statistically significant.

In the 1950s, Edward White and I collected information on the timing of breeding in a series of colonies of Kittiwakes. We found that the laying season in large colonies was more spread out than in small ones, the opposite of Fraser Darling’s contention. While some pairs in the larger colonies started laying earlier than those in the smaller ones, pairs of late-breeding birds (which we now know were mainly young, first-time breeders) were recorded in all the colonies studied.

However, we did discover that breeding in smaller groups of 20–50 Kittiwake pairs within a single colony was more synchronous than in the colony as a whole, and that the average breeding date of each of these groups was earlier as the density of pairs increased. This finding suggested that social stimulation was occurring at the more local level, among groups of breeding pairs, and not in the colony as a whole.

I know of no single pair of Kittiwakes that has been recorded breeding in isolation. As a result, it is difficult for members of the species to form new colonies because this requires a group of individuals to be attracted to, and display at, a new potential breeding site before egg-laying can occur. Up to 20 pairs will collect at the site of a potential new colony for a year or more, and before a few of the pairs succeed in building a nest and laying eggs. However, in subsequent years the numbers breeding increase rapidly. In contrast, Herring Gulls sometimes nest in isolation, but these are birds that have bred before. Young Herring Gulls attempting to breed for the first time do not seem to be able to breed without joining a group already breeding together.

Colonial nesting allows gull species to benefit from the stimulation of their neighbours engaging in courtship activity, and probably encourages earlier nesting in some individuals. This was clearly evident in Kittiwakes when observing a pair reuniting at the colony after one member had been away on a fishing trip (Coulson & Dixon, 1979; Coulson, 2001). The birds engaged in a vocal display of calling, which immediately stimulated similar mutual calling in neighbouring pairs. Where Kittiwake nests were denser, the frequency of this stimulation from neighbours was more frequent. Anyone who has visited a Kittiwake colony will recognise the loud, periodic outburst of calling by groups of pairs as a major feature of the pre-laying period.

New colonies of Kittiwakes and Herring Gulls are highly attractive to potential breeders, and as a result they increase rapidly in size. Part of this effect is a result of the low density of breeding birds in a new colony, with relatively large areas available where recruits can settle without aggression from neighbours. As colonies grow, the area available for new recruits within, and at the immediate edge, decreases relative to their size, and their rate of growth decreases over time. Whether this eventually determines the maximum size for colonies of some species is unclear, but remarkably large colonies are known, numbering tens of thousands of pairs. In contrast, Great Black-backed Gull colonies rarely exceed 200 pairs in Britain (although much larger colonies are known in North America), while large colonies of Common Gulls do occur but are exceptional.

Mixed colonies of Herring Gulls and Lesser Black-backed Gulls are common, but the two species show a tendency to select slightly different areas within the colony. On natural sites, they often nest side by side, but the Herring Gull more frequently nests on open ground with little or short vegetation, while the Lesser Black-backed Gull shows a partial preference for areas with taller vegetation. At urban sites, both species frequently nest side by side and there is no obvious or consistent difference in the sites they select. Great Black-backed Gulls often nest in isolation or in small numbers of pairs in colonies of Herring Gulls or Lesser Black-backed Gulls, where they frequently select sites on raised ground.

Terns often nest in association with Black-headed Gulls, with the latter arriving and laying earlier and taking the first choice of sites. However, this is not always the case – the first Black-headed Gulls to nest on Coquet Island in Northumberland did so alongside Sandwich Terns (Thalasseus sandvicensis), and only arrived after the terns had laid.

Age at first breeding and mate fidelity

Within a species, the age at first breeding is variable and can span three or more years. Smaller gulls tend to breed when they are two or three years old, while many of the large gulls do not normally start breeding until they are four or five years old. This long period of immaturity often allows individuals to visit colonies for one or more years before they breed themselves.

The claim that gulls pair for life is not justified. While the same individuals frequently re-form as a pair in successive years, ‘divorce’ often occurs, with both individuals taking new partners. Change of mate also occurs when one partner dies.

Nests

In general, gulls’ nests are often substantial structures composed of plant material collected by the adults nearby or, in some cases, picked up either from the sea surface, on the tideline or from cliff tops. Gulls that breed on marshes and ponds probably start nest-building earlier than those on drier sites, and they produce a platform above the water level on which most of the displaying and courtship takes place. In situations where the water level rises after rain, there is often an attempt by the pair of gulls to build up the height of the nest to keep the eggs dry. However, this is effective only when relatively small increases in the water level occur.

On several occasions I have seen Kittiwakes flying along the coast carrying nest material as far as 15 km from the nearest colony. This contrasts with Herring Gulls, which usually collect plant material for the nest from within their territory, although if it is not available, such as on roofs of buildings, material is collected from further away. Having collected material, adults shape a central hollow in the centre of the nest by compressing the material with their breasts. This hollow is large enough to hold the usual two or three eggs of the clutch together and prevents the eggs from rolling away by accident.

While nests of most gulls are formed by an accumulation of plant material, the exception is the Kittiwake. The adults collect marine algae and mud to form a base for the nest, and as this dries, it becomes firmly cemented to the rock surface, forming a base on which the rest of the nest is then built. The nest is completed by bringing in grass to form the top. This species is exceptional in that the adults at the nest defecate onto and over the edge of the structure, leaving obvious white streaks on both the nest and the cliff face below, and thus clearly make no effort to disguise the nest. Other gull species move away from the nest to defecate. The Kittiwake also differs from other gulls in that nest-building is often highly social, with a hundred or more individuals repeatedly flying back and forth between a particular source of mud and vegetation and the nest sites. On one occasion when in spring a local farmer spread straw and dung onto a cliff-top field, within a week the nearby Kittiwake colony was entirely draped with long strands of golden straw.

Recently fledged young and immature one- and two-year-old Kittiwakes often pick up potential nest material from the tideline and fly about with it in their beaks, but eventually drop it without taking it into a colony. I have not seen immature individuals of other gull species doing this. Non-breeding three-year-old Kittiwakes often occupy sites in a colony and bring in small quantities of nesting material very late in the breeding season, when most pairs of Kittiwakes have half- or full-grown chicks. In some cases, this forms a base for a nest in the following year. The chicks of most gulls leave the nest when a few days old, seeking cover and protection, but the Kittiwake is again exceptional in that the chicks remain on the nest until they fledge five or more weeks after hatching.

Depending on the position of the colony, some Kittiwake nests are washed off in winter storms. However, those in more sheltered positions remain over winter and until the following breeding season when they are used as the base of new nests. One nest on the Farne Islands was supported in a vertical cleft in the rock face, and as material accumulated year after year, it reached a height of more than 100 cm.

Eggs

All gull eggs have a brown or grey background with numerous spots or streaks of darker pigment, offering a degree of camouflage if they are left uncovered in the nest. The extent to which camouflage prevents or reduces predation of gull eggs has not been investigated, but it can only be one of several factors affecting successful breeding.

The size of egg laid by gulls varies with the average weight of each species, but not in a simple linear manner (Fig. 10). At one extreme, the Little Gull lays an egg that, on average, weighs 18.6 per cent of the normal body weight of females, while the egg of the largest species, the Great Black-backed Gull, weighs only 7.3 per cent of the female’s body weight. The change in egg size as a proportion of the adult weight is a common effect in many bird groups and not just a peculiarity of gulls. Large eggs, irrespective of the species laying them, usually require a longer incubation period, although the reason for this is not immediately obvious. Presumably the body of a chick of a large gull species has more cells and requires more cell divisions before it is ready to hatch. However, that is not the only factor involved, since the eggs of birds in some other taxa, such as the Procellariiformes (petrels and shearwaters), require a much longer incubation period even when the eggs are of the same size.

FIG 10. The relationship between the average weight of adult females of 13 gull species and the average weight of the eggs they lay. The gull species that normally lay two rather than three eggs (Kittiwake, Rissa tridactyla, indicated by the lower of the two filled squares; and the Ivory Gull, Pagophila eburnea, indicated by the upper filled square) sit exactly on the curve calculated for the other species, which normally lay three-egg clutches, indicating that despite having smaller clutches they do not lay bigger eggs.

FIG 11. The relationship between the average weight of adult females of 13 gull species on the British list and the average weight of one of their eggs, expressed as a percentage of the female’s weight.

While the most frequent clutch size in gulls is three eggs, two species that occur in Britain typically lay two-egg clutches, namely the Kittiwake and Ivory Gull. This raises the question as to whether they respond by laying larger eggs. At first inspection, Kittiwake eggs look large, but this is an apparent effect of their paler background in comparison to the eggs of most other gulls (see here). Fig. 11 shows the relationship between female weight and egg weight for a series of species. The relationship is curvilinear, but the Ivory Gull and Kittiwake sit almost exactly on the curve, fitting the data based on the other gull species.

Each egg is laid at about two-day intervals. Usually, the last egg in the clutch is the smallest and tends to differ in shape, being longer and narrower than the rest. The infrequent cases of four eggs in a clutch are probably the result of a second female contributing one or more eggs. This can occur when female–female pairs are formed, caused by a shortage of males. Incubation lasts for more than three weeks and is longer in the larger species. Both members of the pair contribute to the incubation and feeding of the young.

Incubation period

Ideally, the precise measure of the incubation period is the length of time that the eggs are maintained at a temperature just below the adult’s body temperature of about 40 °C. There is a lower temperature (which may be as high as 25 °C) below which the development of the embryo ceases; in temperate regions, this is usually above the environmental temperature. As a result, development soon stops when the eggs are not covered by an adult and the temperature falls below the critical temperature for development. If the incubation of the eggs is frequently interrupted, the time taken for the eggs to complete development to hatching is lengthened.

In practical terms, the incubation period is usually measured as the number of days taken from the start of incubation to the time the egg hatches, but hatching takes several hours and confusion has existed as to whether the end of incubation is when the shell is first pierced by the chick or when it becomes entirely free of the shell. Covering the eggs by an adult gull often takes place before the final egg of the clutch is laid, but in some cases the vascularisation of the brood patches on the adult may not have become fully developed with supplementary blood vessels and the optimal temperature for embryo development may not be reached. Consequently, there may be a delay of a day or so until the eggs reach the critical temperature for development. As a result, the recorded incubation period for a species will vary. The incubation period of birds, and in gulls in particular, is slightly longer in the larger species, but the differences are small and the variation recorded between individual pairs is considerable (Table 6).

In gulls, the incubation of the eggs is often initiated by the female, but overall the sexes tend to share incubation equally in a shift system. Either sex may incubate overnight. In most cases, the incubating individual remains on the eggs until relieved by its partner, so that once incubation starts, the eggs are covered by an adult for about 90–95 per cent of the time.

TABLE 6. Estimates of the incubation period in eight well-studied gull species, presented in order of adult size (smallest first). Data mainly from Cramp & Simmons (1983), Fisher & Lockley (1954) and other sources where appropriate, including personal data for the Black-legged Kittiwake.

Periodically – perhaps five to eight times a day – the incubating adult will stand up and roll the eggs. Egg-turning is believed to facilitate the absorption of the albumen by the embryo and prevents the embryo from adhering to the shell membrane. In domestic hens, the failure to roll eggs in an incubator does not necessarily prevent chicks from hatching, although the success rate is reduced and the chicks often have a lower weight. Egg-turning is unique to birds and the reason why reptile eggs do not need to be rolled is as yet unclear.

In gulls that lay three-egg clutches, it is common for the third egg – which is laid two or three days later than the second egg – to hatch one or sometimes two days after the first two. This asynchronous hatching suggests that effective incubation in these cases starts during the normal two-day period between the laying of the second and third eggs.

Chicks

At hatching, chicks of most gull species have a patterned down that offers some cryptic protection; this is replaced by a pattern of dark feathers that helps to conceal the young birds when they attempt to hide under vegetation. The dark patterning at hatching is missing in Kittiwake chicks, which hatch with a pale grey down and then acquire white, grey and black feathering when older, all of which could also be regarded as aiding camouflage among the whitewashed nests and rocks on the cliffs where they nest.

An effect of the relationship between egg size and adult weight is that, at hatching, the chicks of larger gulls are smaller in proportion to the adults than those of small gull species, and they therefore must achieve more growth in order to reach their adult size and weight. This is presumably one of several reasons why the period of chick growth (and the fledging period) tends to be longer in large gull species than in small ones.

The time between the chick hatching and first flying is known as the fledging period and also shows much individual variation (Table 7). There is a weak trend for it to be longer in the larger species of gulls. The fledging period of the Kittiwake tends to be longer than in other gulls of a similar size. In the case of the Kittiwake, the first flight has to be totally successful from the cliff-nesting site and so is an all-or-nothing event, with the young bird either flying well enough to reach and return to the nest, or to achieve sustained flight immediately, leave the colony and start an independent life.

Fledging is often a gradual process and tends to take longer in the larger species. It is longer in Kittiwakes, where the young cannot fly until they are at least five weeks old and many are at least a week older before their first flight. Unlike most terns, parental feeding of young gulls often ceases at the time of fledging, but sometimes family parties of large gulls stay together for some weeks. During this time they visit feeding sites together and make loud contact calls while moving between areas, and they may even return to the colony in the evening. Exceptionally, a Herring Gull has been observed feeding chicks three months after they fledged.

TABLE 7. Estimates of the fledging period in eight well-studied gull species, presented in order of adult size (smallest first). Data mainly taken from Cramp & Simmons (1983), Fisher & Lockley (1954) and modified from other sources where appropriate. Note that the considerable variation in several species is genuine, and depends on the rate of growth and development of the chicks and whether there had been a stimulus to fly, for example, by the presence of a predator or the observer.

Breeding success

Under favourable conditions, breeding success of gulls is high, with about half the eggs laid producing young that fledged. However, lower success rates occur when there are food shortages or predation occurs. No gulls are known to breed as one-year-olds and they do so only when they have acquired almost adult plumage.

WALKING AND FLYING

Gulls have relatively short legs, and this is particularly true of Kittiwakes and Ivory Gulls. Gulls typically walk or run only short distances, and rapid movement beyond a few metres is usually achieved by flying. The exception to this occurs when gulls are searching for food on grassland and mudflats, when walking – with stops to pick up food – is the norm. Of all gull species, Kittiwakes walk less than any other. During an appreciable part of the year they are oceanic, either flying over or floating on the sea. Immature Kittiwakes that visit land during moult typically remain where they alight, and the adults fly directly onto their nests on precipitous sea cliffs without needing to take more than a step or two as they land. This behaviour is associated with the species having short legs (an adaptation to produce a low centre of gravity to enable the birds to withstand strong gusts of wind while nesting on sea cliffs), while the Kittiwake’s hind toe, which in other birds – including gulls – assists with walking, is reduced to a small protrusion.

The long, narrow wings of gulls, petrels, albatrosses and shearwaters are an adaptation for flight over water, particularly where the sea surface has peaks and troughs that affect wind movement. Many gulls fly with steady wing-beats, while at other times they introduce periods of gliding, between wing-beats. They utilise rising air currents, particularly those over the sea produced by waves or swell, although it is rare for them to fly more than a few metres above the surface of the open sea. In addition, they glide in thermals over land and cliffs, and those produced by buildings in urban areas. Several species of gulls use thermals to rise to considerable heights over land without the need for flapping flight, although the smaller species more rarely soar. Favourable conditions for soaring occur on warm, sunny days. Such apparent effortless flight can be seen in Herring Gulls that breed on the coast but visit the highest ground on Snowdon in north Wales, where they search for food discarded by people walking and climbing here.

MOVEMENTS

Gulls breeding in extreme northern and southern areas of the world are migratory, but this is usually less pronounced or even absent among those living in temperate or equatorial regions. In general, the distances gulls migrate are much shorter than those made by terns, with only Sabine’s and Franklin’s gulls making regular large trans-equatorial migrations. In other gull species, a few individuals may reach or cross the equator, but most remain in the hemisphere in which they breed or were reared. Whether gulls migrate mainly depends on whether food is readily available in winter in their breeding areas.

The seasonal movements of many gulls are better described as dispersal rather than migration, with individuals from the same area or even a single colony moving in different directions and for variable yet often short distances. Immature individuals tend to move further than adults and often do not visit their natal or other colonies where they will ultimately breed for several years.

MOULTING

In all gulls, the 11 primary feathers on the wing are moulted in sequence, with the inner primary being replaced first. The remaining primaries are then progressively replaced outward towards the longest, the 10th, and then finally the small 11th primary on the bastard wing (Fig. 13). With the exception of one species, the flight and tail feathers are moulted and replaced once a year, but some of the body feathers are moulted twice each year.

The exception to this is Franklin’s Gull, which moults all its feathers – including the primaries – twice a year, and as a result adults are missing primaries somewhere in their wing for about two-thirds of the year. This is a surprising pattern, because the efficiency of flight is affected by the loss of some of the primary feathers, particularly the longer outer ones. Most adults will have just completed the post-breeding moult when they start the next primary moult in December, which continues for several months. The information on moult in Franklin’s Gull is based on a small number of specimens and requires further investigation, but the available evidence suggests that the post-breeding primary moult is spread over four months and the pre-breeding moult over a slightly shorter period. Carrying out two primary moults in a year is energetically and nutritionally costly, and no convincing explanation has been offered for it. It could be that exposure to more intense sunlight throughout the year (Franklin’s Gulls winter south of the equator) causes the structure of the primary feathers to deteriorate quicker. However, this is not a convincing explanation, because other gull (and tern) species living in similar areas and making migrations across the equator appear to manage with a single annual primary moult (see also here).

FIG 12. Herring Gull (Larus argentatus) presumably starting its third year of life, still lacking white mirrors and with the new outer primaries and the outer tail feathers still growing. (Mike Osborne)

FIG 13. Above: adult Lesser Black-backed Gull (Larus fuscus) in primary wing moult, with only five of the longest primaries remaining to be moulted and several new, inner primaries in growth. Below: adult Black-headed Gull (Chroicocephalus ridibundus) in primary moult, with the two longest primaries (P9 and P10) not yet dropped and replaced, P8 missing and P7 half-grown. (Nicholas Aebischer)

FOOD AND FEEDING

Gulls are usually classified as omnivores and scavengers, and animal material greatly dominates the diets of all species. The spectrum of food sources taken by gulls is much wider than in terns. Fish and invertebrates are the main food of many species, and small mammals are also taken by the large species. In addition, the larger gulls are often predators on eggs, chicks and even adults of other birds, and several consume eggs and chicks of their own species, although they usually avoid killing their own broods.

Typically, gulls obtain their food from the ground or near the surface of water. Most species can plunge from flight into water in an attempt to capture live food organisms within the top metre or so, but they lack the penetrating diving thrust of Northern Gannets (Morus bassanus) and some terns. Small gulls often pick food items from the water’s surface, either while in flight or phalarope-like when floating on the surface. Gulls breeding inland feed on insects emerging from freshwater bodies and from flat terrestrial areas, and some even catch flying insects such as swarming ants and mayflies.

No gull species regularly and predominantly feeds on vegetable material, although some take berries and grain – for example, Herring and Lesser Black-backed gulls regularly search for animal feed on farmland and also consume grain in the spring and autumn. In some arctic areas, berries have been reported as a component of the diet of gulls, but this source of food is uncommon in more southern areas. That said, there are reports of berries being plucked off Hawthorn (Crataegus monogyna) bushes and even acorns still on oak trees being consumed by Black-headed Gulls. Many stomach analyses of gulls report small quantities of plant material, but these could have been accidentally ingested when picking up animal prey from the ground or even originate from the guts of the animals the gulls consumed. Many gulls will eat bread and other vegetable matter when offered it or when it is discarded by humans, and they often compete for food offered to waterfowl. The late Max Nicholson, who wrote Birds and Men (New Naturalist 17, 1951), was one of many who frequently and regularly fed gulls in winter at lakes, in parks and on the riverside embankment in London in the first half of the twentieth century.

In the last century or so, several species of gull have started to utilise waste materials dumped by humans and they also follow behind tractors during the ploughing of agricultural land. Availability of foods associated with human habitation has attracted gulls to places such as landfill sites and fishing boats, particularly when nets are being hauled in or fish are being gutted at sea. In many areas across Britain, such sites have become a major food source for several species and have played a role in the growth of gull populations. As a consequence, some species are now considered pests and their numbers are managed. As refuse is increasingly being managed in ways that gulls can no longer exploit, and landfill sites previously used by gulls are being closed, this is likely to have an effect on the size of gull populations. Evidence of the magnitude of such changes is yet to be clearly identified, however, and the extent to which gulls can change their feeding habits and find new food sources remains unknown.

Scavenging on carrion or stealing food from other birds (kleptoparasitism) are other commonly employed methods of obtaining food. Kleptoparasitism in gulls is sometimes directed towards their own species and seems more frequent among male gulls. Smaller species of gulls join flock of Lapwings (Vanellus vanellus) and Golden Plover (Pluvialis apricaria), and steal prey items captured by the plovers, particularly when these are too large for them to swallow immediately. Sea ducks, particularly Eiders (Somateria mollissima), are attacked by large gulls when they surface with prey items in their bill, such as crabs, that need to be handled and processed before they can be swallowed. Indigestible items that are eaten by gulls as part of their diet are regurgitated as pellets, and offer a method of identifying some of the food consumed.

DRINKING

All animals need to replace fluid in their bodies; while some obtain sufficient water in their food, others will travel some distance to obtain it. Those species that spend long periods of time at sea or nest on small islands without available fresh water have no option but to drink seawater, although this causes physiological problems owing to its appreciable salt content.

A sailor crossing a large expanse of ocean in a small boat can use a solar still to produce drinking water, and gulls (and many other birds) have the equivalent of this in the form of nasal glands, which are situated between the eyes. When they drink seawater, gulls use these glands to excrete a saline solution that has twice the concentration of the seawater, and by doing so reduce the concentration of salt in their bodies. Not infrequently, gulls are seen with water dripping from the end of their bill. These drops do not necessarily indicate that the bird has recently dipped its beak into water, but are the hypertonic saline solution excreted from the nasal glands. Using this method to remove excess salt from the body involves an energy cost for the birds, so it is not surprising that gulls show a strong preference for accessing and drinking fresh water wherever possible and are frequently seen congregating at freshwater pools and lakes. Herring Gulls at some urban sites even use small pools of fresh water when larger freshwater bodies are not available (Fig. 14). While Kittiwakes depend entirely on seawater and the healthy functioning of their nasal glands in their pelagic wintering areas, in the breeding season some will fly up to 2 km inland from their colonies to drink and bathe in fresh water. Those gulls that breed in colonies where no fresh water is locally available must depend on their nasal glands throughout the year.

FIG 14. Urban Herring Gull (Larus argentatus) attracted to drink at a small source of fresh water. (Mike Osborne)

VOICE

Most gulls are vocal, particularly in the breeding season, and this plays a part in their breeding behaviour. Calls are usually harsh and loud, and are used in territorial defence, as an alarm when birds are disturbed by predators or in greeting a mate. Quieter calls are used during courtship, and specific calls are also uttered, such as the wack-wack call of the Kittiwake during breeding, which tells its mate that it is not moving locally but is leaving and will be away some time on a feeding trip. Other specific calls are used during copulation, to maintain contact in flocks in flight, and to indicate that a food source has been found and that the calling individual is about to feed.

Studies using sonographs to analyse calls for several gull species have shown that there is a wide range of variation in the same call made by different individuals. Playbacks of the calls indicate that birds can recognise the call of their own mate but fail to respond to the same call when it is made by other individuals, suggesting that voice plays a part in individual recognition in addition to visual cues. Chicks of many gull species are able to recognise their parents at a relatively young age, and likewise the parents can identify their own chicks, apparently by their calls and actions – they respond differently to strange individuals. This is particularly important in species where the young wander or group together in a crèche. The Kittiwake is the exception in that adults appear to be unable to recognise their own chicks, although the usual isolated position of the nest on a vertical cliff face forces most chicks to remain in the same nest until they fledge. Occasionally, when two nests are built side by side, a Kittiwake chick will move across to the neighbouring nest and join chicks already there. If the wandering chick is placed back in its own nest, it soon returns to the neighbouring nest again, but such moves seem to be accepted by both pairs of adults.

ROOSTING

Gulls are inactive in the darkness of the night, spending these hours passively, although some species feed locally where there is strong artificial illumination, such as around working trawlers and in town centres. Most gull species spend the night in large flocks, with many roosting on water in sheltered bays at the coast, on reservoirs and on lakes, or on land in places that are free of predators, such as small islands.

Gull roosts are composed of single or mixed species, and may number thousands of individuals. Roost sites are often used year after year if their position excludes mammalian predators. Most species avoid roosting on water exposed to severe wave action, presumably because they do not possess the behavioural adaptations needed to cope with resting and sleeping while sitting on rough or breaking water. Sabine’s Gull, Ross’s Gull and the Kittiwake are totally oceanic outside the breeding season and are the only gulls that regularly spend the night far from shore. Kittiwakes will also roost offshore during the breeding season if they are not incubating or protecting small young in the nest. Before egg-laying, they leave the colony at, or long after, sunset and fly many kilometres out to sea in the dark.

CHAPTER 2
Black-headed Gull

APPEARANCE

The Black-headed Gull (Chroicocephalus ridibundus) is a small gull with a wingspan of about 85 cm. It is common both on coasts and inland, and while it is less abundant than the Kittiwake (Rissa tridactyla), it is much more familiar to the public thanks to its dark head in summer (Fig. 15, left), which gives rise to its common name. The hood is actually dark brown, not black, and while the eye is also dark, the white semicircle around the back of it draws attention to it when the bird is seen at close range. The species is frequently attracted to food in towns and even large cities, both inland and on the coast, but for more than half of the year adults lack the dark hood and are more difficult to identify. At that time, the white head has only small, dark grey patches behind the eyes, but the red bill and legs, and the light leading edge to the wing when in flight, aid recognition (Fig. 15, right). The sexes are similar, with the male being only slightly larger than the female.

FIG 15. Adult Black-headed Gull in summer plumage (left) and winter plumage (right). (Left, Alan Dean, right, Nicholas Aebischer)

Juveniles differ markedly from the adults. They have brown feathers on the wings, on the back of the neck and on top of the head, and these form part of a cryptic patterning that helps to conceal them in vegetation before they fledge. During the first year of life, they lack the bright red legs and bill of the adults, and, although present, the white outer edges to the wings are less obvious. First-years have a black band to the tail, but as this is also present in several other species of small gulls in their first year of life, it is a useful characteristic to age but not to species identification. In the first summer after hatching, the immature birds develop a dark hood later than the adults; this is usually incomplete and mixed with some white feathers. They also retain most of the dark band to the tail. Following the autumn moult, immature individuals entering their second year of life assume the adult plumage, including an all-white tail, but many still have less intensely red legs at this stage.

DISTRIBUTION AND HABITATS

The Black-headed Gull is widely distributed in Europe and breeds across Asia, and there is no evidence of subspecies within this large range. The species extended its western range in the twentieth century, reaching the Faeroes early on at an unrecorded date, Iceland in 1910 (50,000 adults now breed there) and Greenland in 1969. Breeding in North America was suspected as early as 1963 but not proven until 1977, when nesting by 14 pairs was recorded in Newfoundland. More recently, breeding has been suspected at other sites in North America, with recently fledged young being reported at several locations from Labrador to Massachusetts. Several young birds ringed in Iceland have been recovered during winter in Newfoundland and Labrador, but only one ringed individual of this species is known to have crossed from the mainland of Europe to the New World. It was ringed in the Netherlands and recovered 200 km off the coast of Labrador, so it seems likely that those seen in North America are mainly individuals reared in Greenland and Iceland.

Movement to North America has progressed in a series of stages rather by a sudden irruptive transatlantic movement from Europe, with birds breeding or reared in the Faeroes, Iceland and Greenland gradually moving west. This spread, together with a northern movement in Norway, Sweden and Finland, has been interpreted as a response to climate change. However, this theory ignores the spread of breeding to Italy (1960) and Spain (1960), and the fact that the increase within the species’ core area in Europe during the twentieth century has probably been mainly responsible for the spread of the western limits of its breeding distribution.

Today, unknown numbers of Black-headed Gulls nest across Asia and some 4 million to 5 million adults breed in Europe, of which about 250,000 nest in Britain and Ireland. Those breeding here probably represent less than 5 per cent percent of the world population.

The Black-headed Gull is the commonest gull frequenting sports fields, landfill sites, harbours and areas around large shopping complexes, and the birds also seek food in urban streets, domestic gardens and even at bird tables during severe winter weather. In many areas, they are also the gull most likely to be seen following the plough in agricultural fields. The species achieved fame in literature as the character Kehaar in the well-known book Watership Down, written by Richard Adams (1972). Kehaar suffered a damaged wing and took refuge at Watership Down, eventually recovering and returning to his colony.

Numbers of Black-headed Gulls in and around towns and cities can be high, particularly in winter, but this has not always been the case. For example, until the end of the nineteenth century the species was considered scarce in central London, but the birds are now present here in their thousands in winter, where they are abundant in parks, sports fields, lakes and the river Thames, often seen competing vigorously for scraps of food.

The gathering of flocks of Black-headed Gulls around towns and cities varies through the year. Fig. 16 shows their density month by month within a radius of 5 km of Durham, where many were recorded on sports fields, pastures and the river Wear running through the city centre, or patrolling streets and shopping centres in search of discarded food. The gulls retreated to their colonies in March, and since none of these was within 40 km of the city, only an occasional immature bird was recorded in April and none of any age in May and June. There are many accounts of Black-headed Gulls frequenting other areas in winter and being absent there in spring and summer. It is obvious that the distribution of the birds in Britain becomes much more clumped during the breeding season and is limited by feeding range from colonies. At most colonies, first-year (non-breeding) Black-headed Gulls are also infrequent and those that are encountered are seen feeding with adults well away from colonies, commonly on intertidal estuaries.

FIG 16. The seasonal variation in the numbers of Black-headed Gulls within a radius of 5 km of the city of Durham between 1982 and 1984, and in the absence of local breeding colonies. In each year, the gulls were absent in May and June, and the first individuals arrived in July, coinciding with the Durham Miners’ Gala, when a considerable amount of food was dropped by the large numbers of people attending the event. Based on data collected by MacKinnon (1986).

HISTORICAL BREEDING STATUS IN BRITAIN AND IRELAND

The status of the Black-headed Gull in Britain and Ireland in or before the twentieth century is not clear owing to a lack of detailed information. On its website, the British Trust for Ornithology (www.bto.org) says that the species was a rare breeding bird in Britain in the nineteenth century, and the statement made by John Gurney Jr in 1919 that the species was in danger of extinction here cannot be true. Both opinions lack supporting evidence and are contradicted by the extensive egg collecting that occurred at that time (here), and by the historical evidence accumulated by Michael Shrubb in his book Feasting, Fowling and Feathers – a History of the Exploitation of Wild Birds (2013). It is likely that the movement of colonies was misinterpreted as a decline in numbers, and that the ‘disappearances’ of the birds were probably movements of adults elsewhere and the establishment of new colonies in response to nest predation. Such movements of colonies are also frequent, and better known, in the Little Tern (Sternula albifrons), where they are closely related to poor breeding success induced by predators or by human disturbance.

There can be little doubt that the numbers of Black-headed Gulls breeding in Britain increased in the period between 1900 and 1973, as did those of many other gull species breeding here. However, Black-headed Gulls are not easy to count accurately. Colonies of very varying sizes occur both on coasts and inland, and the latter are often particularly difficult to count, with nesting taking place in marshes, on islands in lakes and tarns, and sometimes among tall or floating vegetation in boggy areas. In addition, colonies on moorland are often small and scattered, and new ones can remain overlooked for some time.

The first census of Black-headed Gulls in England and Wales was made in 1938, with further surveys in 1958 and 1973. The report of the census made in 1973 concluded that there were between 200,000 and 220,000 breeding adults, and that since 1938 there had been an increase of 177 per cent, while the number of colonies containing more than 2,000 adults had doubled since 1958. However, part of the reported increase might be attributed to a better coverage of colonies in 1973. In Scotland, a census made in 1958 suggested that there had been little recent change in abundance, but that numbers had continued to be influenced by uncontrolled egg collecting. The national censuses of Britain and Ireland in 1970 and 1985 counted only coastal-nesting Black-headed Gulls and did not consider the appreciable numbers nesting inland. The census in 2000 included inland colonies for the first time and resulted in an estimate of about 336,000 breeding adults in Britain and Ireland, of which 44 per cent were nesting inland.

FIG 17. Map of the breeding distribution of the Black-headed Gull in Britain and Ireland in 2008–11. Reproduced from Bird Atlas 2007–11 (Balmer et al., 2013), which was a joint project between the British Trust for Ornithology, BirdWatch Ireland and the Scottish Ornithologists’ Club. Channel Islands are displaced to the bottom left corner. Map reproduced with permission from the British Trust for Ornithology.

Between 1970 and 1985, a 71 per cent decline in coastal-breeding Black-headed Gulls in the Republic of Ireland was reported, but the numbers there had recovered by the time of the 2000 census. It is possible that an appreciable proportion of the missing birds in 1985 had not, in fact, died, but had moved to new breeding sites not visited at that time.

The general impression from these censuses is that the numbers of breeding Black-headed Gulls in Britain and Ireland increased during most of the twentieth century, and then numbers showed little overall change in Britain between 1985 and 2000. The meagre and incomplete data available since then hints that numbers have been maintained.

The BTO Bird Atlas 2007–11 data, collected between 1968–72 and 2007–11, reported a reduction of 55 per cent in the number of 10 km squares in Britain and Ireland containing breeding Black-headed Gulls. Whether this reflects a decline in overall numbers is uncertain, as it could be the result of a trend towards fewer, larger colonies. Sample counts made at some coastal sites by the Joint Nature Conservation Committee (JNCC) show little evidence of a national change in numbers between 1986 and 2015, and this is supported by the national censuses in 1985 and 2000, which (although the earlier census was incomplete) did not suggest any major overall numerical changes within Britain and Ireland.

There has been a trend over time towards decreasing numbers of Black-headed Gulls breeding at inland sites, while numbers nesting at coastal sites have increased in England and Wales. However, this change does not seem to have occurred in Scotland. This movement probably reflects that birds breeding on coastal islands are often better protected from both humans and Foxes (Vulpes vulpes) than those breeding inland. The impact of the dramatic increase of Fox numbers in Britain in recent years is not frequently appreciated. Stephen Tapper (1992) found that numbers of this predator had increased fourfold between 1961 and 1990, and suggested that the effect of Fox predation on Black-headed Gull colonies may be greater and more widespread than that caused by American Mink (Neovison vison).

COLONIES

Colony sizes

The Black-headed Gull is a colonial breeder, nesting on a wide range of sites, including lakes, reservoirs, small moorland pools and tarns, marshes, sewage farms, clay pits, dunes, saltmarshes and industrial ponds. While numbers of nests in some colonies are easily counted, others are difficult to measure as the nests can be concealed by vegetation or lie on boggy ground, making access difficult or risking damage to the vegetation. Good vantage points do not exist near many colonies, and different methods are therefore needed here to estimate their size.

The basic unit used to measure the size of a colony is the number of nests, applying the realistic assumption that two adult birds are associated with each nest. In very large colonies, sampling of nest density has been used and then extrapolated across the total area of the colony as delineated from aerial photographs or detailed large-scale maps. At some sites, pair and hence nest numbers have been estimated through the less reliable method of counting numbers of birds in the air when disturbed and then using a conversion factor, such as dividing by 1.55. Most Black-headed Gull census counts have errors, which should be considered when making interpretations of the national status of the species and possible changes in abundance over time.

FIG 18. Part of the colony of Black-headed Gulls nesting with Sandwich Terns (Thalasseus sandvicensis) on Coquet Island, Northumberland. (John Coulson)

Data for years since 1980 on Black-headed Gull colony size recorded in the UK Seabird Colony Register allows for an in-depth assessment. While the species is essentially a colonial breeder, solitary breeding pairs on the day of counting comprised 10 per cent of the sites in Scotland and 8 per cent in England. Whether many of these pairs had been solitary for the entire breeding season is uncertain, because most of the nesting sites were visited by counters only once. It is possible that some single pairs were the sole remainders of a group present earlier in the breeding season that had suffered predation or disturbance, and had deserted the site prior to the observer’s visit. Single pairs of Black-headed Gulls can probably breed without the need of stimulus of other pairs, although when they do so, they nest late and their breeding success is usually low.

In Britain and Ireland, the numbers of pairs of Black-headed Gulls estimated at breeding sites (subsequently referred to as colonies for convenience, and including sites with single pairs) between 1980 and 2015 ranged from only one pair to more than 10,000 pairs. Overall, a third of colonies in Britain contained between 10 and 100 nesting pairs (Fig. 19).

At the same time, the average size of colonies differed between Scotland and England, with 84 per cent in Scotland having fewer than 100 pairs (Table 8), while the comparable figure for England was a third lower, at 54 per cent. Colonies with 100 to 1,000 pairs were twice as frequent in England than in Scotland, while colonies of more than 1,000 pairs formed 13 per cent of all sites recorded in England, but only 2 per cent of those in Scotland.

These differences reflect the type of nesting sites used in the two regions, with the majority in Scotland being inland on upland sites and usually associated with relatively small waterbodies, while in England large coastal colonies were more frequent and were sometimes extremely large. The distributions of colony sizes in Wales and Ireland were intermediate between those in England and Scotland, with 72 per cent of colonies in Wales and 62 per cent in Ireland having fewer than 100 pairs (Table 8).

FIG 19. The size distribution of Black-headed Gull colonies (including sites with only one pair) in England and Scotland between 1980 and 2015, based on Seabird Colony Register data for 364 colonies surveyed in Scotland and 260 in England.

TABLE 8. The proportion of Black-headed Gull colonies in England, Scotland, Wales and Ireland with fewer than 100 breeding pairs, based on data from 1980–2015. Sites with a single pair present have been included.

TABLE 9. The percentage of Black-headed Gull colonies of different sizes in England and Wales in 1938, 1958 and 1980–2012. Sites with only one pair are included.

A comparison between the size of colonies reported in the 1938 and 1958 censuses made in England and Wales, and more recent data collated in the Seabird Colony Register, show progressive changes in colony sizes over time (Table 9). There has been an overall decrease in the proportion of colonies with fewer than 100 pairs since 1958, while the proportion and total numbers of large colonies with more than 1,000 pairs has more than doubled over the same period. Many colonies – particularly small ones in England and Wales – have disappeared for various reasons, including disturbance, predation and drainage, while others – particularly large ones – are now protected within nature reserves, although even a few of these have disappeared.

Persistence of colonies

While some Black-headed Gull colonies persist for many years, in general they are not as permanent as those of the Kittiwake or Herring Gull (Larus argentatus). Over the past hundred years, many Black-headed Gull colonies have disappeared while new ones have become established. Small colonies are particularly susceptible to decline, and this is especially true of those on upland moors, where the birds nest around small bog pools, reservoirs and lakes. A consequence of this is that some of these mobile groups can be missed during census work.

Various reasons have been suggested for the desertion of colonies. In some cases these are speculative, and include drainage, military activity, competition from increasing numbers of large gulls, aggression by Mute Swans (Cygnus olor) and Canada Geese (Branta canadensis), egg collecting, keepering, and predation by Foxes and American Mink. Rats and Foxes have been a persistent problem for Black-headed Gulls nesting at Blakeney Point in Norfolk, and no young were reared at the colony in 2000. In 2014, Fox predation at the same site resulted in very few chicks being reared by the 2,200 pairs present, while in 2016 an appreciable increase of rats caused the gulls to abandon the colony and, presumably, move elsewhere.

Ravenglass colony

The site of the very large Black-headed Gull colony on the coast of Cumbria at Ravenglass is owned by the Muncaster Estate. There has been continuous collection of eggs there since the seventeenth century, a practice that was recorded as being extensive in 1886. Del Hoyt has suggested the colony had an annual yield of 30,000 eggs over many years. Neither Michael Shrubb’s searches (2013) nor my own have uncovered documentation to confirm or contradict this statement, but the colony was certainly used consistently as a source of freshly laid eggs for much of the twentieth century by the estate, and workers also took some young from time to time. The collection of eggs was well organised and was stopped each year on an agreed date, which left time for successful breeding by birds laying late or with repeated clutches. David Bannerman recorded that the estate collected 72,398 eggs at Ravenglass in 1941 and 24,568 in 1951, when a further 6,000 eggs were taken (presumably illegally) by others.

In 1954, the site was leased to Cumberland County Council as a local nature reserve and egg collecting ceased. The gulls were protected and studied in the ensuing decade by Niko Tinbergen’s research group from Oxford University. During the period 1954–68, the colony size remained between 8,000 and 12,000 pairs, but during this time predation on eggs, chicks and adults occurred from time to time and breeding success was low. In 1968, a full-time warden was appointed, and he reported that 8,100 pairs nested. From then onwards, a marked decline continued each year; Neil Anderson’s study in 1984 found only 1,300 pairs nesting, and the colony was totally deserted in 1985 (Anderson, 1990). Five pairs nested there in 1988 and three pairs in 1989, but none since (Fig. 20). Predation by mammals over several years, particularly Foxes, was thought to be the cause of decline and ultimate desertion. The abandonment of the Ravenglass colony probably involved a movement of Black-headed Gulls to several other colonies, possibly those in the Ribble Estuary and Sunbiggin Tarn, because no new or established colonies were nearer.

FIG 20. The numbers of nests (pairs) of Black-headed Gulls nesting at Ravenglass in 1939–90.

The history of the Ravenglass colony and other similar sites illustrates two important points. First, egg collecting over many years that was well organised and had an early cut-off had no adverse effects on the colony and did not lead to its desertion. Second, the creation of a nature reserve without effective predator management does not prevent colony desertion; indeed, in recent years there have been other instances of colonies within nature reserves disappearing.

Sunbiggin Tarn colony

The history of the large inland Black-headed Gull colony at Sunbiggin Tarn, near Orton in Cumbria, parallels that of Ravenglass. In 1891, a thousand eggs were said to have been collected from the edge of the colony. The site was estimated to contain 400 pairs in 1938, 1,200 pairs in 1958 and 2,500–3,000 pairs in 1973. The colony continued to grow and reached about 12,500 pairs in 1988 (see below), maintaining similar numbers in 1989, when Foxes were seen. Numbers of gulls then showed a noticeable decline in 1990, and only a few pairs bred in 1991. The colony persisted with a few pairs, and by 2000 had increased to 1,200 pairs, but by 2010 gulls had ceased nesting at the site altogether (Fig. 21).

Direct counting of all nests proved to be impossible at the large Sunbiggin Tarn colony because of the boggy areas within the tarn and the high vegetation, which would be damaged by trampling during a search for nests. A count of the colony was not obtained for the national census in 1985–87, and in 1988 a novel method was used to measure its size. This involved capturing a large sample of the breeding adults in late May and dyeing their tails yellow. A total of 1,010 adults were captured and marked during two weeks by attracting them with food placed about 1–2 km from the colony and then cannon netting them. When marking was completed, the proportion of dyed birds in the colony was obtained by counts of 4,540 adults passing in and out of the site along flight lines. Of these, 186 – or one in 24.4 – had yellow tails, so multiplying the total number dyed by this proportion (24.4) gave a measure of the total number of breeding birds in the colony. This produced an estimate of 24,644 adults (rounded off to 12,500 pairs), identifying it as the largest inland colony in Britain at the time. Unfortunately, Clare Lloyd et al. (1991), in their book reporting the status of seabirds in Britain and Ireland in 1985–88, erroneously recorded this census as 25,000 pairs of Black-headed Gull, not 25,000 individuals.

FIG 21. Part of the large Black-headed Gull colony at Sunbiggin Tarn, Cumbria, during an exceptionally dry spring in 1984. (John Coulson)

Part of the colony at Sunbiggin Tarn could be viewed by people from the nearby road, and they often fed the gulls, which made attracting and capturing the birds in cannon nets much easier. The public’s interest in the well-being and size of this colony was considerable, and not without humour. The investigators netting adult birds had to explain to onlookers what they were doing and that there would be an explosion when the cannons were discharged. On two occasions, the observers remarked that they were puzzled because they could not find a gull in their field guides with a yellow tail, and asked whether this was a rare species! Members of the public often also liked to guess how many gulls were nesting, generally coming up with figures that were widely lower than the actual numbers calculated – most likely because the extent of the colony could not be viewed from a single vantage point. Incidental to the main study, adults with yellow tails were seen feeding in the Lake District and several places on the river Eden up to 40 km from the colony a few days after marking.

FIG 22. Numbers of Black-headed Gulls nesting on the Sands of Forvie, 1986–2017. Data mainly from the Seabird Colony Register.

Ythan Estuary (Sands of Forvie) colony

Black-headed Gulls have nested at the Sands of Forvie at the mouth of the river Ythan, near Aberdeen in north-east Scotland, for many years. In the late 1980s, about 500 pairs nested there annually, along with Sandwich Terns (Thalasseus sandvicensis), Common Terns (Sterna hirundo) and Eiders (Somateria mollissima). Fox predation became intense in about 1990, when breeding Sandwich Terns and Black-headed Gulls abandoned the area and the number of young Eiders declined. In 1994, steps were taken to reduce and then exclude Foxes from the breeding areas, and in 1998 the first Black-headed Gulls returned to breed. Numbers increased rapidly in the following years and reached more than 1,500 pairs in 2002. This level was not maintained, but numbers exceeded 500 pairs each year up to 2012, when the colony size increased annually until 1,921 pairs were recorded 2016 (Fig. 22).

Foulney and other colonies

The colony at Foulney in north-west England (Fig. 23) showed a pattern of decline spread over a few years in the late 1980s and early 1990s, before breeding there ceased altogether sometime in or before 1996. As elsewhere, predation of eggs and nestlings was considered the main cause.

FIG 23. The decline and subsequent total desertion of the Black-headed Gull colony at Foulney, a coastal site in Cumbria. Data mainly from the Seabird Colony Register.

FIG 24. The rapid build-up of a Black-headed Gull colony at the RSPB reserve at Saltholme, near Stockton-on-Tees, between 2005 and 2017. Data mainly from the Seabird Colony Register.

There are many other examples of Black-headed Gull colonies that showed appreciably smaller numbers each year before breeding stopped entirely. The national census in 2000 listed 12 large colonies that had disappeared since 1985, plus a further five that had lost more than 89 per cent of the maximum numbers of breeding pairs over recent years. In contrast, nine colonies showed major increases within a short time period, including the colony at Langstone Harbour in Hampshire, which in the 15 years between 1985 and 2000 increased more than 60-fold. Many new colonies had become established and subsequently grew rapidly, such as those at Hamford Water in Essex, Coquet Island in Northumberland, Larne Lough in Northern Ireland, Loch Leven in Scotland, the Ribble Estuary in Lancashire and Saltholme (Fig. 24), a nature reserve near Stockton-on-Tees, County Durham, that has been managed by the RSPB since 2007.

Reasons for colony declines

Egg collecting, particularly if stopped early in the season, has carried on year after year at several Black-headed Gull colonies without resulting in abandonment. And at Ravenglass, Neil Anderson’s investigations (see above) ruled out a possible link between the decline there and contamination associated with the nearby Sellafield nuclear fuel reprocessing and decommissioning plant. In many areas – particularly the fenland of eastern England – colonies have been lost due to drainage and expanding agriculture, which have removed otherwise suitable nesting sites. However, it seems that the main cause of decline and disappearance of Black-headed Gull colonies is predation of eggs, young and sometimes adults. The presence and increase in numbers of mammalian predators, particularly Foxes and American Mink, have been associated with the decline of the colonies at Sunbiggin Tarn, Ravenglass and the Sands of Forvie. During his studies at Ravenglass, Hans Kruuk found that four Foxes killed 230 Black-headed Gulls in one night, and a total of 1,449 adult gulls were killed by Foxes in two consecutive breeding seasons. Studies made at the colony by Ian Patterson during the period of appreciable mammalian predation activity found that scattered and outlying pairs of gulls were entirely unsuccessful, while those in the central core areas, although having a very low success rate, did succeed in rearing a few young (Patterson, 1965).

Once colonies have started to decline, the impact and pressure from the same numbers of Foxes or American Mink produce a vicious spiral that has an increasing impact on the remaining birds. Another predator at Ravenglass was the Hedgehog (Erinaceus europaeus), which had developed the habit of consuming the flesh of live gull chicks that had not reached fledging age. Several of the colonies referred to in the past as sites where eggs were collected have now ceased to exist, and some authors have attributed these to uncontrolled egg collecting extending throughout the entire breeding season. It is ironic, therefore, that it was only after egg collecting had stopped at Ravenglass that the colony declined and then disappeared, presumably owing to the lack of efficient predator control.

When a colony disappears in a matter of a few years, it is unlikely that the adults have died, but rather that they have moved to other sites and colonies. The decline at Sunbiggin Tarn following predation resulted in some adults moving to Killington Reservoir alongside the M6 motorway in Lancashire, a move confirmed by a few ringing recoveries of adults previously marked at Sunbiggin Tarn. However, this colony contains nowhere near the numbers of birds recorded breeding at Sunbiggin Tarn, which implies that many adults from Sunbiggin probably moved considerable distances and to several different sites, as no other single large colony was established within 80 km of it at the time of its decline. This is also true of many small, ephemeral colonies on upland moorland in northern England and Scotland. A consequence of this is that mobile groups of Black-headed Gulls are often overlooked, and new colonies can grow rapidly through immigration in only a few years – as has happened at Saltholme (Fig. 24).

Exploitation of eggs and young

In the past, both the eggs and young of Black-headed Gulls were extensively exploited in Britain by humans. Michael Shrubb gives a detailed account of this in his 2013 book, Feasting, Fowling and Feathers, from which I have extracted some of the information given below.

Through the combination of Black-headed Gull numbers, their extensive inland breeding often close to human populations, and the relatively easy access to many nests compared to those of gull species that nest on coastal islands and sea cliffs, the eggs of this species have been exploited more extensively than those of other gulls in Britain and Ireland for the past 500 years and probably longer. In some places in the sixteenth century, nearly fledged young were collected by driving them into nets, and they were then housed and fed until required for human consumption. Some were purchased by the wealthy for 4–5d each, (equivalent to about £7 at today’s prices) or given as gifts, just as we now give flowers. Oxford college records at the time also list Black-headed Gull eggs being bought for a fraction of a penny each.

In the seventeenth century, one landowner is said to have made a profit of £60 in one year (equivalent to about £7,000 today) by collecting and selling gulls’ eggs. Exploitation of young birds probably decreased over time, but they continued to be sent to London markets nonetheless, and egg collecting became more frequent and extensive. Exploitation continued into the eighteenth century, and there are more records at this time of eggs being collected throughout England and Scotland. There are also records of gull colonies disappearing, the causes of which were attributed at the time to excessive egg collecting.

In the nineteenth century, egg collecting for human consumption throughout Britain and Ireland was extensive. In some areas it was organised by the landowners, but elsewhere it appears to have been uncontrolled. Well-established markets existed in London, and eggs collected in bulk in Norfolk and Hampshire were sent there for sale. That the Black-headed Gull was widely distributed as a breeding bird in southern England is evident from records of the large numbers of their eggs being collected and sold in the London markets. For example, in 1800 some 8,000–9,000 eggs were collected at Scoulton Mere in Norfolk, while in 1864 and later, 2,000 eggs a year were collected. From 1858, 700 eggs were taken each year at Hoveton in Norfolk, and in 1864 alone, 2,000 eggs were collected. According to The Spectator (17 April 1897), in 1834 at Stanford in Norfolk, a ‘tumbrel-load’ of eggs was collected in a day and sold for 3d a dozen.

It is probable that landowners enforced a degree of protection to the gull colonies in the nineteenth and early twentieth centuries owing to the value of the egg trade. In some cases, collecting was limited to taking eggs only from nests that held one or, occasionally, two eggs, to ensure that those collected had not been incubated, and at some but not all sites, collecting ceased on 15 June. As a result, clutches of three eggs were often left untouched, while cessation of collecting in June permitted some gulls to re-lay and breed successfully from late, repeat clutches.

An account of annual egg collecting in Ayrshire, Scotland, in the first 40 years of the twentieth century is given by Ruth Tittensor in a 2012 edition of Ayrshire Notes. The eggs were collected for local bakeries and domestic use, and even for egg fights between youths. Interestingly, the article includes a personal account of egg collecting by a person who later became a senior member of the Nature Conservancy! Elsewhere, the sale of eggs also continued into the twentieth century. In the 1930s, for example, more than 200,000 gull eggs were sold annually at Leadenhall Market in London. Egg collecting increased extensively during the two world wars, with large numbers collected to supplement rationing. An archived Pathé News ciné film shows several Land Girls based at Muncaster Estate during the Second World War collecting baskets of eggs at the large Ravenglass colony and boxing them up in crates ready for dispatch to London. Hugh Cott describes eggs being on sale in Cambridge in 1951 at 9d each. A London market for Black-headed Gull eggs existed for the whole of the twentieth century, with some being sold as ‘plovers’ eggs’.

Today, despite current bird protection Acts, systematic collecting of Black-headed Gulls’ eggs is still permitted in England by licence holders, although there is little general knowledge of the numbers of licences and eggs collected. The licences are issued annually by Natural England for collecting eggs for food consumption. As a result of a request for information, Natural England responded in 2016 by stating that ‘Licences are only issued where evidence of hereditary/ancestral/traditional family rights to collect eggs can be given’. The public body also stated that it seeks ‘to ensure that collection of eggs is sustainable in relation to numbers collected and colony size’. However, I still await a response as to how this is achieved and where the records are deposited, since they do not appear in the Seabird Colony Register or local county bird reports. An accurate census of a colony where many eggs are collected is often difficult to carry out because of the greater spread of laying and repeat clutches.

A freedom of information request granted by Natural England in 2016 revealed that it had issued licences recently for egg collection at six sites in England: the North Solent nature reserve in Hampshire; Lymington, Pylewell and Keyhaven marshes, also in Hampshire; Barden Moor in North Yorkshire; and the Upper Teesdale National Nature Reserve in County Durham. Currently, up to 40,000 eggs are collected annually and sent to London. These details are not publicised and I found no information in the published literature. A further freedom of information request to Natural England revealed that in 2015 it issued 24 licences to collect Black-headed Gull eggs in England (22 in Hampshire, one in Yorkshire and one in County Durham). Table 10 shows that the numbers of eggs collected in Hampshire remained at about the same level over the period 2009–15. Those taken in North Yorkshire increased 17-fold between 2011 and 2012, and since then the numbers collected have continued to increase, with more than 32,000 eggs taken in 2015.

No mention of these annual egg collections appear in recent county ornithological reports, while Natural England made the surprising statement that information prior to 2008 ‘has not been retained’. This public body should not destroy historical information on colony sizes and the numbers of eggs collected, and such data should be archived. Natural England also refused to identify the colonies from which eggs were collected, or to whom licences were issued, thereby preventing further research on these activities. How this quango carries out the annual monitoring of the colonies – as it claims to do – remains unknown, and why this information is not deposited in the national Seabird Colony Register, which it supports, requires investigation. It is likely that eggs laid by Mediterranean Gulls (Larus melanocephalus) are being inadvertently taken at some of the colonies for which licences are issued.

TABLE 10. The number of egg-collecting licences issued and Black-headed Gull eggs collected in Hampshire, North Yorkshire and County Durham in 2009–15, as reported by Natural England. The public body stated that data for earlier years have not been retained.

Natural England has indicated that the licences place restrictions on the dates of collecting, these normally covering the period 1 April to 15 May each year. They also restrict the length of time collectors can spend in the colonies on each visit. Natural England revealed that the County Durham licences authorise collection of eggs from three large colonies of gulls, which therefore must be in Upper Teesdale, but the quango is not willing to disclose which colonies are involved.

Black-headed Gull eggs collected in England are currently sold by up-market London stores and find their way onto the menus of several prestigious restaurants. The eggs are normally served hard boiled or lightly boiled and seasoned with celery salt, and a serving containing three eggs may cost anything from £7.50 to £22. One restaurant in 2015 was offering omelettes made from three gulls’ eggs filled with lobster meat.

WINTER ABUNDANCE

Few British-reared Black-headed Gulls leave the region in winter, with most making only local movements. Of those that do migrate, a higher proportion move to Iberia from south-east England than those reared in north-east Britain (Fig. 25). In contrast, large numbers of Black-headed Gulls move from the Continent to winter in Britain and Ireland.

FIG 25. Movements of Black-headed Gulls ringed as nestlings in south-east England and recovered outside the breeding season, based on 976 recoveries. Right: Similar mapping, but based on 447 recoveries of Black-headed Gulls ringed in north-east Britain. Note that fewer birds moved to Iberia from the north-east and that birds from the two regions have different winter distributions in Ireland. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

The BTO Winter Gull Roost Survey in 2004 estimated that more than 1.6 million Black-headed Gulls were using winter roosts in the United Kingdom and Northern Ireland., and since the total did not include all winter roosts, there are probably more than 2 million individuals wintering here. This number can be compared with 280,000 breeding birds in the United Kingdom and Northern Ireland, which with immatures added comes to about 360,000 individuals. As most of these remain here in winter, it suggests that some 77–82 per cent of wintering Black-headed Gulls recorded in 2004 had moved here from the Continent. Gabriella Mackinnon and I obtained a comparable figure of 71 per cent, derived from ringing recoveries up to 1980 (Fig. 26). Both percentages are rough estimates, but it is clear that a major proportion of the wintering Black-headed Gulls in Britain and Ireland originate from mainland Europe. Without new estimates, it is not known whether these numbers from the Continent have declined in more recent years as a result of the relatively mild winters.

Some readers may find it strange that, despite the satisfactory status and abundance of the Black-headed Gull as a breeding species in Britain, it has been Amber-listed here since 2015 as a species of conservation concern. This is based on the belief that numbers of wintering Black-headed Gulls in Britain have decreased by 33–50 per cent over the last 25 years, based on counts at winter night roosts. Since the British breeding numbers do not appear to have reduced (here), a possible explanation is that a major decline has occurred in northern Europe – the area from which many of the British wintering gulls originate. However, there is little evidence of this apart from Denmark. An alternative explanation for the decline in winter numbers in Britain is that more individuals are remaining on the European mainland during the milder winters encountered in recent years. A preliminary analysis of recent winter ringing recoveries of Black-headed Gulls marked on mainland Europe suggests that the change of wintering areas is more likely to be the explanation. If this is correct and the species is not declining appreciably in Europe as a whole, but is modifying its wintering areas, then is it really a matter of conservation concern? In contrast to the assessment in Britain, the International Union for Conservation of Nature (IUCN) still regards the Black-headed Gull worldwide as a species of Least Concern.

FIG 26. The percentage of Black-headed Gull recoveries reported in Britain and Ireland in each month that were originally ringed in mainland Europe. Note the trend for first-year birds to return to the Continent later. The recoveries of a few Continental birds in May and June may represent rings reported some time after finding.

Arrival of wintering birds

The numbers of Black-headed Gulls arriving in Britain and Ireland for the winter boosts the breeding season population by a factor of about four. These visitors come from a wide range of countries in northern Europe, with most starting their journey at locations around the Baltic Sea (Figs 27 and 28). Their arrival is spread over several months, from late July to early November (see Fig. 26).

The countries of origin of Continental Black-headed Gulls visiting Britain varies slightly with their final destination, with birds flying south-west or west to reach their wintering area (Figs 27 and 28). The exceptions are individuals reared in Iceland, some of which winter in Scotland and Ireland, but most of which avoid England and Wales. Other Black-headed Gulls reared in Iceland have been recovered from the east coast of North America, a route apparently taken by all of those reared in Greenland, although this wintering area is totally avoided by birds breeding in mainland Europe and in Britain.

Fig. 29 shows the number of recoveries of foreign-ringed Black-headed Gulls in Britain and Ireland for each month of the year up to 1985 in relation to those reported monthly from December to February, a period during which it is assumed that all individuals have arrived at their wintering areas. An appreciable proportion of foreign adults, possibly mainly failed breeders, arrive in Britain during July, but only a few young of the year have crossed the North Sea by the end of the month. A small proportion of first-year individuals reared abroad remain in Britain in the following summer, having failed to return to the Continent in the spring. These birds, entering their second year of life and already in Britain in July, are joined by more similar-aged immature birds moving here from the Continent.

FIG 27. Black-headed Gulls ringed in winter in north-east Britain (to the right of the thick black line) and recovered abroad in a subsequent breeding season. Note the five Iceland recoveries. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

FIG 28. Black-headed Gulls ringed in winter in south-east Britain (to the right of the thick black line) and recovered abroad in a subsequent breeding season. Note the single Iceland recovery. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

FIG 29. Recoveries in Britain and Ireland of Black-headed Gulls ringed in mainland Europe expressed as a percentage of the monthly recoveries between December and February. Note the high proportion of birds entering their second year of life (which moult early) reported in Britain in July, and the late departure of first-year birds in spring, with a few apparently remaining in Britain during the summer.

Numbers of all ages cross to Britain in August. By the end of that month, about half the adults that winter here have already arrived, but only 15 per cent of first-year birds have arrived. The early movement from countries bordering the Baltic Sea perhaps reflects the less favourable feeding conditions there. For example, the small tidal changes in the Baltic markedly restrict the extent of inter-tidal areas available for foraging, while the warmer and drier continental weather makes earthworms less available there in late summer.

There is evidence of a lag in new adult arrivals in Britain in September, probably because most adults are still completing their moult and growing their longest primaries. The lack of, or reduction of, these long feathers must impair their ability to make long flights. A second peak of arrival occurs in November, and by the end of that month most of the adults that winter here have arrived, apart from a few birds that possibly turn up in December. The maximum numbers of Black-headed Gulls are present in Britain for only three months, from December to late February.

The timing of immigration of first-year and adult Black-headed Gulls in autumn to Britain depends on the country of origin. Dates after 1 July by which the first 25 per cent of recoveries of young birds and adults from abroad are recovered have been used as an index of the arrival times of first-year and adult Black-headed Gulls from different countries in Europe (Table 11). In general, adults and young birds arrive on different average dates, but this is not constant for each country of origin and varies by more than 60 days. Young birds arrive earlier than adults from countries closer to Britain, but the reverse applies for those from more distant countries. This suggests that first-year birds start their migration earlier than adults but move much slower, perhaps making a series of short journeys interspaced with stopovers at rest areas lasting a number of days. In contrast, adults appear to make much longer flights to their wintering area and arrive in Britain from European countries at less variable dates. The interpretation of the trend line in Fig. 30 suggests that first-year birds on migration take about 43 days longer than adults to travel 1,000 km.

TABLE 11. Date by which the first 25 per cent of adult and first-year Black-headed Gulls ringed abroad have been recovered in Britain in each 12-month period, starting 1 July. Based on MacKinnon and Coulson (1986).

FIG 30. Difference in the dates of the first 25 per cent of recoveries in Britain of first-year and adult Black-head Gulls of Continental origin (data from Table 8) in each 12 months from 1 July, plotted against the distance from country of origin to central England. Note that the first-year birds from countries close to England arrived earlier than the adults, but that the converse was true for those travelling further.

Faithfulness to wintering areas

There is extensive and convincing evidence that some adult Black-headed Gulls from the Continent return to the same immediate wintering area year after year. The best example is a gull ringed in Poland, whose ring numbers were recorded in the same central London park over eight consecutive winters. However, such faithfulness to wintering areas is not absolute, as there are many instances of birds from the Continent wintering in Britain one year and then remaining in mainland Europe (usually in the Netherlands or Denmark) in the following winter (Fig. 31). It has been suggested, but not yet proven, that relatively mild conditions in western Europe in some recent winters have resulted in fewer Black-headed Gulls migrating here; this effect has also been reported for several species of waders and other waterbirds.

Evidence of the extent to which Black-headed Gulls use the same wintering area in successive years comes from individuals wing-tagged in north-east England, as these had a high probability of being seen if they returned in the following winter. Most tagged birds were breeding on the Continent, and so the annual return would involve many individuals migrating considerable distances. In the winter after tagging, those that returned were seen on an average of five different days, so it is unlikely that many others were missed. The study detected that 48 per cent of adults and 32 per cent of second-year birds, but only 17 per cent of first-year birds, returned to the same area in successive winters (Table 12). Allowing for mortality in the intervening year (estimated at about 15 per cent for adults and 30 per cent for first-year birds), 56 per cent of the surviving adults returned to the same wintering locality, with 38 per cent of those in their second winter when marked returning for a second time, but only 24 per cent of those marked in their first year.

FIG 31. Black-headed Gulls ringed in winter in Britain and recovered more than 20 km away in a subsequent winter. Many individuals returned to Britain, but note the appreciable number that failed to do so in a subsequent winter and were recovered on the Continent. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

TABLE 12. The proportions of marked Black-headed Gulls that returned to the same wintering area the following year.

A total of 38 marked individuals that did not return to the study areas were seen by other observers elsewhere in the winter following marking. These were reported mainly on the east coast of Britain, south of the study areas. Two extreme cases were recorded; one bird 450 km away on the south coast of England, and the other 300 km north at Aberdeen in Scotland. In addition, one remained in Denmark. It is evident from this that only a proportion of individuals of all age classes return to the same wintering area, but that faithfulness is stronger (although not complete) in adults. The reason why some birds return while others fail to do so remains unknown, but is not primarily related to their sex.

Return of birds to the Continent

The return migration from Britain to the Continent is much more synchronised than the arrival and begins in late March, with most adults migrating in April (Fig. 32). First-year birds tend to depart later than adults and some do not leave until May, while others (perhaps 10 per cent of immature birds wintering here) remain throughout the summer. These birds join the feeding adults associated with colonies in Britain (since they leave their wintering feeding areas) and presumably do not return to the Continent until they have overwintered here for a second time.

FIG 32. The month of arrival of Black-headed Gulls entering Britain from the Continent in autumn (white bars) and leaving in spring (blue bars), based on monthly change in numbers of ringing recoveries of foreign birds in Britain. Note that for all age classes, the spring departure is much more synchronised than the autumn arrival dates.

By the end of July, numbers of second-years have arrived in Britain and joined the few that spent the summer here. These birds are not constrained by breeding and moult about four weeks earlier than adults, so this might explain their early crossing of the North Sea. More arrive in late October and early November, and many late arrivals originate from further away, east of the Baltic.

Distribution of Continental wintering birds

The Continental Black-headed Gulls that winter in Britain do not disperse randomly across the country, and are much more abundant in eastern and southern regions. The proportions of young gulls ringed in Britain or on the Continent and recovered from December to February in each of 10 regions of Britain and Ireland (4,267 recoveries in total) were used to produce an index of the proportions of geographical distribution in each area (Fig. 33). The percentages of Continental birds in these different regions ranged from 7 per cent to 80 per cent, based on at least 115 recoveries in each region and, in most cases, many more. The wide range of percentages obtained suggests that they are a close approximation to the actual proportions of birds of Continental origin. Ireland, Scotland and Cumbria had low proportions of Continental gulls, while the highest proportions occurred in south Wales, southern England, and eastern England from Yorkshire southwards. Overall, about 71 per cent of the wintering gulls in Britain and Ireland originated from the Continent – a high value, but consistent with the much higher overall numbers of Black-headed Gulls reported here compared with estimates in the breeding season.

FIG 33. Estimates of the percentage of Continental Black-headed Gulls present in 10 regions of Britain and Ireland in winter based on numbers of recoveries of British and Continental ringed birds in each region between early December and the end of February.

Sex ratio of wintering birds

While the sex ratio of Herring Gulls and Great Black-backed Gulls (Larus marinus) captured in Britain during the winter approaches equality, that of wintering Black-headed Gulls shows a marked skew, with many more males being present. The data in Table 13 show that birds identified at breeding sites in northern England in May and June contained a minor excess of 108 males per 100 females. However, the sex ratio of wintering birds captured in England changed considerably, with a threefold excess of males in October and November, followed by the reversion of proportions to near equality between December to February. In the winter samples, most individuals were visitors from the Continent, and the skewed sex ratio in October and November suggests that either migrating males were arriving earlier than the females by an average of a month or more, or that males and females were feeding at different types of sites not sampled in those months, but were feeding at the same sites later in the winter. A total of 20 males and 11 females wing-tagged in the autumn were subsequently reported to have returned to the Continent, which gives some support for the suggestion that the bimodal pattern of the arrival time of Continental gulls (Fig. 32) represents males migrating earlier than females.

TABLE 13. The sex ratio of adult Black-headed Gulls cannon netted at landfill sites between October and February in north-east England, and near large colonies in northern England in May and June. Males were identified by a head and bill measurement of 82 mm or longer.

BREEDING BIOLOGY

Black-headed Gulls build their nest on the ground in areas with low-growing vegetation of varying density, ranging from sand dunes with much bare ground, to taller and floating vegetation around tarns and lakes. However, a few exceptions have been reported. At Seamew Crag, a small islet on Lake Windermere, Clive Hartley and Robin Sellers recorded several of the 50 pairs in the colony there nesting on top of low bushes over several years following 2009 (pers. comm.). In East Anglia in 1947, an entire colony numbering more than 300 pairs switched to nesting 2–3 m above ground level in young spruce trees, apparently in response to the flooding of their usual nest sites on the ground (Vine & Sergeant, 1948). Unlike some other gull species, there is only one old record of Black-headed Gulls nesting on a building, but in 2015 Robin Sellers found three small groups doing so, two near Perth and the other at Montrose, both in Scotland (pers. comm.).

Philopatry and colony faithfulness

Most Black-headed Gulls are two years old before they breed for the first time and a minority are a year older before they breed. Exceptionally, one-year-old individuals attempt to breed, although their success is very low. Many of those that have survived to maturity return to breed in the colony in which they hatched as chicks (called philopatry), but others move to other colonies, often some distance away. The high proportion returning to the natal colony indicates that the young birds retain a good memory of where they were reared. However, it is easier to find marked individuals that have returned to their original colony than those that have moved elsewhere and consequently the extent of philopatry is often exaggerated. A realistic estimate of the extent of philopatry is obtained by measuring the proportion of those ringed as chicks and have reached adult age that were recovered during the breeding season less than 20 km from their natal site. In the case of the Black-headed Gull, about 60 per cent of the young that survive to breeding age are philopatric and the remaining 40 per cent move to other colonies and in a few cases, across the North Sea, to breed on the Continent. There have also been exchanges of individuals between Ireland and Britain (Fig. 34). Once adults have bred in a colony, their attachment to it becomes high, and at least 85 per cent of those surviving to the following year return to it to breed. The few adults that move elsewhere to breed are mainly from colonies that are in the process of being deserted, in some cases as a reaction to repeated nesting failure and the presence of mammalian predators.

FIG 34. Movements of more than 20 km of Black-headed Gulls ringed as chicks in Britain and Ireland and recovered in the breeding season when of breeding age. Many returned to breed at or near where they were reared, but a similar proportion apparently moved to other colonies. A few moved to the Continent to breed, mainly between northern France and Denmark, but one moved to Germany, another to Norway and a third to northern Sweden. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

Annual reoccupation of the colony

Black-headed Gull colonies are first visited by groups of adults in March each year. They arrive at irregular intervals during the morning, flying over the site without landing, and do not remain long at the colony or in the vicinity. Eventually, on one such visit some birds do land, but they exhibit a degree of nervousness and are easily disturbed. These visits become more frequent, but will be curtailed or prevented by cold and windy weather. As the days pass, the daily presence of the birds at the colony lasts much longer and spreads into the afternoon, but the colony is always vacated before sunset. The gulls usually leave by a synchronous ‘up’ or ‘panic’ flight, which sees them all suddenly rising high above the colony as if alarmed, yet without an obvious stimulus such as the appearance of a predator. As April progress, the amount of time the gulls spend at potential nest sites extends to the greater part of the day, but the colony is still deserted each night and reoccupied early in the morning, sometimes well before sunrise.

Pairs are formed early and courtship displays become common (Fig. 35). In the meantime, nesting material is collected locally and brought in to the colony. Birds nesting in some coastal colonies often collect substantial and untidy quantities of brown seaweed, while those at inland sites collect dry grass locally and carry it to the selected nest site to construct the nest. More material is usually added to the nest during incubation. Colonies and nesting sites are usually close to water, and nests are often substantial structures that raise the eggs above the local water levels.

FIG 35. Female Black-headed Gull (right) courtship-begging for food from a male. (Norman Deans van Swelm)

The density of nests varies according to the size of the colony and the nature of the nesting site. Nests are 1–2 m or more apart where plant growth is sparse, but only 50 cm apart in dense vegetation, presumably because this tends to conceal the neighbouring pair, moderating the extent of aggression between close neighbours.

Eggs and incubation

Black-headed Gull eggs are brown with spots of a darker colour. They are laid in mid- to late April (the date of the first egg laid in different years at Ravenglass ranged between 12 April and 26 April), but even at this time the colony is still often deserted at night, with the adults moving to night roosts and leaving the early eggs unprotected. As clutches are completed and incubation begins, increasing numbers of birds remain in the colony throughout the night; it is at this point that predation by Foxes on incubating adults may occur.

In Britain, the peak of laying in Black-headed Gulls is reached at the end of April or in early May, which is earlier than in other gull species breeding here. Fig. 36 shows the date distribution for the first eggs in each clutch recorded by Ian Patterson at Ravenglass (1965). There is considerable laying synchrony by the majority of pairs, but there is often a distinct tail or a secondary late peak of birds that re-lay after losing their first clutch or delay laying owing to inexperience.

The typical clutch comprises three eggs, although two-egg clutches are common. Occasionally, four-egg clutches occur, but whether these are laid by only one female has not been investigated. Late-laying birds produce clutches of just one or two eggs. Average clutch size within a colony varies considerably, from 2.3 to 2.8 eggs. Nils Ytreberg (1956) recorded a high average of 2.9 eggs from 411 nests in Norway, but in a later year reported a smaller average of 2.62 eggs based on a sample of 100 nests (Ytreberg 1960). The average clutch size tends to be lower in years when laying starts late, such as in small colonies and those further north and in the uplands. Eggs are laid at variable times throughout the day and the interval between eggs is usually about two days.

FIG 36. The spread of the laying season at Ravenglass, recorded by Ian Patterson in 1965. The secondary peak at the end of May/early June probably comprised repeat clutches produced by pairs that had lost their first clutch or by pairs of young, inexperienced birds.

Incubation of eggs and feeding of young is shared by both members of the pair. The eggs are covered by an adult irregularly while the clutch is still being laid, with the first egg probably protected rather than incubated. Obviously, incubation does not take place at night on occasions when the adults desert the colony in the evening. Intensive incubation starts at variable stages during laying of the clutch and often before the third egg is laid, but the brood patches of incubating birds are not fully vascularised for the first few days and initially the covered eggs may not reach a high enough temperature to facilitate development. This results in variable incubation periods, and the early start of incubation is sometimes enough to cause the third egg to hatch a day or so after the first two. Ivan Goodbody (1955) recorded incubation periods from the laying of the last egg of between 23 and 26 days at different nests.

Rearing young

A study by Roland Brandl and Ingrid Nelsen (1988) reported that Black-headed Gulls feed their chicks at intervals of about 45 minutes during daylight (but not at night), and that chicks receive at least 20 feeds each day. Each parent made five to six feeding trips per day, and obviously retained food from each trip for at least two feeding bouts. The authors found that this rate was mainly independent of the nestlings’ age or brood size. Adults regurgitate food onto the ground for the chicks and often re-swallow any surplus.

The newly hatched young have cryptically marked down, and as they grow, the feathers are patterned in shades of brown. The chicks remain in the nest for about a week if undisturbed and are brooded by their parents for a decreasing length of time each day as they grow older. Brooding at night often continues for a day or two longer after daytime brooding has ceased. Chicks eventually move a short distance away from the nest and shelter in nearby vegetation, where they receive some protection from both adverse weather and predators. One parent remains at the nest site most of the time, feeding the chicks by regurgitation and defending them against neighbouring adults and attacking wandering young from other pairs.

Once chicks reach 20 days old, they develop an interest in searching the ground for items they can pick up and swallow. Although they probably find little that is edible this way, the behaviour seems to develop their ability to search for food. Young birds can fly when they are about 34 days old and leave the colony area soon after fledging, often accumulating in small groups in open areas nearby. There, they search for food, later joining feeding flocks of adults in fields or on the shore. There is no evidence that the young are accompanied or fed by their parents once they leave the colony, nor do they return to the nest site to be fed after their first flight.

Breeding success

Breeding success in Black-headed Gulls is highly variable and depends on whether the colony is protected from predators, including humans. American Mink, rats, Badgers (Meles meles), hedgehogs, Foxes and herons are major predators at some sites, and are suspected to have caused the desertion of several colonies. Intensive predation can result in very few young fledging from a colony. For example, in a sample area at Ravenglass studied in detail by Ian Patterson, 2,213 pairs each fledged an average of only 0.06 young in 1961, while in 1963, 2,290 pairs each fledged an average of 0.10 chicks (Patterson, 1965). This poor breeding success was mainly caused by Fox predation on eggs, young and adults late in the breeding season.

At colonies where predation has been typically much lower than at Ravenglass, Black-headed Gulls probably fledge about 0.6–0.7 young per pair each breeding season. On its website, JNCC reports an average production of about 0.6 young per pair annually based on a few sample colonies studied between 1987 and 2015, but in the best year 1.2 chicks per pair fledged. Studies of several colonies over many years in France reported an average of 1.4 young fledged per pair (Péron et al., 2009). Once the last few young fledge, which is usually in late July or early August, colonies are rapidly deserted until the next breeding season.

FOOD AND FEEDING

Despite being small, the Black-headed Gull has been recorded feeding on a wide range of items, although in general these are smaller than those taken by the larger gulls. Animal material dominates, with invertebrates being by far the most frequently consumed food. Earthworms and the adult and larval stages of beetles and flies predominate in food obtained in fields. In coastal areas, marine crustaceans, molluscs and worms are frequently consumed when mudflats are exposed at low tide, or small food items are taken from the sea surface (Fig. 37). In addition to the consumption of animal matter, the gulls ingest a wide range of plant materials from time to time. Some of these are ingested incidentally with animal food, but seeds and grain are intentionally consumed – in autumn, for example, acorns are occasionally plucked from the tops of oak trees and Hawthorn (Crataegus monogyna) berries are picked off hedgerows by birds momentarily alighting on branches. A wide range of animal and vegetable materials, including bread, are taken around human habitation and at landfill sites. In some areas, bread and food scraps are regularly supplied by the public, particularly in parks and by lakes, and these form a major part of the winter diet in urban areas.

The gulls look for potential food either by flying 1–2 m over the ground or water and landing to pick up items, or (more frequently) by landing, usually as a dense flock in a field, where they spread out by walking over the ground. This latter habit is used extensively just after dawn, as soon as the gulls arrive from their night roosts and when earthworms are still active on or near the ground surface. Small fish and riparian insects are captured and consumed on or near freshwater sources. Insects such as swarming winged ants are caught in flight on calm days in late summer. At times, adults bring large quantities of hairy caterpillars collected from moorland to feed to their young, but the hairs of some of these species are irritants and many chicks regurgitate them in numbers, such that they sometimes litter the ground within the colony. Occasional small mammals and amphibians are consumed, but these form a minute part of the diet.

FIG 37. Black-headed Gulls feeding on small fish larvae. Note the unusual head position. (Norman Deans van Swelm)

Kleptoparasitism

Kleptoparasitism is common in Black-headed Gulls, whereby the birds steal from members of their own species and, particularly, flocks of Lapwings (Vanellus vanellus). Golden Plovers (Pluvialis apricaria) feeding on agricultural land in winter also regularly have their food stolen by the gulls. Lapwings and Golden Plovers feed mainly on earthworms and insect larvae obtained by actively searching the ground, and they are often joined by Black-headed Gulls (and sometimes Common Gulls, Larus canus). However, rather than searching for worms, some gulls just stand nearby or follow the plovers; when a plover finds a large food item that requires manipulation before it can be swallowed, it is rapidly and often successfully challenged by one of the gulls, and frequently the food item is stolen. In other cases, a plover may take off with the worm or insect in its bill, only to be closely pursued by a gull, which attempts to force it to drop the prey. When this happens, the gull takes the food as it falls or quickly turns to pick it up from the ground.

Such kleptoparasitism of Lapwings and Golden Plovers decreases the waders’ own rate of food consumption, but this is partially mitigated by feeding at night under full moon conditions when the gulls are absent. The fact that Black-headed Gulls resort to kleptoparasitism suggests that they are not as efficient as plovers at searching and finding their own earthworms.

Kleptoparasitism by Black-headed Gulls appears to be a specialised feeding behaviour used by a minority of individuals, and experience is important in successfully obtaining food in this way. Even by their second year of life, young Black-headed Gulls have still not achieved the efficiency of adults when carrying out kleptoparasitic attacks (Hesp & Barnard, 1989).

In urban areas in winter, the tables are often turned on Black-headed Gulls. Those birds finding large food items that they fail to swallow immediately are frequently pursued by Common Gulls or even members of their own species, until they are forced to drop the food to the benefit of the pursuers. Such pursuits are particularly frequent in urban areas during severe winter weather, when large numbers of Common Gulls move from snow-covered high ground and congregate at much increased densities at urban feeding sites.

Feeding areas

In the breeding season, the feeding areas and ranges of Black-headed Gulls are limited by the location of the colony, and well-defined flight lines are often evident to and from large colonies. Many feeding adults forage within 10 km of the colony, although some will travel 40 km or more to favoured sites with a regular supply of food provided through the activities of humans. These may be fields where ploughing is in progress, landfill sites, large car parks (particularly near food stores), coastal picnic sites or riverbanks in towns where people feed bread, chips and other items to birds. Food supplements put out for farm animals are also exploited. Few Black-headed Gulls hunt far from the shore at coastal sites, and only rarely do they plunge-dive and submerge to capture fish.

In the 1960s, along the 18 km tidal reaches of the river Tyne (which was little more than an open sewer at the time), Black-headed and Common gulls were the commonest birds feeding in winter at sewage outfalls and on sewage items floating in the river. Once the sewage was piped separately to treatment plants to clean the river up, the numbers of Common Gulls reduced dramatically, but Black-headed Gulls remained abundant and were still able to find food in the river. This is because the outflow of filtered water passing from sewage plants into the river still contained small items, which were attractive to the Black-headed Gulls but not the Common Gulls.

The habit of large numbers of Black-headed Gulls following tractors ploughing fields is both widespread and spectacular, with many individuals competing for position just behind the plough blades, and diving to catch and consume worms or insects exposed when the soil is turned over. Similar feeding occurs when grass is being cut for hay. The introduction of grass cutting earlier in the year for silage attracts many Black-headed Gulls and has had a marked effect in some areas where the birds feed. Although silage production was introduced many years ago, it became far more extensive from about 1969, and by 1993 it had increased sevenfold as a means of producing winter cattle feed. Grass is now frequently cut twice a year for silage and much earlier in the season than for hay. This has allowed feeding by Black-headed Gulls on insects in these fields to be spread over a much longer period of the species’ breeding season than was once the case, and presumably has been of benefit to the gulls.

Feeding at landfill sites in winter attracts flocks of hundreds and even thousands of Black-headed Gulls. It is a highly social activity and occurs at irregular intervals, often with a resting, inactive flock remaining nearby for long periods. From time to time, one or two individuals leaving the flock will then fly over the landfill working area and alight there or pick up a food item while in flight. This act of feeding by one or two individuals is a major stimulus to those birds in the resting flock, and the great majority will then leave the roost and stream over to the feeding site, land on the refuse and search for food – several hundred individuals may be highly active within a small area. Suddenly, feeding will stop, perhaps as the result of a loud noise, the approach of a person or the arrival of a lorry. On other occasions and for no apparent reason for alarm, the flock will rise as one and return to the roosting site, staying away from the landfill working area for many minutes or even hours.

Food-collecting techniques

Black-headed Gulls use several techniques when collecting food, summarised below:

1. Aerial searching Individuals patrol suitable areas, turning and swooping to pick up items discovered. Used on farmland and in urban areas.

2. Walking over fields of short grass The birds are usually spread out and search for food items on the ground.

3. Feeding while floating on water The birds swim buoyantly, turning rapidly to pick small items such as larval fish from the water’s surface, phalarope-like (Fig. 37). Head-dipping into the water is less frequent and diving does not take place.

4. Feeding frenzies Many individuals converge on an appreciable food source, such as food discarded by the public, a freshly ploughed field or refuse at a landfill site. In all three cases, individuals are attracted from some distance by the erratic flight and calling of those birds already scrambling for food.

5. Feeding on flying insects This is most frequent in autumn, when winged ants take to the air on calm, warm days. The gulls ‘hawk’ after the insects, flying slowly and frequently changing direction.

6. Consumption of items from trees, bushes and low plants This is an infrequent method, with gulls approaching branches or low plants, landing on them momentarily, and plucking abundant insects from the leaves and thin branches. Similar methods are used to obtain acorns from oak trees and berries from Hawthorn bushes.

7. Paddling in shallow pools on intertidal mud This technique is used to disturb, detect and capture small invertebrates, including marine worms. While Herring Gulls also use this method on areas of short grass, this has not been reported in Black-headed Gulls.

8. Kleptoparasitism The birds steal food from Lapwings and Golden Plovers when the waders are feeding on soil invertebrates in pastures.

Use of winter feeding areas

The next sections are based on data gathered through extensive ringing and wing-tagging studies of Black-headed Gulls wintering in north-east England. Observing coloured leg rings on Black-headed Gulls was difficult in resting flocks and when they were feeding on grasslands, so to overcome this problem, plastic wing tags were used (here). These proved more successful in identifying and following the behaviour of individuals throughout the day. The collaborative research involved identifying feeding areas and catching and marking birds prior to and during three years of intensive field studies on the species made, and was carried out by Gabriella MacKinnon at Durham University as part of her doctoral thesis (MacKinnon, 1986). It involved us in many cold early-morning starts in winter to reach the feeding areas and set cannon nets before the gulls arrived at about sunrise from their night roosts.

Most of the wing-tagged individuals studied in north-east England frequently returned to feed at the same sites during the whole winter (Table 14), but some moved away during this period. A marked difference in the behaviour of the gulls was detected between those using coastal sites and those feeding inland. At the coast, both adult males and females were particularly faithful to the same area throughout the winter, but at inland sites both sexes were less faithful, with adult females nearly twice as likely to move elsewhere than adult males. This difference between coastal and inland sites probably relates to the difficulty of accessing food in fields during periods of cold, frosty weather, which is less of a problem at coastal and intertidal areas as they are less susceptible to freezing. First-year birds were variable in the proportions that returned to feed at the same site, but overall, they were less faithful to sites than adults.

Sightings of wing-tagged birds that moved 10 km or more from the study areas in north-east England in the same winter and reported by other observers revealed that all moved south, remained on or near the east coast of England, and moved less than 100 km, apart from one bird, which moved more than 350 km. Of the 254 adults marked in winter in north-east England, 19 were seen in Britain during the breeding season. Nine of these birds were recorded within 50 km of the winter study areas, while 51 were recorded in colonies on the Continent, confirming that most of the marked birds studied bred outside Britain.

That some individuals return to the same feeding areas each day is remarkable – in some cases during the field studies, birds commuted up to 25 km twice a day to and from a large overnight roost on land containing at least 10,000 birds. Observations of the Black-headed Gulls leaving their night roost in the early morning revealed that individuals from several parts of the roost left in successive waves spread over the 30 minutes before sunrise. Some of those leaving from different parts of the roost circled, formed flocks and departed inland. The birds arriving at sunrise at an extensive series of sport fields in Durham 25 km away gathered in one or two flocks, and invariably included several wing-tagged birds that were previously marked when feeding there. Yet some of these tagged birds (and probably the others) did not roost close to each other at night. How individuals dispersing from the night roost among thousands of others managed to gather into a flock returning to the same feeding grounds is a fascinating mystery to which I have no answer. How do birds regularly flying to the same feeding area manage to group together into a flock on leaving the large roost? This is an interesting aspect of behaviour that requires more research, which might be assisted through the use of small modern transmitters (here).

TABLE 14. The proportion of wing-tagged Black-headed Gulls in north-east England recorded in subsequent weeks during the same winter in a different feeding area. Data summarised from MacKinnon (1986).

Age structure in different feeding areas

First-year and adult Black-headed Gulls are not distributed in the same proportions in different feeding habitats within the same area. During the studies carried out in north-east England, first-year birds formed only 1–5 per cent of the Black-headed Gulls present near river mouths and on the coast (Table 15), and were more than four times as frequent feeding inland on pastures and grassland, averaging 22 per cent of individuals present. At landfills, first-year birds increased from 11 per cent in autumn to 20 per cent by midwinter. The reason for these differences is not known, but since birds feeding on the coast and those feeding inland used the same night roost, it would appear to be the result of variation in the feeding ability of birds of different ages (Table 16). This suggests that coastal feeding may require greater experience in order to become proficient, as has been shown in other studies of Black-headed Gulls relying on kleptoparasitism as a source of food (here).

TABLE 15. The proportion of first-year Black-headed Gulls in winter counts near the mouths of four rivers in north-east and eastern England, and on the north-east coast. Proportions of first-year birds showed little variation between days or between years.

Interestingly, the cannon-net captures of Black-headed Gulls at landfill sites were composed of up to twice the proportion of first-year birds than those counted at nearby loafing sites before making a capture (Table 15). This probably indicates that the young birds take greater advantage of feeding frenzies and possibly have a greater risk of capture because of their less efficient feeding ability, which causes them to stay and feed for longer at landfill sites.

Feeding rates of Black-headed Gulls differ considerably between different sites (Table 16). Items are taken much more rapidly at sewage outfalls, as these sites offer an abundance of readily available food. Following the plough is less successful as it involves a degree of searching or gaining good positions, while walking over short grass on playing fields or pasture involves considerable searching before each food item is discovered. The figures in Table 16 do not take into account the size of each item consumed, although they were likely to be similar on fields and at coastal sites, and smaller at sewage outfalls.

TABLE 16. Feed rates measured as numbers of swallows per 12 seconds by Black-headed Gulls at different feeding sites. First-year birds were slower feeders than adults at all three sites. Data mainly collected by Gabriella MacKinnon and personal unpublished records.

Weight changes

The studies of Black-headed Gulls in north-east England confirmed that adult males are about 17 per cent heavier on average than females throughout the year, and that both sexes are 15–17 per cent heavier in winter than in summer (Fig. 38), reaching a peak weight in December when daylight is shortest. From February onwards, both sexes gradually lose weight, reaching their lowest weight in summer. Recently fledged first-year birds in July are much lighter than adults, but soon catch up, so that by September they have almost reached the weight of adults of the same sex. They then remain marginally lighter throughout the rest of the autumn and winter. The heavier weights in winter are, presumably, caused by an increased layer of subcutaneous fat beneath the skin, which reduces heat loss and acts as a reserve to tide individuals over difficulties encountered in feeding during severe weather.

The low weights of adults in summer does not seem to be a direct response to the stress and effort of breeding, because first-year birds, which do not breed, follow the same trend from November to March. Lower summer weights are achieved by adults by March, before they migrate and have started to visit their colonies. Between March and August, adults maintain the lower weight, and there is only slight evidence that breeding causes further weight loss. One effect of lower body weight in summer is that the energy cost of flying is reduced. This is at a time when food reserves protecting against cold weather and insulation to reduce heat loss are less necessary, and so there is no need for birds to carry the associated extra weight.

In the studies, only minimal differences were apparent in the weights of Black-headed Gulls caught in July and August just after dawn (before they have eaten their first feed of the day) compared to those caught in the afternoon (Table 17). However, as day length becomes shorter and the period when the gulls cannot feed increases, their overnight weight loss increases progressively. By December and January, the overnight weight loss recorded reached 18.3 g, which is an appreciable 6 per cent decrease of the afternoon body weight, at a time when body weight is at a seasonal maximum (Table 17 and Fig. 38).

FIG 38. The monthly average weights of adult and first-year male and female Black-headed Gulls in north-east England. Based partly on data analysed by MacKinnon (1986) and on unpublished personal data, totalling 1,953 individuals. Note that few first-year birds were weighed in April, May and June.

There is a widely held view that winter is the period when birds face the highest risk of food shortages, leading to loss of weight and increased mortality. In the case of the Black-headed Gull, however, this is not so. Prolonged periods of hard frost do little to affect the average weight of the birds. Table 18 shows the deviation in weight of Black-headed Gulls from the monthly average in relation to the number of consecutive days with hard ground frosts before capture. Short periods of frosty weather had little impact on their weights, and even with more than four days of continuous hard frosts, their weights had declined by only 3.7 g, or about 1.2 per cent of the body mass expected at that time of year.

TABLE 17. Weight change between dawn and afternoon of adult Black-headed Gulls between July and January. Weights are the average for males and females recorded in north-east England. Significant overnight weight losses are indicated by an asterisk (*).

TABLE 18. The effect of consecutive days with hard ground frosts on the weight of Black-headed Gulls. Results are shown as the deviation from the average weight of adults of each sex in the month of capture in north-east England.

FIG 39. The percentage of the total number of ringed adult Black-headed Gulls reported dead in each month of the year. The distribution suggests that mortality is relatively low during the winter months and then peaks at about twice the winter rate during the breeding season (May to July). Figures are based on 2,575 recoveries of adults and include individuals recovered abroad.

With Black-headed Gulls achieving high body weights in winter and sustaining these during cold weather, this begs the question as to when the risk of mortality is highest in the species. This was first investigated by Jim Flegg and Christopher Cox in 1956–74 (Flegg & Cox, 1975), and the amount of related data have increased since then following the recovery of many more British ringed birds found dead from 1975 to 1984 (Fig. 39). In fact, the seasonal pattern of deaths has changed little since the original analysis, with the numbers reported dying each month during the breeding season (April to July) double those in each month between October and March. In the British climate, Black-headed Gulls are at a greater risk of being killed by predatory mammals while breeding (as found by Hans Kruuk at Ravenglass and Clive Craik in the Hebrides), than by food shortages in winter. While the birds are breeding and are attached to a particular colony site, their feeding areas are restricted. Agricultural fields planted with root or grain crops become unsuitable as feeding areas as the plants grow, while those where grass is allowed to grow tall for hay and silage production also become unsuitable until the first cuts are made. Once the grass is cut, the speed at which the fields are then visited by gulls strongly supports the belief that food may be in short supply during the breeding season.

ANNUAL SURVIVAL RATES

Considering that the Black-headed Gull is so abundant, there is relatively little information on the survival rates for members of the species. The first estimates were published in 1975 by Flegg and Cox, who suggested that the adult survival rate in the south of England was 76 per cent per year. This value was probably low because some recoveries of old birds were still to be reported, and in addition, doubts now exist about the durability of the rings then in use, which may have reduced the estimated survival rate (see below). More recently, Anne-Caroline Prévot-Julliard, Jean-Dominique Lebreton and Roger Pradel (1998) estimated a 90 per cent annual adult survival rate in France.

Gabriella MacKinnon and I used British BTO ringing recoveries to estimate the annual survival rates of Black-headed Gulls ringed as nestlings in the UK. The rate in the bird’s first year of life proved to be low, followed by a constant rate of 84 per cent for the next 10 years of life (MacKinnon & Coulson, 1987). Thereafter, the annual survival rate declines progressively, with the oldest bird reported at 22 years of age; this is well below a maximum age of 33 years reported by Klaas van Dijk and Rob Voesten in 2014. There are two possible explanations for this pattern. One is that senility sets in after Black-headed Gulls reach about 10 years of age. The second is that the rings on the gulls start to be lost or become illegible after 10 years, resulting in fewer reports of old birds that are of this age in the population. We encountered a similar problem with aluminium and Monel rings used on Kittiwakes and Herring Gulls in the past, and examined both aluminium and Monel rings used by BTO ringers recovered during our studies on Black-headed Gulls in north-east England. We concluded that ring wear and loss was a problem and that a few individuals probably outlived the life of the rings used to mark them. As a result, survival rates were probably underestimated. In addition, based on information available for other seabirds, it is considered unlikely that senility occurs in 10-year-old individuals. We also found that Monel rings, which were introduced to overcome the wear found on aluminium rings, actually lost weight at a similar rate to the aluminium ones and were therefore little improvement.

Fortunately, methods have been developed that allow survival rates to be calculated based on age-truncated recoveries. MacKinnon and I used these in our studies (with help from Nicholas Aebischer and David Jackson) to determine annual survival rates for each year from 1950 to 1980 using recoveries of birds up to 10 years old (Fig. 40). This method excluded information on survival rates for the oldest 20 per cent of Black-headed Gulls living in the wild, but avoided bias produced by ring wear and whether senility was having any effect. Using this method on the accumulated BTO recoveries, annual survival of adults fluctuated around 83 per cent between 1950 and 1955, and then progressively increased to about 87 per cent from 1974 to 1980. The survival of first-year birds in the 1950s was about 60 per cent and increased progressively to about 70 per cent during the 1970s.

FIG 40. Estimates of the annual survival rates of adult and first-year Black-headed Gulls in Britain from 1950 to 1980. The adult survival estimates were based on birds recovered within 10 years of ringing to avoid bias. Each year runs from July to June, and the years shown relate to the start of each 12-month period. Both trend lines differ significantly from a constant survival rate over the period considered, and survival has gradually increased from 1950 to 1979. Note the low survival rate from July 1962 to June 1963, when adult survival declined to 71 per cent and that of the young fledged in 1962 decreased to 44 per cent. The winter during these 12 months was the most severe of the 30-year period.

The results show a number of interesting features. First, the annual survival rate has increased overall in both adults and first-year birds in more recent years. Second, the first-year survival rates show much more between-year variation. Third, in only one winter (1962/63, the severest during the range of years used in the study), do the birds show clear weather-related decreases in survival, with first-year birds managing only 44 per cent survival and adults 71 per cent survival in that year. And finally, the high (84 per cent) survival rate of first-years and above-average survival rate of adults in the 1974/75 winter coincided with the mildest weather at this time of year in England since 1869.

Regional survival rates

The survival rates of Black-headed Gulls ringed as pre-fledged chicks or as adults in different regions of Britain (Table 19) show little regional variation, and the small differences are all within the tolerances of the methods used. In general, the mortality rate in the first year of life is about double that of the birds once they become adults. However, the estimates for Ireland indicate that Black-headed Gulls here had both the lowest first-year and adult survival rates, and the lowest adult life expectancy. These figures are consistent with the appreciable current decline in adult numbers nesting there. The highest adult survival rate and adult life expectancies were in Scotland, which is consistent with the increase in numbers reported nesting there between 1985 and 2000.

TABLE 19. Estimates of the annual survival rates of adult and first-year Black-headed Gulls, and life expectancy of adults, in Britain and Ireland according to the area in which they hatched. Data are restricted to those birds ringed between 1950 and 1980.

MOULTING

There is surprisingly little information on the progress of the primary wing moult of Black-headed Gulls. The start of moult commences at the innermost primary, with the old feather usually dropped by adults in June (the earliest recorded date is 26 May), at a time many have already hatched chicks. Since the inner primaries are relatively short, the start of moult at this stage will have little effect on flight and the ability of parents to search for food for the young. After incubation is complete, the primary moult progressively spreads along the wing, until the last (outer) primaries are dropped in September and replaced by fully grown feathers by mid-October. In all, the primary moult takes three to three-and-a-half months from the first feather being shed to the longest primaries becoming fully grown.

The moult of the primary feathers by immature one-year-olds starts about a month earlier than in adults and is completed by September – a three-month period, which is generally shorter than that taken by the adults. The period when the outer and longer ninth and 10th primaries feathers are missing (August for first-year birds and September for adults) probably interferes most with efficient flight, and so this may inhibit long-distance migration until the new outer primaries have grown.

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