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 © Mike Toms, 2019 Photographs © Individual copyright holders Mike Toms asserts his moral right to be identified as the author of this work Cover art 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: 9780008164744 Ebook Edition © April 2019 ISBN: 9780008164768 Version: 2019-04-03

THIS BOOK IS DEDICATED

TO CHRIS MEAD AND DAVID GLUE,

WHOSE KNOWLEDGE OF GARDEN BIRDS

AND PASSION FOR THEIR STUDY, IS MUCH MISSED.

EDITORS
SARAH A. CORBET, SCD
DAVID STREETER, MBE, FIBIOL
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

Dedication

About the Editors

Editors’ Preface

Author’s Foreword and Acknowledgements

1 Gardens and Birds

2 Foods and Feeding

3 Nests, Nest Boxes and Breeding

4 Opportunities and Risks

5 Behaviour

6 Birds, Gardens and People

7 Species Accounts

References

Species Index

General Index

The New Naturalist Library

About the Author

About the Publisher

Editors’ Preface

GIVEN THE EXPLOSION OF INTEREST in garden birds, their habits and feeding since the 1960s, this is an addition to the New Naturalist Library that was crying out to be written. Many, perhaps most of us, are familiar with a range of garden birds on a day-to-day basis. That range may be large or small, but familiarity engenders an interest in how the birds are faring and an enthusiasm for taking part in censuses and surveys: ideal seeding ground for what are now called citizen science projects. Against this background, the Editorial Board was fortunate to be able to call on an author tailor-made for the task in Mike Toms. Mike will be familiar to NN readers as the author of the very popular and very readable text on owls (NN 125, 2014). He has spent most of his working life with the British Trust for Ornithology. Over and above his personal fascination with owls, he has been involved in the organisation and analyses of the various BTO Garden Bird studies and, thus, is the ideal author for this authoritative text.

The text opens with an analysis of the role of gardens in the ecology and general prosperity of our commoner land bird populations and an assessment of the diversity that we have created. Interestingly, though we think of gardens as ubiquitous, the land area that they occupy, at just over 400,000 ha, is only slightly larger than the county of Suffolk, and less than 10 per cent of British land area with some form of statutory protection. That said, the potential for interest and fascination that they offer, far outweighs those statistics. In the second chapter, Mike Toms explores the phenomenal growth of garden bird feeding since the end of the Second World War and the equally amazing diversity of foodstuffs now available, particularly seeds. Long gone are the days of throwing out a handful of scraps from the table to satisfy our feathered friends!

Subsequent chapters cover the provision of nesting opportunities, nest boxes and other aspects of the breeding season (Chapter 3), and the aptly-named ‘Opportunities and Risks’ (Chapter 4) outlines the disease health hazards (sometimes devastating) that birds face at feeding stations. Enthusiastic gatherings squabbling at feeders become vulnerable to lurking sharp-eyed predators like Sparrowhawks Accipiter nisus. Chapter 5, ‘Behaviour’, emphasises the opportunities (and privileges) on offer in gardens, to watch birds interacting at close quarters, and demonstrates how simply watching can develop into (equally enjoyable) studying, far more easily than in the general open countryside. Equally absorbing and fascinating are the varied aspects of garden bird watching in Chapter 6, ‘Birds, Gardens and People’, which, among other things, emphasises the many benefits, including to physical and mental health, that people can derive from the interactions.

The last chapter is an extremely useful compendium of succinct accounts of the most regularly observed garden birds, with, in each case, tabulated biological data (clutch size, incubation period, breeding attempts per annum, etc.) followed by an account of the particular relevance of the species to our gardens. It lists, without fear or favour, ‘the good, the bad and the ugly’ of garden birds and items of fascination too numerous to note, other than to offer examples. Few outside the southeast of England would have considered the Ring-necked Parakeet Psittacula krameri as a garden bird, but these raucous and colourful aliens (although now with a self-sustaining feral population) can be a domineering force disrupting the feeding and nesting of other species. The Magpie Pica pica, not a popular garden bird, has its evil reputation as a predator of nests and nestlings accurately assessed, and moderated. Perhaps strangest of all, the Red Kite Milvus milvus – which most of us would consider an unlikely garden bird – is now numerous enough near its centres of reintroduction to visit garden feeding stations routinely, and some populations are showing signs of dietary imbalance, the natural food of carcasses, road kills etc. being substantially better for them than the household waste scraps usually on offer in gardens.

Much else of equal fascination awaits – now please do read on and enjoy Garden Birds to the full.

Author’s Foreword and Acknowledgements

PRIVATE GARDENS PROVIDE OPPORTUNITIES FOR engagement with the natural world, delivering connection with a broad range of different plants and animals. Birds are one of the most obvious and accessible components of the community of species using our gardens, visiting bird tables and hanging feeders, and occupying the nest boxes that we erect for them. Familiar species, like Robin, House Sparrow and Blackbird, provide a common currency, the basis to conversations had between neighbours, relatives and friends. That we choose to engage with garden birds, and talk about their visits and behaviour, underlines the central place that they play in our lives. For those who never venture into the wider countryside, garden birds can provide a window onto the natural world, helping to restore lost connections and contributing to health and well-being. But what about the birds themselves; what role do gardens play in their ecology and behaviour? What do we really know about the risks and benefits that shape how and why birds use gardens and the resources present within them?

Over the past two decades, I have been fortunate enough to have worked on garden birds, and with those who chose to watch them in their gardens. Through ‘citizen science’ projects, most notably the weekly BTO Garden BirdWatch scheme, I have been able to study the different ways in which wild birds use our gardens. I have been able to examine garden use in relation to the wider landscape, exploring both the seasonality of garden use and the longer-term patterns that have followed changes in how we manage the countryside. Thanks to the efforts and support of the volunteers participating in citizen science schemes, I have been able to explore some of the questions surrounding garden use and the role that gardens and their resources play in the lives of visiting birds, addressing the benefits of food provision, the risks of disease transmission and the threats posed by predatory species.

Such research underlines that the ecological processes operating within the garden environment are similar to those operating elsewhere, providing us with opportunities to learn about the wider world and to recognise the role that gardens play within the broader suite of habitats that make up our countryside. Gardens and the wider built environment pose some particular challenges for wild birds; they also offer unique opportunities. The process of urbanisation means that more land will be occupied by urban landscapes and we need to understand the implications of this if we are to deliver built environments that can also work for birds and other wildlife, rather than just for ourselves. Much of the work outlined in this book, carried out by researchers around the globe, delivers the scientific evidence and understanding that we will need as we seek to inform decision-makers and support conservation practitioners. I am fortunate enough to have played a small part in this work, and I hope that reading about it excites you as much as it excites those of us carrying out the research and citizen science.

This book would not have been possible without the help, support and generosity of many people. In particular I would like to thank those with whom I have been fortunate enough to have worked on garden birds: Margaret Askew, Andrew Cannon, Dan Chamberlain, David Glue, Tim Harrison, Donna Hobbs, Becky Lawson, Dave Leech, Amy Lewis, Chris Mead, Kate Plummer, Jacky and Alic Prior, Kate Risely, Clare Simm, Gavin Siriwardena and Paul Stancliffe. I am most grateful to Myles Archibald, Hazel Eriksson, Julia Koppitz and the staff at William Collins; to Jim Flegg and the New Naturalist Editorial Board, and to Tom Cabot for his work on design and layout. Once again, I have been extremely fortunate to have my words graced by cover artwork produced by Robert Gillmor. The photographs that grace this book and bring the text to life were very generously and kindly provided by Mark Grantham, John Harding, Jill Pakenham, Hazel Rothwell, Tom Streeter and Moss Taylor, to all of whom I am very grateful. Thanks are also due to the staff of the BTO, its volunteers and those who have supported the work on garden birds through projects like Garden BirdWatch. The BTO, its staff and volunteers are truly inspirational and I am very grateful to have found such a wonderful place of employment. Finally, I would like to thank Annabel Hill for her love and support, and for helping to make this book possible.

Mike Toms

November 2018

CHAPTER 1
Gardens and Birds

PRIVATE GARDENS ARE A FEATURE of most of the dwellings found within the UK. A study by Davies et al. (2009), using data from a number of different datasets, put the proportion of households with an associated garden at 87 per cent. The researchers then used this information, alongside figures for mean garden size and the number of households, to calculate the total area of gardens within the UK. The resulting figure, some 432,964 ha, represents an area larger than the county of Suffolk but still an order of magnitude less than the 4.7 million ha of UK land that is under statutory protection.

While such figures underline that gardens only make up a relatively small part of the UK land area, they are the habitat within which many of us engage with birds and other forms of wildlife. This gives them special importance, not least because gardens provide the best opportunities to empower UK citizens with the conservation and research understanding needed to support sustainable planning, lifestyle and land management decisions. Birds are of particular importance in this context because they are one of the most visible, accessible and appreciated components of the wider natural world. If we can understand how they use gardens, and how this use is perceived by the people who have access to those gardens, then we can begin to provide the evidence and messaging needed to support public engagement with the natural world more widely.

FIG 1. Private gardens vary considerably in their size, composition and use, presenting a series of different opportunities for birds. (Mike Toms)

We also need to recognise that gardens come in many different forms; at the simplest level they have tended to be split into those that are urban, suburban or rural in nature, but there is substantial variation within these broad categories. Many gardens fall within urbanised landscapes, so we also need to understand the relationships that exist between gardens and their surroundings. We know, for example, that city-centre gardens are just one form of urban green space and that birds, being mobile, will move between a city’s many different patches of such space. But how important is the spatial arrangement of these different patches or the temporal pattern of feeding resources available within them? Many of the same questions can be asked of those birds using rural gardens, perhaps bordered by arable farmland or woodland plantations. This underlines that gardens should not be viewed in isolation but rather as a part of a wider landscape over which birds may range. In some cases these ranging movements may take birds not just beyond the boundaries of the gardens and often urbanised landscapes within which they sit, but also beyond the borders of countries or even continents.

FIG 2. The Robin is perhaps the most familiar of our ‘garden birds’ but in reality it is a species of woodland edge habitats. (John Harding)

It is for this reason that this book on garden birds starts by looking at the nature of the garden habitat and the garden’s place within a wider landscape context. Inevitably, this will require us to look at urban ecology and the processes that shape the urban environment, and to look into a future where the ongoing process of urbanisation sees an ever-growing number of us living within towns and cities. We will also use this chapter as an opportunity to ask ‘what is a garden bird?’ and to examine the nature of garden bird communities. Are they, for example, just those species that happen to be generally common and widespread across the wider landscape, or is there something special about garden bird species and the communities that they form.

THE URBAN ENVIRONMENT

One of the most shocking statistics relating to the human population is that showing the proportion of the global population now living within urbanised landscapes (see Figure 3); this figure, which passed 50 per cent in 2008, is projected to reach 66 per cent by 2050 (United Nations, 2014). This increase brings with it an associated increase in the amount of land under urban cover. Urbanisation is an ongoing process and considered to be one of the greatest threats facing species and their ecosystems. It is of particular concern because some of the most intensive urban development is projected to occur within key global biodiversity hotspots (Elmqvist et al., 2013). The growth in the numbers of households globally, and within biodiversity hotspot regions in particular, has been more rapid than aggregated human population growth, reflecting that average household size continues to fall. This is relevant because the reduction in average household size is thought to have added 233 million additional households to biodiversity hotspot countries alone between 2000 and 2015 (Liu et al., 2003). It seems all the more important than ever to understand the implications of urbanisation for biodiversity; to some extent this urgency has been recognised by the research community, with increasing numbers of studies helping to unravel how birds and other forms of wildlife make use of the urban environment and revealing what urban expansion means for their wider populations. Urbanisation also has implications for the ways in which the human population interacts with the wider natural world, of which it is a part, so we also need to learn about these relationships and what they mean for both birds and people; this is something that we will explore in Chapter 6.

FIG 3. The proportion of the human population living within urbanised landscapes passed 50% in 2008 and is predicted to reach 66% by 2050. Redrawn from United Nations data.

The process of urbanisation involves the conversion of natural or semi-natural landscapes – the latter including farmed land – into ones that are characterised by high densities of artificial structures and impervious surfaces. Urbanised landscapes contain fragmented and highly disturbed habitats, are occupied by high densities of people and show an elevated availability of certain resources. Associated with the process of urbanisation is the modification of ecological processes, particularly those linked with nutrient cycling and water flow. The density of bird species within such landscapes is best explained by anthropogenic features and is often negatively associated with the amount of urban cover. This underlines the importance of urban green space, especially gardens and (for some species) urban parks and woodland. Although many bird species decline in abundance once an area has become urbanised, some are able to take advantage of the new opportunities that have been created. Because different species respond to urbanisation in different ways, we typically see a dramatic shift in the structure of avian communities living within urban landscapes and the habitats, like gardens and urban parks, found within them.

Alongside the structural changes seen, which may alter nesting opportunities or the availability of food resources, urbanisation also results in increased levels of disturbance, noise, night-time light and pollution, all of which may impact birds and other wildlife. While some of these impacts may be generally negative across species, there may be instances where such effects are not felt equally. Noise pollution can impact on some species more than others, for example, perhaps because of the frequencies at which such noise tends to occur; bird species whose songs are pitched towards lower frequencies may be affected more than those whose high-pitched songs can still be heard above the background noise. Other species may show a behavioural response, perhaps moving the time at which they sing or even altering the characteristics of the song itself. It has been found, for example, that Robin Erithacus rubecula populations breeding in noisy parts of urban Sheffield sing at night, with the levels of daytime noise experienced by these individuals a better predictor of their nocturnal singing behaviour than levels of night-time light pollution, the latter previously considered to have been the driver for nocturnal song in this species (Fuller et al., 2007a).

Not all of the changes that occur during urbanisation are necessarily negative; the provision of food at garden feeding stations (Chapter 2) is regarded as being generally beneficial, buffering the temporal variation in food availability that is typical of natural landscapes and thought to limit populations. The availability of anthropogenic food, either at garden feeding stations or present as food scraps, is thought to be one of the main factors driving the structure of urban bird communities, something to which we will return shortly. A number of studies have found that urbanisation stabilises both the richness and composition of bird communities, perhaps because there is greater predictability of, and less variation within, the climate and resource availability of more urbanised landscapes (Suhonen et al., 2009; Leveau & Leveau, 2012).

As we’ll also discover later in this chapter, some species are better able to cope with or respond to the impacts and opportunities of the built environment than others, and there are particular traits within bird families or species groups which may lead to them being more likely to adapt to the urban environment (Blair, 1996). This raises the question of just what is a garden bird.

WHAT IS A GARDEN BIRD?

Some people take the term ‘garden bird’ to mean a species that is common and widespread, adaptable and found just as commonly in other habitats. While this is certainly true of many garden birds, it is not true of every species found to use gardens on a regular basis. The phrase ‘common or garden’ – meaning something that is common and consequently of little value – may have also added to the sense that garden birds have little conservation value and are thus of little interest. This may be exacerbated by the sense that gardens themselves are an artificial habitat, highly modified and managed and thus greatly removed from more natural habitats and processes. However, it needs to be remembered that many of the habitats that we consider to form the countryside – such as farmland and woodland – are also often highly managed and very different to wholly natural landscapes untouched by human activities. Once we recognise this, then it becomes possible to accept garden birds less as a distinct type of bird and more as simply a subset of a wider population using a broad range of habitats.

Look at any book on UK garden birds and you will see species that are summer migrants (Swift Apus apus, House Martin Delichon urbicum, Spotted Flycatcher Muscicapa striata), winter visitors (Redwing Turdus iliacus, Waxwing Bomybycilla garrulus, Brambling Fringilla montifringilla, Fieldfare Turdus pilaris), insectivores (Goldcrest Regulus regulus, Wren Troglodytes troglodytes), omnivores (Carrion Crow Corvus corone), cavity nesters (Blue Tit Cyanistes caeruleus, Great Tit Parus major) or have low dispersive ability (House Sparrow Passer domesticus), all underlining the variety of ecological traits encountered within our garden bird community. Certain ecological traits may come to the fore if you look at particular components of the wider community; for example, an examination of the birds using garden feeding stations may suggest the dominance of granivorous and omnivorous species rather than insectivores, a reflection of the types of food being provided (Shochat et al., 2010). A different pattern may be seen in winter from summer, when looking at some aspect of the breeding community, or indeed when looking at the same component (e.g. cavity-nesting) but in different geographical locations. We know, for example, that cavity-nesting species are well represented within the UK garden bird community, having adapted to nest boxes, but are far less common in that of Australia, where native cavity nesters rely on natural tree cavities rather than boxes (Shanahan et al., 2014).

Globally, certain species are well represented within garden bird communities, including Rock Dove/Feral Pigeon Columba livia, House Sparrow and Starling Sturnus vulgaris (Aronson et al., 2014). Also widely represented – at least within the wider urban environment – are introduced or ‘invasive’ species such as Mallard Anas platyrhynchos, Canada Goose Branta canadensis, Collared Dove Streptopelia decaocto and Ring-necked Parakeet Psittacula krameri, plus (in those places beyond their native range) House Sparrow, Starling and Rock Dove/Feral Pigeon. As a group, corvids appear to be well represented, as do pigeons and doves, leading some authors to suggest that urban bird communities globally are becoming more homogenous in their composition (either directly, in terms of particular species, or indirectly, through different species occupying similar ecological niches). This process is something that we will discuss in the next section.

FIG 4. Blackbird is best considered as an ‘urban adopter’, a species that is opportunistic in the way that it uses gardens and the resources that they offer. (John Harding)

The species of birds associated with the urban environment – of which gardens are a key component – can be divided into three main types. These are the ‘urban exploiters’, the ‘urban adopters’ and the ‘urban relicts’. Exploiters are those species, like Feral Pigeon, that are typically abundant within urban areas and which depend upon the anthropogenic resources available within the built environment. Adopters also make use of the resources but are more opportunistic in how they do this, and many of our familiar garden birds can be regarded as being of this kind – think of the Blackbirds Turdus merula and Siskins Spinus spinus that move into gardens during the winter months. Urban relict species are those whose population has managed to hang on within a fragment of their former habitat that is now contained within a wider urbanised landscape; such species tend to be found outside of western Europe, where new cities have emerged quickly within formerly wildlife-rich habitats. Another term that may be encountered when discussing urban bird populations is ‘urban avoiders’, those species that are absent or very poorly represented within the urban bird community.

The Rock Dove/Feral Pigeon is one of the oldest and most cosmopolitan commensal species, whose huge global population reflects early domestication and the subsequent transportation and introduction to sites across the world. The very high Feral Pigeon densities encountered in many cities is, to a large degree, a consequence of supplementary feeding and discarded human food, and the presence of buildings and other structures with an abundance of suitable nest sites. In some cities, such as Singapore, these populations are derived from the rapid expansion of a genetically homogenous group of founder individuals (Tang et al., 2018), underlining how a species can very rapidly colonise an urban area given favourable conditions. Similar patterns may be seen in introduced populations of House Sparrow, Starling and Collared Dove.

The notion that species found in a high proportion of the world’s cities, such as Starling, are successful generalists and able to breed anywhere, does require some refinement. Work by Gwénaëlle Mennechez and Philippe Clergeau has, for example, revealed that while the abundance of breeding Starlings does appear to be similar throughout the urbanisation gradient, the degree of urbanisation still has a measurable negative impact on its breeding success. Working in western France, Mennechez and Clergeau (2006) found that the amount of food delivered to Starling nestlings in more urbanised areas was significantly lower than that delivered elsewhere along the urbanisation gradient, resulting in smaller nestling masses. These urban Starlings were found to produce fewer young, each of seemingly lower quality; while they were able to maintain breeding populations in these highly urban habitats, the species was not as successful as a simple measure of breeding abundance might suggest. This is something to which we will return in Chapter 3.

URBAN BIRD COMMUNITIES

Surprisingly perhaps, one in five of the world’s 10,000 or so bird species is found in the highly urbanised landscapes of major cities, including 36 species that have been identified by the IUCN global Red List as threatened with extinction. (Aronson et al., 2014). Globally, the species richness of urban bird communities ranges from 24 species to 368, with a global median of 112.5 species (Aronson et al., 2014). As just noted, a number of studies, typically involving work carried out within a single city, have suggested that the process of urbanisation results in urban bird communities that are becoming increasingly similar over time, and dominated by a few key species. This process is known as homogenisation and is thought to come about because urbanisation not only extirpates native species from an area but also promotes the establishment of non-native species capable of adapting to the new conditions (Luck & Smallbone, 2010). Although these new conditions tend to be similar in cities across the world, they do not in themselves promote the same species winners; instead, it is our preference for certain species (transporting them to new settlements) that has resulted in the homogenisation seen at a global scale (McKinney, 2006).

In their review of the bird communities of 54 cities, spread over 36 countries and 6 continents, Aronson et al. (2014) found that cities tended to retain similar compositional patterns within their distinct biogeographic regions. While certain non-native species were shared across many of the cities studied, the urban communities had not yet become taxonomically homogenised at a global scale – they retained the characteristics of their local region pool. Four species were found in more than 80 per cent of the cities examined; these being the familiar Feral Pigeon, House Sparrow, Starling and Barn Swallow Hirundo rustica. With the exception of Australasia, the proportion of non-native species found within each urban community was similar (at 3 per cent), Australasia being somewhat higher because of the large number of non-native species introduced into New Zealand by settlers. Work within Europe (Jokimäki & Kaisanlahti-Jokimäki, 2003; Clergeau et al., 2006) suggests that urbanisation here might bring about homogenisation by decreasing the abundance of ground-nesting species and those preferring bush-shrub habitats, but also noting that it is difficult to generalise because of the effects of latitude and diversity in the urban habitats studied.

What is interesting about species seemingly well adapted to the urban environment is that they tend to share a number of traits (Leveau, 2013). They tend to be omnivorous in their diet, largely sedentary in habits and able to utilise a range of artificial nesting sites (Máthé & Batáry, 2015). Table 1 highlights the broad ecological traits of the garden bird species considered in this book. As we have seen, omnivores and granivores (seed-eating species) tend to dominate the garden bird community, with insectivores often poorly represented. This suggests that invertebrate populations within gardens and the wider urban environment may not be sufficient to support populations of insect-eating birds. It also has implications for breeding success more widely, since most small birds feed their chicks on invertebrates. Work by Croci & Clergeau (2008) also suggests that ‘urban adapter’ species are those that are sedentary in habits and omnivorous, though with the additional traits of preferring forest environments, being widely distributed, and being high-nesters with large wingspans. Croci & Clergeau (2008) also note that ‘urban avoider’ species tend to be those that allocate more energy to their breeding attempts than ‘urban adaptor’ species, a trait that might make it difficult for them to adapt to new environments. Logically, given the absence of larger and older trees with natural cavities, you might predict cavity-nesting birds to be less common in urban environments. However, the presence of substantial numbers of nest boxes appears to help at least some of these species, most notably the tits, though those requiring large cavities often lose out.

TABLE 1. Common garden birds, their ecological traits and status. * Goldcrest and Long-tailed Tit are open-nesting species, whose nest is fully domed. Conservation status is derived from the Birds of Conservation Concern (BOCC) List, which classifies species as Red-, Amber- or Green-listed based on a number of criteria. Non-native species with feral populations are not assessed by BOCC.

That wild birds can take advantage of the opportunities afforded by human activities is evident from changes in urban-nesting gull populations within the UK, which have increased substantially over a short period of time; this increase has occurred despite the fact that coastal populations of the same species have been in decline. It is thought that much of this increase has been driven by the availability of food scraps on our urban streets and at landfill sites, coupled with the relatively predator-free nesting opportunities available (Ross-Smith et al., 2014).

Interestingly, much of the ecological work looking at urban birds has tended to adopt a binary approach, separating species into urban and non-urban classes and then looking for differences in their ecological and/or physiological traits (e.g. Møller, 2009). Such work has underlined, for example, that insectivores, cavity nesters and migrants are under-represented within urban bird communities, while species showing high rates of feeding innovation, high annual fecundity and high adult survival rates are over-represented. However, such an approach – where species are lumped into one or the other of these two groupings – fails to account for important differences in how individual species may respond to urbanisation, or to differences in how a species may respond in different parts of its geographic range. When you consider these differences, then many of the identified traits characteristic of one or other community disappear (Evans et al., 2011).

The nature of urban bird communities has been the subject of study and review for a number of years and several common patterns emerge. It appears, for example, that local factors are more important in determining the species richness of these communities than factors operating at a wider regional scale, and that urban communities respond positively to the availability of supplementary feeding and the structural complexity of local habitat, but negatively to the degree of human disturbance (Evans et al., 2009a). While there is likely to be continuing debate around this topic (e.g. Møller, 2014), it does appear that, at the species level, generalists are better suited to the urban environment than specialists, as are those species that nest off the ground. Generalist species are known to be less susceptible to, and may even benefit from, environmental disturbance of the sort associated with urbanisation (McKinney & Lockwood, 1999), and they are also the species that appear to be coping best with a changing global climate (Davey et al., 2012), something that may also be behind their apparent success in our towns, cities and gardens. Broader environmental tolerance is something that has been demonstrated in urban birds, most clearly through a study by Frances Bonier and colleagues, who compared the elevational and latitudinal distributions of 217 urban birds found in 73 of the world’s largest cities with the distributions of 247 rural congeners. The results of this work showed that urban bird species had markedly broader environmental tolerance than rural congeners, suggesting that a broad environmental tolerance may predispose some species to thrive within urban habitats (Bonier et al., 2007).

UK GARDEN BIRDS, THE SIZE OF THEIR POPULATIONS AND COMMUNITY STRUCTURE

The BTO/JNCC/RSPB Breeding Bird Survey (BBS) is the core scheme used to monitor the population trends of a broad range of breeding bird species across the UK. Some 3,000 participants visit randomly selected 1-km survey squares and record the bird species that they encounter there. The birds seen are recorded in a series of distance bands running out parallel to the line transects along which each observer walks. This approach, coupled with the collection of habitat information for each of the transect sections, allows researchers to calculate density estimates by habitat for each of the bird species commonly recorded. These can then be multiplied up by the area of each habitat type nationally to calculate population estimates based on habitat type, which in turn provide a sense of the bird populations associated with gardens and wider urbanised landscapes (Newson et al., 2005).

FIG 5. Data from the BTO/JNCC/RSPB Breeding Bird Survey underline the sizeable populations of certain species breeding in urbanised landscapes. Redrawn from data presented by Newson et al. (2005).

This approach suggests that some of the national population estimates previously given for familiar species, like Blackbird, Starling, House Sparrow, Greenfinch Chloris chloris, Jackdaw Crovus monedula and Woodpigeon Columba palumbus, have been underestimated because the built environment and its gardens had not been properly taken into account. The BBS habitat-based approach indicates the importance of urban, suburban and rural human habitats for species like House Sparrow and Starling, which have 61.5 per cent and 53.9 per cent respectively of their breeding population found here. The survey has also enabled the production of figures suggesting that the habitats directly associated with human sites (urban, suburban and rural) could represent 27,919 km2 (2,791,900 ha) or 10.9 per cent of UK land area. This can be further broken down as urban (2.2 per cent), suburban (5.1 per cent) and rural human sites (3.6 per cent). The figure of 10.9 per cent is somewhat higher than the 6 per cent figure that has been derived from the Corinne Land Cover map. Given the different assumptions involved, it seems likely that the true figure lies somewhere between the two approaches and possibly closer to the 6 per cent derived from satellite imagery. Using the figure of 432,964 ha, presented at the top of this chapter and calculated by Davies et al. (2009) for the area of UK gardens, and the two figures just mentioned (Newson et al. 2,791,900 ha, Land Cover map 1,454,970 ha) for the amount of built-upon land, suggests that gardens might represent between 16 per cent and 30 per cent of the land area present within UK cities, towns and villages, and 1.79 per cent of total land area.

Gardens and their associated houses are of particular importance to House Martin and Swift, two species that national monitoring schemes like the Breeding Bird Survey struggle to monitor. While these two summer visitors are clearly dependent upon buildings for the nesting opportunities that they provide, the nature of the surrounding gardens is much less important. Resident species, such as House Sparrow and Starling, also make use of domestic dwellings for their nesting opportunities but are more closely tied to the nature of the gardens within which these dwellings are located. The presence of shrubby cover is important for House Sparrow, while Starlings appear to favour properties where there is access to nearby areas of short vegetation – such as garden lawns or amenity grassland. Another summer migrant, the Spotted Flycatcher, also appears to be dependent on the nature of the gardens it occupies, seemingly preferring rural or larger suburban gardens with mature trees and an abundance of small flying insects.

A number of bird species use gardens at a particular time of the year, arriving to take advantage of feeding opportunities when conditions elsewhere become less favourable. This is something that we will examine in greater detail in Chapter 2 but it is worth noting here how the early winter arrival of migrant thrushes and finches boosts resident populations and sees birds taking windfall apples and the fruits of berry-producing shrubs. Joining these less obvious visitors (which look the same as year-round residents, such as Blackbird and Chaffinch Fringilla coelebs) are more obvious migrants, including Brambling, Redwing, Fieldfare and Waxwing. Such species tend to forage over large areas, responding to the availability of favoured foods, and so are able to take advantage of the seasonal resources present in many gardens.

COMMUNITY STRUCTURE AND GARDEN TYPES

As the figures produced by Newson et al. (2005) illustrate, the garden bird community contains a significant component of the wider breeding populations of a number of key species. In addition, Newson et al.’s work underlines that many garden species also occur alongside one another within other communities – such as the farmland bird community and the woodland bird community. There may be differences between these communities in terms of species interactions, such that one species does better than another in one habitat but not elsewhere. Such differences in community structure can also be seen from smaller and more focussed studies. Work on tit populations across different UK habitats reveals that Great Tits usually outnumber Blue Tits in woodland populations, often by 2:1 or more. In a suburban population studied by Cowie & Hinsley (1987), the situation was reversed, with Blue Tit outnumbering its larger relative by 3:1 or more. This suggests that Blue Tits might be better suited to the urban environment than Great Tits.

Gardens vary greatly in their size and structure, and consequently in their use by birds. Although we often categorise gardens into urban, suburban and rural, this is a rather simplistic approach and fails to adequately account for the variation that exists both within individual gardens and in the wider habitat framework within which they sit. Within the UK, as much as 7 per cent of land area may be located within towns and cities with a human population in excess of 10,000 people. Some 80 per cent of the UK population lives in these areas, with 40 per cent of the population living within London and our other major conurbations. In an attempt to document the extent and structure of the gardens associated with these conurbations, Alison Loram and colleagues at the University of Sheffield took a detailed look at the cities of Edinburgh, Belfast, Oxford, Cardiff and Leicester (Loram et al., 2007). The researchers surveyed a sample of at least 500 properties from each of the five cities, revealing that 99 per cent had an associated garden. The size of the gardens – whose median areas varied across the different cities, from 96.4 m2 (Belfast) to 213.0 m2 (Edinburgh) – was closely related to the type of housing present; the general pattern revealing that garden size roughly doubles as you move from terrace housing, to semi-detached to detached. Relatively small gardens (<400 m2) were much more numerous within the cities than larger gardens, contributing disproportionately to the total garden area present. This has important consequences for engaging householders in nature conservation through practices such as wildlife-friendly gardening (see Chapter 6), because a small garden might not seem particularly important in a wider context to a householder. I have often heard the phrase ‘what difference can I make; my garden is only small’ but it is important to remember that individual gardens do not exist in isolation; they are instead part of a wider ecological network.

FIG 6. Blue Tits typically outnumber Great Tits in the urban environment, by as much as 3:1, but are the less numerous species within traditional woodland habitats. (John Harding)

Loram’s work also revealed the spatial pattern of gardens within each of the cities, reflecting variation in the density of the human population and the associated densities of housing stock. The history of each city, together with the geography of its location, contributed to the pattern of housing seen; the notion of a simple gradient in garden size, from the leafy suburbs with their large gardens through to courtyard gardens in the urban centre, was disrupted by these and other factors. This also meant that there was no clear relationship between garden size and distance to the edge of the conurbation within any of the five cities studied. This is important when we look at urban bird populations in more general terms as, for example, examined through the work of Chamberlain et al. (2009a) and others.

At a wider spatial scale, it is possible to use data from the BTO/JNCC/RSPB Breeding Bird Survey to examine the densities of bird species across the broader landscape and, as was done by Tratalos et al. (2007), to look at the relationship between avian abundance, species richness and housing densities within urban areas. Tratalos et al. (2007) found that total species richness – and that of 27 urban-indicator species – increased from low to moderate household densities, before then declining as you reached higher household densities. Avian abundance showed a rather different pattern, increasing across a wider range of household densities and then only declining at the highest household densities. The researchers were also able to look at the patterns in abundance seen within individual species, highlighting some interesting differences between them. Importantly, however, most of the species showed a hump-shaped relationship with household density, declining at the highest densities. A somewhat worrying finding, highlighted by the team, was that avian abundance almost invariably began to decline at a point below the density of housing required by the UK government for new developments.

A number of studies have highlighted that within cities it is the suburbs that support the greatest variety of bird species, something that is reflected in the hump-shaped distribution of the relationship between species richness and housing density mentioned above. A widely accepted hypothesis for the shape of this relationship is that the observed pattern is due to the increased habitat heterogeneity present at intermediate levels of urbanisation – i.e. that the suburbs are more diverse than either the more urbanised town centres or the farmland habitats that so often surround our towns and cities. However, it may also be the case that there is a higher availability of food resources in our suburbs than elsewhere (see Chapter 2) and that this, rather than habitat heterogeneity, shapes species richness.

Where a garden is located along the rural to urban gradient may influence the community of garden birds associated with it. This is something that we have been able to examine by using data from the BTO’s Garden BirdWatch project (Chamberlain et al., 2004a). Looking at nearly 13,000 garden sites contributing weekly data on garden birds between 1995 and 2002, we examined the extent to which the occurrence of individual garden bird species at a site was determined by features within the garden itself or by the nature of the surrounding landscape. Garden size was taken into account because we found that, in most cases, species were more likely to occur in large gardens (and larger gardens were more likely to occur in rural habitats and to have high tree and hedge cover). Exceptions to this were House Sparrow, Collared Dove, Black-headed Gull Chroicocephalus ridibundus and Starling, all of which were more likely to occur in small gardens.

FIG 7. Analysis of BTO Garden BirdWatch data revealed that while most bird species were associated with larger gardens, Starling was one of four species found to be more likely to occur in smaller gardens. (Jill Pakenham)

Once we had controlled for the effects of garden size and the provision of food at garden feeding stations, we found that the likelihood of many species occurring in gardens was dependent on the nature of the surrounding local habitat rather than on features within the garden itself. We found that 12 of the 40 species examined were most likely to occur where the habitat outside the garden was rural in nature – these included open-nesting passerines associated with woodland or woodland edge (Wren, Robin, Blackbird, Blackcap Sylvia atricapilla and Chaffinch), hole-nesting (Great Spotted Woodpecker Dendrocopos major, Blue Tit and Coal Tit Periparus ater) or farmland (Pied Wagtail Motacilla alba, Rook Corvus frugilegus, Greenfinch, Goldfinch Carduelis carduelis and Yellowhammer Emberiza citrinella). Just seven species showed a probability of occurrence that was highest in gardens located within urban habitats, these being House Sparrow, Feral Pigeon, Starling, Magpie Pica pica, Black-headed Gull, Woodpigeon and Collared Dove.

The features within a garden may sometimes be more important than those in the surrounding landscape, as work carried out in Hobart, Tasmania, demonstrates. Daniels and Kirkpatrick (2006) examined how garden and wider landscape features influenced the abundance and species richness of bird species in 214 suburban gardens and found that the percentage cover of shrubs had a very important influence on the garden birds present, with the presence of shrubby vegetation favouring small native woodland birds and the New Holland Honeyeater Phylidonyris novaehollandiae in particular. The same study also underlined the importance of native vegetation for the nectar- and fruit-eating bird species native to Tasmania, but did note that such species also made use of non-native plants. In contrast, introduced bird species – such as Blackbird, House Sparrow and Goldfinch – only used non-native vegetation.

A piece of UK work looking more broadly across a single urban area – the city of Bristol – identified three different bird communities: one composed of species associated with broadleaf woodland and/or inland waterbodies, one associated with high-density housing and residential gardens, and one intermediate to these two and associated with low-density housing and variable amounts of woodland (Baker et al., 2010). More detailed examination of these three communities suggests that the diversity of bird species present within urban areas may be dependent on the availability of blocks of natural and semi-natural habitat, with areas dominated solely by residential gardens supporting fewer species. However, despite the lower species diversity of residential gardens, these areas may support particularly high densities of those species that are present. This is something that is reinforced by other work (e.g. McKinney & Lockwood, 1999).

The spatial arrangement and pattern of gardens and other forms of urban green space also has consequences for the extent to which birds use different patches. Connectivity of suitable habitat patches may be less of a problem for birds than it is for less mobile species, but it has still been shown to be of importance. Esteban Fernández-Juricic, for example, highlighted the importance of wooded streets as potential corridors for birds living in urban Madrid (Fernández-Juricic, 2000), while work in North America has revealed that even the movement of urban-adapted species is influenced by the structure and composition of urban habitats (Evans et al., 2017).

FIG 8. The presence of green cover within urban areas is of particular importance to urban birds, shaping both their distribution and their ability to move between different parts of the built environment. (Mike Toms)

While there has been relatively little work examining how features within and around gardens influence the bird species present, rather more work has been done on other components of urban green space (Jokimäki, 1999). Some of the findings from these studies may provide an indication of how similar features may influence garden bird communities. Work on areas of public green space within London has, for example, underlined the importance of patch size, the presence of waterbodies and the provision of areas of scrubby cover and weedy patches (Chamberlain et al., 2004b). Importantly, some of this work has also underlined the importance of gardens in promoting the species richness of the bird communities using other types of urban green space, such as parks (Chamberlain et al., 2004b). Interactions between gardens, urban green space and the wider built environment may be complex, with the non-biotic components of a city (such as building height and architectural style) also a potential influence on the nature of the bird communities present. Work in Paris, for example, has demonstrated that building height can influence the abundance of particular avian guilds, such that the abundance of omnivorous species was found by Vincent Pellissier to be influenced by the interaction between building heterogeneity and the proportion of low and medium-height buildings present (Pellissier et al., 2012). The findings of this work have implications for urban planning.

Although our work examining the BTO Garden BirdWatch dataset (see Chapter 6) underlines the greater importance of features outside of a garden in determining its use, there do appear to be some garden characteristics that shape the extent to which a garden is visited by particular bird species. Several finch species appear to favour those gardens (and garden feeding stations) that are associated with the presence of tall and mature trees (either within the garden or nearby). Greenfinches, Chaffinches and Goldfinches appear to prefer to fly into tall trees before dropping down to garden feeding stations to feed. Similarly, the presence of thick shrubby cover near to feeding stations appears to increase their use by House Sparrows. The presence of berry-producing shrubs and trees will influence whether or not a garden is visited by wintering thrushes or Waxwings, while the presence of a pond appears to be a particularly attractive feature during the summer months for many different garden bird species.

HOW THE URBAN ENVIRONMENT IMPACTS ON BIRDS AND THEIR POPULATIONS

Over recent years there has been an increasing push to understand the finer-scale effects of the urban environment – of which gardens are a significant component – on the ecology, health, behaviour and physiology of birds. Such effects have been studied both at the level of the individual bird and at the level of the population or community. As we shall see in this section, some of these effects can have significant consequences, determining which species thrive in our town and city gardens and which struggle.

Three of the most striking ways in which urban habitats and gardens differ from more natural landscapes are in the availability, predictability and novelty of key resources, most notably food (provided at garden feeding stations) and nesting opportunities (provided through nest boxes). As we will discover in Chapter 2, the presence of food at garden feeding stations can lead to the creation of a more predictable environment (both through time, and spatially across the city or town). This can have profound consequences for urban birds, as work on Northern Cardinal Cardinalis cardinalis populations in Ohio has demonstrated. Rodewald & Arcese (2017) found that female cardinals, breeding in urban habitats within their study area, were more similar in their contribution to the next generation than was the case for those breeding in rural habitats, where a pattern of winners and losers was more evident. Importantly, this difference occurred despite comparable variation in body condition across the habitats studied. The urban cardinal population, then, is more homogenous in terms of its breeding performance than the rural population.

Living within an urban environment is thought to impact on the health of the organisms found there and we know, for example, that urban pollution elevates the levels of oxidative stress (see Chapter 4) seen in humans and birds (Isaksson, 2010). Researchers have measured the levels of oxidative stress in urban populations of study species and compared these with individuals living in other, more natural habitats. Other researchers have looked instead at the levels of particular hormones, again using these to tease out possible health effects. More recently, attention has turned to telomeres; these are the nucleoprotein structures found at the end of chromosomes. Telomeres are thought to promote genome stability and there is good evidence to associate the length of a telomere with lifespan, mortality rate and disease risk. This suggests that telomeres may prove a useful ‘biomarker’ for phenotypic quality and ageing. If the pressures of urban living modify an individual’s oxidative balance, then this may result in the shortening of telomere length and indicate a health effect.

FIG 9. Being raised in an urban environment significantly reduces telomere length in Great Tits. Telomeres are the nucleoprotein structures found at the end of chromosomes and thought to promote genome stability. They have been linked to health and longevity. (Jill Pakenham)

Through an experiment in which nestling Great Tits were cross-fostered, Salmón et al. (2016) found that being raised in an urban environment significantly reduced telomere length compared to that of nestlings raised in a rural environment. However, this finding could not be replicated by a similar study in France (Biard et al., 2017). A larger study, this time of Blackbirds, also found that urban-dwelling individuals had shorter telomeres than those living elsewhere, adding evidence to this growing area of research interest (Ibáñez-Álamo et al., 2018). While Clotilde Biard’s study failed to find differences in telomere length between urban and rural populations of Great Tits in France, the work did reveal differences in chick growth (rural chicks were larger and heavier) and in plumage colouration (the yellow carotenoid-based plumage was more colourful in rural chicks – see also Chapter 2), suggestive of developmental constraints within the urban populations. Measures of ‘damage’ in relation to the impacts of urbanisation have been shown to vary between species with, for example, Salmón et al. (2017) determining that sparrow species show greater overall damage than that seen in tits.

There is the suspicion then, that gardens and the wider urban habitat are sub-optimal for many bird species (though not all). If this is the case, then we might expect to see the presence of three features that have been found to be characteristic of sub-optimal habitats elsewhere. These are:

i) a greater proportion of young breeders and more floating individuals;

ii) less stable populations, whose numbers fluctuate from year to year; and

iii) low breeding density.

The idea that populations living within sub-optimal habitats are more likely to fluctuate from year to year is linked to the notion of ‘source-sink’ dynamics, where populations living within high-quality habitats produce a surplus of young each year, but competition for breeding territories in these high-quality habitats is high, resulting in many young and less dominant individuals moving away to occupy territories in less optimal habitats. This also explains the first characteristic that we mentioned – the greater proportion of young birds in sub-optimal habitats. We certainly see this in suburban tit populations (Cowie & Hinsley, 1987; Junker-Bornholdt & Schmidt, 1999) but it isn’t always the case. Work on farmland Blackbird populations has revealed the presence of greater numbers of young males in this habitat (Hatchwell et al., 1996a), and David Snow – in his classic treatise on urban Blackbirds – speculated that urban populations act as a source population for a rural sink (Snow, 1958). Snow had demonstrated that his urban Blackbirds were generating a significant surplus of youngsters each year, some 1.7 yearlings per pair when only 0.7 yearlings per pair were needed to maintain a stable population.

Urban living may also lead to greater exposure to humans, perhaps resulting in increased tolerance of their presence and reduced levels of ‘fear’. Work on urban Great Tits has highlighted that they are less neophobic and show shorter flight initiation distances (see Chapter 6) than their rural counterparts (Møller et al., 2015; Charmantier et al., 2017). They also appear to have a more proactive coping strategy when dealing with stressful situations (Senar et al., 2017a).

DISEASE AND THE URBAN ENVIRONMENT

Chapter 4 explores the diseases affecting garden birds in detail, but it is important to give a brief overview here of how disease risks differ between urban and rural environments. Comparisons between the diseases of urban and rural populations of birds have yielded mixed results when it comes to seeking general patterns of occurrence and prevalence. Some studies, such as the work of Grégoire et al. (2002), Fokidis et al. (2008) and Evans et al. (2009a), have found reduced parasite loads in urban populations of garden bird species, including Blackbird. Others, such as Giraudeau et al. (2014) have found higher levels of disease in more urban areas. Giraudeau’s work revealed that the severity of coccidian infection in House Finch Haemorhous mexicanus and the prevalence of pox virus were both inversely related to the proportion of undisturbed habitat within the Phoenix metropolitan area. Patterns of disease infection along urban to rural gradients, and across the different habitats within the urban environment, may differ between diseases, in part a reflection of variation in how different diseases are transmitted. In the case of parasites, transmission may require an intermediate host that is absent from the urban environment (Sitko & Zales´ny, 2014), but for other diseases – including those transmitted between individuals through contaminated food – transmission rates may increase in gardens and other urban sites because of the high densities of birds attracted to garden feeding stations (Lawson et al., 2018). The provision of food at garden feeding stations may also influence disease dynamics (Galbraith et al., 2017a).

FIG 10. Garden feeding stations have been implicated in the transmission of disease between different species of garden bird, but the presence of supplementary food also has positive benefits for birds like this Brambling. (Jill Pakenham)

ARE GARDENS IMPORTANT?

As the figures presented within this chapter demonstrate, gardens and the wider built environment support significant proportions of the breeding populations of certain bird species. In addition, gardens and their associated resources may be important for particular birds at other times of the year or during periods when the availability of key resources within the wider environment is at a seasonal low. This underlines that gardens are important, and not just for species flagged as being of conservation concern. How we manage our gardens and the resources they contain has consequences for bird populations and, as we shall see later in this book, may also help to drive evolutionary change. It is important to remember that garden bird populations do not exist in isolation, since birds are well able to move between different habitats or regions. While gardens may not be as suitable as certain other habitats for breeding, they may be better in other ways, at least for some species.

We are, however, far from fully understanding the role that gardens play in a wider context, particularly in relation to source-sink dynamics, and it is certainly too early to be able to quantify the true value (or cost) of garden living for bird populations here in the UK, in Europe or North America, let alone elsewhere in the world. The mobility of birds makes it difficult to follow individuals throughout their full life cycle; in turn, this prevents us from being able to determine why particular individuals use gardens and the consequences or benefits of this use on future events in their lives. For a young Great Tit, raised in a piece of mature deciduous woodland bordering a city’s suburbs, the presence of suitable food in garden bird feeders may enable it to survive its first winter when it would otherwise have died. While this fortunate individual may fail to secure a prime woodland breeding territory the following year, and instead end up making a failed breeding attempt in a garden setting, it may still have the opportunity to occupy a woodland territory in a subsequent year. In the end, this individual’s lifetime reproductive success may still be better for having used a garden and its resources, than would have been the case had it only ever lived within a woodland site.

FIG 11. Gardens across the globe vary greatly in their structure and in the plants they contain, something that can reflect both cultural differences and local conditions. (Mike Toms)

CONCLUDING REMARKS

Before we leave this chapter and turn to look in more detail at particular aspects of the garden environment, it is worth reminding ourselves that the gardens present here in the UK, and across much of western Europe and North America, are often very different from those in other parts of the world. Thinking about gardens in a more global context forces you to move away from the predominantly recreational basis to gardens located within western Europe and North America. Many home gardens elsewhere are very different and provide families with space to engage in food production for subsistence or small-scale marketing. Such gardens may also play an important social or cultural role, perhaps acting as spaces within which knowledge related to agricultural practices can be shared. The management of these spaces creates structures and microclimates that are typically very different from the surrounding countryside; in this respect they can be considered alongside the more familiar urban and suburban gardens of western Europe, even though they look and act very differently (Guarino & Hoogendijk, 2004). It is known, for example, that ‘home gardens’ support high levels of inter- and intra-specific plant diversity, making them important in a global context, but far less is known about the role that they play for wider biodiversity and, in the context of this book, wild birds (Galluzzi et al., 2010). For the most part, however, we will just consider the gardens of western Europe, North America and Australasia in this book, something that also reflects where the greater amount of research has been carried out. The pattern of research worldwide isn’t just linked to particular geographic areas, since it has also been demonstrated that there is a positive and significant association between the degree of urbanisation of a species and how frequently it has been the subject of scientific study (Ibáñez-Álamo et al., 2017).

This chapter has highlighted the fact that gardens come in many different forms and that this has consequences for the communities of birds associated with them. The birds present in our gardens are the species that have, for the most part, adapted to the process of urbanisation to take advantage of the resources and other opportunities that gardens and the wider built environment provide. While some of these species populations are resident within the built environment, others make use of our gardens on a seasonal basis. Most UK gardens are located within highly urbanised landscapes; with urban land cover globally predicted to triple between 2000 and 2030 (United Nations, 2014), we can expect to see future changes in our garden bird communities. Such changes are part of an ongoing process that has altered the distribution of bird species, changed the composition of avian communities, brought about local extinctions and altered behaviour. It is these features and processes that we will examine of the following chapters, starting with feeding opportunities (Chapter 2), then moving on to an examination of breeding behaviour (Chapter 3), disease risk (Chapter 4) and behaviour (Chapter 5).

CHAPTER 2
Foods and Feeding

FEEDING WILD BIRDS IS UNDOUBTEDLY a popular pastime here in the UK and a great many of us put out mixed seed, sunflower hearts and fat- or suet-based products for our garden birds. The addition of such food to the environment represents a substantial supplementation of the resources available to wild birds, yet we still lack a clear understanding of its effects. In a wider research context, we know that food supplementation can increase survival rates (Brittingham & Temple, 1988a), change community and population structure (Galbraith et al., 2017b), alter behaviour (Saggese et al., 2011; Plummer et al., 2015) and impact on wider biodiversity (Orros et al., 2015a). It has also been linked to disease transmission in birds (Lawson et al., 2018) and to health and well-being benefits in people.

Through this chapter we will explore how and why supplementary food is provided to garden birds, how provision varies across countries, cities and cultures, and the consequences that such provision has for the birds that visit our gardens to partake of this significant resource. Understanding why people feed wild birds and the extent to which they appreciate its costs and benefits will be central to our exploration, as will a review of the scale of food provisioning, which is where we will start this chapter.

THE PROVISION OF FOOD FOR WILD BIRDS

It has been estimated that between one-quarter and two-thirds of households across major parts of Europe, North America, Australia and New Zealand provide food for wild birds (Thomas, 2000; Ishigame & Baxter, 2007; United States Fish & Wildlife Service, 2011; Davies et al., 2012). Within the UK, and using figures from the English Housing Survey and other studies, Davies et al. (2009) reported that 48 per cent of households, and 51 per cent of households with a garden, participated in feeding wild birds. Perhaps more interestingly, the study also reported that 28 per cent of households, and 23 per cent of households with a garden, specifically used bird feeders. An earlier and often quoted study by Cowie & Hinsley (1988a) put the figure at 75 per cent, but was based on a small sample of households (in Cardiff) and did not properly account for those questionnaires that were not returned. Work in Reading, Berkshire, using a face-to-face questionnaire approach outside supermarkets, came up with a figure of 55.3 per cent for the proportion feeding wild birds, of which 65 per cent reported that they fed all year round (Orros & Fellowes, 2015b). Marketing work by the RSPB, using two separate telephone surveys of randomly selected groups of 1,000 people and carried out in summer and winter 2004, indicated that 56 to 61 per cent per cent of people over the age of 16 had fed their garden birds during the past year (RSPB, unpublished).

FIG 12. Suet-based foods, such as fat balls, have become very popular with UK householders, their use targeted at small garden birds like tits and Starlings. (John Harding)

A rough calculation by Zoe Davies and colleagues, based on the population sizes of birds known to use seed feeders, suggests that there is at least one bird feeder for every nine potentially feeder-using birds. This does not allow for the fact that many households provide food in more than one feeder, or that many bird feeders are sitting empty at any one point in time. RSPB data, again from their marketing survey, suggest that on a typical summer’s day one in every seven gardens has at least one empty bird feeder. Even so, it does underline the potential scale of food provisioning taking place within the UK – something Davies et al. (2009) have suggested could equate to a standing crop of 2,580 tonnes of bird food. Figures from the Pet Food Manufacturers Association put UK bird food sales at c. 150,000 tonnes annually, representative of an annual consumer spend in excess of £200 million. O’Leary & Jones (2006) suggest that in excess of 500,000 tonnes of food is provided annually across the UK and US combined, while United Nation’s figures from 2005 put the global bird food industry’s value at $5–6 billion, with growth of c. 4 per cent annually since the 1980s (Lin, 2005).

Thanks to the periodic National Survey of Fishing, Hunting, and Wildlife-Associated Recreation, we know that expenditure on wild bird foods within the United States has doubled over a 20-year period, exceeding $4 billion per year by 2011 (United States Fish & Wildlife Service, 1991; 2011). We lack a comparable survey within the UK, and securing commercially sensitive sales information from the wild bird care sector is problematic, but it is thought that the current annual UK spend on wild bird care products (food, feeders and nest boxes) is between £220 million and £500 million (Fuller et al., 2012).

FIG 13. The proportion of BTO Garden BirdWatch participants presenting different foodstuffs at their garden feeding stations each week across four of the project years (2014–17). Note the increase in fat and suet provision during the winter months. ‘Live’ foods include dried mealworms. Data reproduced with permission from BTO.

Information from the weekly submissions made by BTO Garden BirdWatchers provides a measure of the change seen in different types of bird food (see Figure 13) but these data come from a subset of the national population (see Chapter 6) and do not reveal the actual quantities provided. Having said this, as part of BTO’s work on microevolution in Blackcaps (see later in this chapter), we were able to estimate that the total volume of sunflower hearts and fat products provided by BTO Garden BirdWatchers during the winter months more than doubled from 6.35 tonnes to 17.26 tonnes over the course of a 12-year period (Plummer et al., 2015).

WHY DO PEOPLE FEED WILD BIRDS?

While bird feeding is clearly influenced by external factors, such as socioeconomic status and having access to a garden, it is also shaped by intrinsic and motivational factors. At its heart, bird feeding may be seen as a humane act, an act of kindness reflecting a wish to help a fellow creature. However, this act may be further shaped by other motivations, such as guilt or a wish to learn. Despite the obvious commercial advantages to be gained by understanding motivations for feeding wild birds, there has been relatively little work in this area and many of our assumptions about why people provide supplementary food for wild birds remain untested (Jones & Reynolds, 2008). Understanding motivations requires a considered and rigorously scientific approach; it is all too easy to inadvertently bias a response from a study subject by asking a question in the wrong way.

The work that has been done suggests that people feed wild birds for a range of different reasons driven by underlying environmental, cultural and philosophical perceptions (Davies et al., 2012). Many people derive pleasure from feeding wild birds and in many instances the provision of food is simply a reflection of this. Others feed because they are concerned about wild birds (and other creatures) and wish to nurture and support them. Some provide food because of the experiential knowledge that is gained, while others seek to counter the guilt that they feel over wider human impacts on the environment.

Examination of a random sample of 1,000 participants in BTO’s weekly Garden BirdWatch scheme (Schreiber, 2010) found that ‘pleasure’, ‘contributing to the survival of wild birds’ and ‘studying behaviour’ were the top three justifications given by respondents for feeding. ‘Responding to environmental degradation’ and ‘teaching children’ were also given as reasons for feeding. Of course, participants in a citizen science scheme like Garden BirdWatch are unlikely to be representative of the wider UK public; they have sufficient interest in garden birds to have become involved in a rigorous monitoring scheme and will have been exposed to articles delivering information and messages on the practices associated with feeding wild birds. At this point it is just worth noting that the provision of supplementary food is sometimes practised for other reasons; conservation practitioners use supplementary food to enhance the survival and reproductive success of endangered species, while others use it to reduce predation pressure by provisioning predators; some use it to bring animals to sites where they can be observed or even hunted.

FIG 14. Peanuts were one of the first foods provided specifically for wild birds visiting UK gardens. Originally presented loose or in net bags, peanuts are now typically provided in mesh feeders, though their use has fallen with the arrival of new high-energy seeds. (Jill Pakenham)

The provision of food at garden feeding stations, and indeed the wider participation in wildlife-friendly gardening, may also be a response to peer pressures or the development of a shared social ideal, or to the opportunities provided by having the time and/or financial resources required. The ‘luxury effect’ noted by Hope et al. (2003), Kinzig et al. (2005) and others, where wealthier neighbourhoods support greater levels of vegetation cover and have a greater richness of vertebrate taxa, may also apply to the provision of food for wild birds. We’ll return to this as we move through the next two sections.

As well as seeking to understand the motivations for feeding, it is also important to understand any concerns that those feeding wild birds may have about the practice. While some individuals may only see feeding as being beneficial – for their own interest or for the welfare of the birds themselves – others may continue to feed even if they have concerns about particular aspects of the practice. The sample of BTO Garden BirdWatch participants just mentioned were also asked whether they had any concerns about feeding and, if so, what these might be. Just under half of the respondents noted that they did have concerns; those highlighted being the risk of disease transmission, the risk of attracting predators, and the risk of attracting unwanted species to the garden.

THE VARIATION IN FOOD PROVISION

Across northern Europe and North America the feeding of wild birds appears to have once been a winter practice, suggesting that food was provided in recognition of the challenging winter conditions being faced by birds. To some extent feeding remains a winter practice; as data from BTO Garden BirdWatch reveal, the proportion of its participants providing supplementary food shows a strong seasonal pattern with increasing provision during the winter months. Up until the 1980s, the prevailing view was that feeding wild birds during the breeding season would lead to adult birds feeding their young on unsuitable foods and perhaps lead to nestlings choking on ‘indigestible foodstuffs such as peanuts’. Despite this, a pronounced move towards feeding year-round was made during the 1980s, when the advice provided by both the RSPB and BTO switched from winter only to feeding throughout the year.

The switch in BTO’s position was evidence-based and derived from work carried out by Patrick Thompson, who organised a special spring extension to the BTO’s Garden Bird Feeding Survey (Thompson, 1987). The study sought to compare the numbers of feeding birds in the spring with those seen in winter and to determine which species made use of garden feeding stations during this period. Some 181 participants took part in the winter component, with 113 contributing to the spring extension. While fewer species were recorded in the spring component than in the winter, the same species were present in the top 12 in both studies; for some species the level of feeding in spring was as high as, or higher, than that seen in autumn or winter. In House Sparrow, for example, feeding activity peaked in late spring. Other species, e.g. Robin, Blue Tit and Great Tit, used provided food extensively through into the first month of the spring survey, but after this their use of this resource dropped markedly. This drop in use was thought to reflect a shift towards natural prey, which generally become more abundant in spring. That seed-eating species, like Dunnock Prunella modularis and Greenfinch, did not move away from provided food quite so quickly perhaps underlines a lack of seed availability more widely until later into the year.

The second component of Thompson’s study looked at the breeding season use of supplementary food (peanuts) by Blue Tits and Great Tits across 13 gardens at Ashridge Park, Hertfordshire. This population had been studied by the BTO’s Chris Mead since 1977. Chris was a key figure in the development of several projects studying garden birds. The garden-nesting Blue Tits bred significantly earlier than those breeding in nearby woodland and laid larger clutches. There was no difference in the timing of laying between the two Great Tit populations but the garden-breeding birds laid significantly smaller clutches than their woodland counterparts, and there was no significant difference in the fledging success of either species between the two habitats. The feeding habits of the adult tits were also carefully monitored. While Great Tits were very rarely seen to feed their young on peanuts (less than 7 per cent of visits), the Blue Tits were never seen to do this. Importantly, no young tits were reported to have died from ingesting nuts.

FIG 15. While the food available at garden feeding stations appears to be important to young Blue Tits once they have left the nest, it is only rarely provided to nestlings by adult birds. (John Harding)

Elsewhere, both Cowie & Hinsley (1987) and Perrins (1979) have noted a lower fledging success in tits using gardens compared to woodland habitats, with much of this down to chick starvation, despite the fact that supplementary foods are available in this habitat. This underlines the reluctance of adult tits to feed their young on supplementary food. Collectively, the evidence from Thompson’s study and the wider literature convinced BTO to change its advice on year-round feeding, recognising that the extra food may provide a ‘valuable food source to adult birds’. The advice to switch to year-round feeding in Germany was made even more strongly by Peter Berthold and Gabriele Mohr, both respected researchers, whose book Vögel füttern, aber richtig – loosely translated as ‘feeding birds, but right’ – made a scientifically based case for year-round feeding. Although initially criticised in some scientific circles, the book was well received by birdwatchers, shifting 50,000 copies within two years of first publication. Figures from the UK, taken from the RSPB’s marketing study, suggest that winter and summer feeding levels are now fairly similar, at least in terms of the proportion of people providing food (53 per cent summer and 59 per cent winter).

DIFFERENCES BETWEEN INDIVIDUALS

The suspicion that some people are more likely to put out food for birds than others has proved difficult to test, in a large part because of the methodological difficulties in securing a random and representative sample of respondents when carrying out questionnaire-based studies. The scale at which the data are collected can also cause problems for their interpretation. Fuller et al. (2012), for example, found that three different measures of socioeconomic status – household income, the age of householders and the number of individuals comprising the household – were poor predictors of bird feeding behaviour at the national level; however, all three were found to be strongly related to both the presence of bird feeding and the spatial density of bird feeding stations when they just looked at the city of Sheffield. It appeared that by aggregating data at a wider scale, the researchers had averaged away important fine-scale differences. The Sheffield data revealed that bird feeding and the spatial density of bird feeders increased as household income increased but showed an interesting ‘humped’ relationship with the age and number of householders.

Provisioning may also vary between individuals in terms of how often they put out food, the types of foods they provide and where the food is presented. Roughly a quarter of those interviewed by the RSPB put out food daily, with half putting out food at least once a week, and food was presented roughly equally across bird feeders (33 per cent), bird tables (27 per cent) and on the ground (30 per cent). Davies et al. (2012) found that just 29 per cent of the households they studied provided food at least once a week. The work in Reading, mentioned earlier in this chapter, had a more detailed second component; this looked at a subset of people feeding birds within the city, charting what they provided over a two-year period. As well as the finding that a significantly higher proportion of the individuals feeding year round provisioned daily, compared to those who only fed the birds during autumn and winter, the Reading work also collected valuable information on the amount of food being given. This revealed that each day the median amount provided per garden was 127 g, though ranging from 18 g to 3,573 g. Knowing what food was being provisioned, and in what quantities, enabled Orros and Fellowes to calculate a daily energy value of 628 kcal provisioned per garden per day. In addition to variation in the quantities of food provided, there was also variation in the types of food provided, with 91 per cent of individuals provisioning one to three types of food. Mixed seed was the most commonly provisioned food, by both mass and kcal; interestingly, 43 per cent of individuals still provisioned household scraps.

FIG 16. Various members of the crow family, including Carrion Crow, have taken advantage of the feeding opportunities available at garden feeding stations. Work in North America has highlighted how the nature of these opportunities may vary regionally. (John Harding)

Work in the US has also examined the question of who feeds wild birds, revealing that older people are more likely to provide supplementary food than young people (Lepczyk et al., 2012), and are more likely to be women and to have achieved higher educational qualifications than those not participating (Lepczyk et al., 2004a). Mark Goddard, working on why people in Leeds garden for wildlife, also found that the frequency of bird feeding increased with age (Goddard et al., 2013). The RSPB marketing study, which is probably the study that is most representative of the wider UK population, found that interviewees 35 years of age or older were more likely to feed their birds than those under 35 (RSPB, unpublished). Of course, the characteristics of those provisioning wild birds may sometimes confound attempts to examine the relationships between food provision and other features – for example, housing density (something we touched on in the previous chapter).

REGIONAL DIFFERENCES

Cultural differences in attitudes to birds may explain many of the regional differences in the types and quantities of supplementary food provided for garden birds. The biggest markets for wild bird food and feeding products are based in Europe and North America but even within these regions there are differences between countries and peoples. Within Europe, for example, the feeding of wild birds is common in Germany, Poland, Finland, Switzerland, the UK and the Netherlands, but not in France, Spain or indeed other Mediterranean countries. That cultural differences might be behind the patterns seen within Europe could be suggested by the differing attitudes towards the hunting of small wild birds, still a common practice in the south of Europe but not in the north. Could the differences within Europe also relate to geography and the harsher winter weather encountered further north within the continent?

As Lepczyk et al. (2012) found, some of these regional differences in provisioning practices within North America do indeed relate to climatic conditions. Through a series of parallel studies Christopher Lepczyk and colleagues found that householders in Michigan and Arizona had differing attitudes towards the provision of supplementary food for birds; while 66 per cent of respondents in Michigan provided food, just 43 per cent of those in Arizona did so. The authors put this down to the fact that Michigan is a region that experiences severe winter weather, during which birds face harsher environmental conditions, while Arizona has a more favourable winter climate. In addition, Lepczyk found that Michigan residents were more likely to provide high-quality supplementary foods (such as thistle and sunflower seeds) than those living in Arizona. One further pattern of interest can be seen in in Lepczyk’s study: relatively fewer urban residents engaged in the provision of high-quality food than was the case for non-urban residents; they were, instead, more likely to provide food of a lower quality.

FIG 17. Granivorous (seed-eating) species, like Yellowhammer, dominate at UK garden feeding stations, while insect-eating species tend to be less common. (Jill Pakenham)

Similar findings were reported from Poland, where Tryjanowski et al. (2015) found that urban and rural areas differed in the availability of food, offered intentionally or unintentionally to wild birds during the winter months. Both intentional and unintentional food provision was greater within urban areas than it was in rural locations. While the number of bird feeders and feeding stations differed significantly between the two habitats, the proportion of feeders containing food was similar. Tryjanowkski found differences between the two habitats in the availability of several foodstuffs, with seed feeders and waste food significantly more common in the urban area and animal fat more common at rural sites.

There also clear regional differences in the types of food being provided for wild birds at garden feeding stations in different countries. While the main foods provided at garden feeding stations in the UK and elsewhere in Europe are seed-based, in North American backyards we see the additional use of sugar solution feeders for hummingbirds, and in Australia we see many more high-protein foods provided (such as meat and cheese), reflecting the differing range of species more commonly encountered in Australian backyards.

A BRIEF HISTORY OF FOODS AND FEEDERS

Quite where our willingness to feed wild birds originated is unclear but it is likely that food scraps have been provided to wild birds over hundreds of generations. Perhaps the earliest written evidence of material encouraging the feeding of wild birds is that provided in ancient Hindu writings from the Vedic period (c. 1500–500 BCE). This material describes the practice of Bhuta Yajna, which was one of the five great sacrifices used to develop the spiritual growth of a man. Bhuta Yajna involved the placing of food offerings known as bali on the ground; these being intended for ‘animals, birds, insects, wandering outcastes and beings of the invisible worlds’.

Wild bird feeding in the form that we know it today receives very little mention in historical documents and it is not until the 1800s that we see references encouraging or supporting the practice. Allen (1967) refers to the ornithotrophe, a device developed by John Freeman Milward Dovaston – a minor romantic poet and regular contributor of articles to Loudon’s Magazine of Natural History. The device was essentially a modified trough, fitted with rows of perches and filled with household and farmyard scraps. A man with an obvious talent for self-promotion, Dovaston promoted the device and its success in attracting birds through letters to the magazine penned under the pseudonym of Von Osdat. Reference to the device also made an appearance in notes to the preface of the first volume of Bewick’s A History of British Birds. Bird tables were certainly in use by the 1850s, and promotion of feeding of urban birds is mentioned as early as 1875, through a piece that appeared in the 16 February edition of the Edinburgh Evening News. This article noted that a society for feeding birds in winter had established 22 feeding stations across the town, recognising their important role in controlling insect pests.

As Darryl Jones notes (Jones, 2018), perhaps one of the most important events in the development of bird feeding within the UK was the 1908 publication in English of a German book on how to attract and protect wild birds. Written by Martin Hieseman, the book was based on the work of Baron Hans Freiherr von Berlepsch, a nobleman who had spent a considerable amount of time conducting ornithological experiments on his large estate. Included within the publication were several ‘appliances’ that had been developed to provide wild birds with food. Of these, the ‘food bell’ and ‘food house’ would be recognisable today; a somewhat more outlandish appliance – though you can see where the Baron was coming from – was the ‘food tree’. This was a small spruce or fir tree over which a mixture of meat, bread, poppy flour, millet, oats, elderberries and seeds, bound together with beef or mutton fat, was poured. A smaller version of this appliance – the ‘food stick’ – presumably was aimed at those with less opportunity to produce small conifers quite so readily.

FIG 18. The use of coconut shells, filled with fat or suet, is still popular with both garden birds and those who feed them. (Mike Toms)

Over the ensuing decades more publications followed, some of which proved instrumental in shifting us towards today’s pattern of year-round feeding. The growing interest in feeding wild birds was certainly being recognised as a characteristic of the UK population; a 1910 article in Punch magazine identified bird feeding as a national pastime and included a number of adverts for feeding devices. A major development in bird feeding occurred when Droll Yankees’ 1960 A-6F tubular bird feeder was produced. This device supported the delivery of seed, particularly sunflower seed, keeping it dry and secure. More recent changes have largely sought to modify the successful Droll Yankees’ design, for example by looking at the shape and positioning of the feeding ports and their associated perches.

THE TYPES OF FOODS PROVIDED

It wasn’t just the development of feeding devices that proved to be important. In the US, the Kellogg Seed Company, which had been selling birdseed since the end of the First World War, brought a seed mix – the ‘Audubon Society Mixture’ – to market, a mix that had been developed in association with the Audubon Society following a series of experiments to determine which foods were preferred by wild birds. Five years later, ‘Swoop’ Wild Bird Food appeared in the UK and soon after that came new forms of sunflower seed and the tiny black seeds of Niger Guizotia abyssinica. While the main markets for wild birdseed are in Europe and North America, the key production areas lie elsewhere, in eastern Europe (particularly important for sunflower seed), China, India and Myanmar.

The foods provided at garden feeding stations differ in their composition, both in terms of macronutrients like carbohydrate, fat and protein, and micronutrients like calcium, carotenoids and vitamin E. All are important for birds, though in differing amounts and some may be more important at particular times of the year than at others. Protein, for example, plays an important role in female birds as they prepare for the demanding production of a clutch of eggs. When you look at the foods most commonly provided at garden feeding stations here in the UK, you soon discover that they typically have a higher fat content relative to that of protein: black sunflower seeds (fat 44.4 per cent : protein 18.0 per cent), peanuts (44.5 per cent : 28.7 per cent) and peanut cake (70.5 per cent : 17.1 per cent) (Jones, 2018). In addition to their carbohydrate, protein and fat content, foods like peanuts and sunflower seeds may be important sources of micronutrients, such as vitamin E. As will become clear later in this chapter, these micronutrients are not just important for the adult bird; they are also important for eggs and their developing embryos.

Sunflower seeds and sunflower hearts

The sunflower seeds used for bird feeding have a relatively high edible oil content, something that results from several decades of work using selective breeding. The plant itself originates in North America and was initially cultivated by Native Americans, the seed probably arriving in Europe through Spain. Russian agronomists took a great deal of interest in sunflower seeds and began a programme of work selecting for those that were high in oil, successfully delivering an increase in edible oil content from 20 per cent to almost 50 per cent. These high edible oil lines from Russia were reintroduced into North America after the Second World War, rekindling interest in the crop. It is from here that sunflower seeds appear to have entered the bird food market, although eastern Europe and Russia – which account for just under half of the worldwide production – are now the most important source for sunflowers entering the UK market. Worldwide production is about 40 million tonnes.

FIG 19. Sunflower hearts, with their high edible oil content, are very popular with garden birds like Goldfinch. The popularity of these seeds has as much to do with Russian agronomists as it does with American entrepreneurs. (John Harding)

Sunflower seeds and their oil have a range of market uses, with the black-husked oilseed type a staple for bird food; this was introduced into the UK in the 1970s. Sunflower hearts were first introduced to the UK market during the early 2000s, their use proving popular with both wild birds and with the people putting out food. Unlike black sunflower seeds, hearts are already de-husked so leave less mess under bird feeders that then needs to be cleaned up. The lack of a husk also reduces handling times for feeding birds, which makes the hearts energetically more attractive that the traditional black sunflower seeds. At 6,100 kcal per kg, the calorie content of sunflower hearts is better than that of peanuts (5,700 kcal per kg); because the hearts have been de-husked, by weight they are also significantly higher in kcal than black sunflower seeds (5,000 kcal per kg).

Peanuts

Peanuts, also known as monkey nuts or groundnuts, are the edible seeds of Arachis hypogaea, a cultivated leguminous plant originating from a genus that developed in southwest Brazil and northeast Paraguay, where the most ancient species in the genus still grow. Available evidence suggests that Arachis hypogaea itself emerged within the hunter/cultivator communities present in Peru and/or Argentina. Peanut shells dated to 1800–1500 BCE have been excavated from sites near Casma and Bermejo in Peru and in the High Andes of northwest Argentina. There is also evidence of early presence in China, suggesting that mariners from China visited the South American region and returned home with peanuts. Evidence from shipwreck remains off the South American coast adds further support to this hypothesis.

In recent history, and up until the 1960s, peanut production was dominated by countries within the sub-Saharan region of Africa, but this changed rapidly following low yields, change in domestic policies and a reduction in market pull (Pazderka & Emmott, 2010). Over the same period (see Table 2), China became the dominant producer of peanuts, securing 37 per cent of the producer market share by the mid-2000s and benefitting from internal agricultural reforms and the development of its market economy. Production within some countries (e.g. India and Indonesia) is more targeted towards internal markets than export and there has also been an interesting development in the market for value-added prepared peanuts, which is why the Netherlands features so prominently in the export table.

TABLE 2. Peanut production and exportation at a global scale, 2001–07. Data from Pazderka & Emmot (2010).

Peanuts can vary in their quality, both between regions and in relation to local climatic factors. There is, for example, a general acceptance that those produced in China are of better quality than those originating in India because of the higher oil content that they contain. With more than 100 countries cultivating peanuts, of many different varieties, it is easy to see how the wild bird food peanut market now has global reach. As we will discover in Chapter 4, peanuts are not without their problems; the presence of aflatoxin is a major threat to the market and to peanuts destined for wild bird food. In some years and areas, whole crops can be lost because of the presence of aflatoxin – which is toxic to both humans and birds – and this is why, for example, US producers can spend in excess of $27 million annually in order to ensure that their peanuts meet the agreed standards for aflatoxin control.

Peanuts were once a staple at UK garden feeding stations, initially presented in mesh bags, and to some extent still are for many of those who feed wild birds. However, the development of new seed mixes and the popularity of sunflower seeds and hearts with both birds and bird feeders have reduced their prominence. It is not unusual for those providing food to comment that the peanuts in their feeders often go largely untouched, except for visiting Great Spotted Woodpeckers and occasional Nuthatches Sitta europaea.

Seed mixes

A wide range of seed mixes is now available for use in feeding wild birds and these can vary greatly in their composition. In addition to seed-only mixes, some also include fat- or suet-based material or have invertebrate protein or fruit added to them. Watch a Greenfinch feeding at a feeder containing a seed mix and you’ll soon discover that the bird will typically select certain seeds and drop others. This pattern of selection reflects the fact the different seeds and grains vary in both their nutritional content and in the time required to process them. A feeding bird is seemingly able to balance these two components and make an appropriate selection.

Recognising the different nutritional content of different types of seed, and wishing to secure an advantage in what is a highly competitive market, a number of wild bird care companies have signed up to the Birdcare Standards Association, an industry body that is governed by a set of guidelines. The guidelines for wild birdseed (see Table 3) effectively identify certain foodstuffs as being largely unsuitable. These, which include buckwheat and whole oats, sometimes feature prominently in cheaper seed mixes, where they effectively provide a bulking agent, which is little used by the birds. When looking for a seed mix, you often find that you get what you pay for, the better-quality mixes commanding a premium. However, you also need to consider the species that will be feeding on the mix; if, for example, the main recipients will be House Sparrows, then a mix with larger seeds and grains is likely to be better used than one which is made mostly of smaller seeds which the sparrows find difficult and time-consuming to process.

TABLE 3. The Birdcare Standards Association sets the following compulsory standards for seed mixes carrying the Birdcare Standards Association logo.

Niger

The small seeds of Niger were originally used to produce a cooking oil by peoples living within eastern Africa, and cultivation probably first occurred in the Ethiopian highlands. The seed was renamed ‘Nyjer’ in the US, in part to clarify its pronunciation and avoid unfortunate associations with a similar-looking slang word, being registered as a trademark of Wild Bird Feeding Industry, a now well-established US company, in 1998. It is sometimes referred to, incorrectly, as ‘thistle’ seed. As a flower, Niger had certainly been introduced to British gardens by 1806, and the species has been known in the wild since 1876. There is a suggestion from national botanical surveys that its occurrence in the wild is increasing, perhaps because of its use in bird food. As a commercial crop, Niger is mainly grown in India, Ethiopia and – to a lesser extent – Myanmar, underlining the global scale of production that ends up on UK garden centre shelves.

Use as a supplementary food for wild birds came rather later, the seed starting as something of a niche ‘conditioning’ product used by cage bird enthusiasts. It was known to be popular with American Goldfinch Spinus tristis and Pine Siskin Spinus pinus in the US in the 1960s, and it was the association with Goldfinches here in the UK that helped its popularity to increase. The seed’s small size leads to it being favoured by fine-billed species like Goldfinch and Siskin, with larger-billed species finding it too delicate to bother with. The small size also requires the use of a special ‘nyjer feeder’, whose small feeding ports prevent the seed from flowing out of the feeder and onto the ground – which is what happens if you inadvertently put the seed into a standard feeder. Initially, it seems that the provision of Niger seed at garden feeding stations encouraged the arrival of Goldfinches, with some participants in the BTO Garden BirdWatch scheme commenting on how they had never had visiting Goldfinches until they started provisioning the seed. Others, however, failed to attract them to Niger, even where it was provided alongside other foods at garden feeding stations. One of the interesting patterns of Niger use by Goldfinches appears to be the move away to sunflower hearts over recent years; this may be linked to the decline in UK Greenfinch populations following the emergence of finch trichomonosis (see Chapter 4) and the release of Goldfinches from competition with this larger and more dominant species. Niger seed may still be an important food for Siskin and Lesser Redpoll Acanthis cabaret, the latter species now being seen more commonly at UK garden feeding stations.

FIG 20. With their fine bill, Siskins are one of the species to have taken to Niger seeds, though they seem to prefer sunflower hearts if these are available and there is little competition from larger species like Greenfinch. (John Harding)

Fat and suet products

Although highly variable in terms of their content, a high-quality fat product may have in excess of 8,500 kcal per kg, something that makes these products particularly attractive for use during the winter months. Fat- and suet-based products come in a diversity of forms, probably the most familiar of which are ‘fat balls’ and square or rectangular blocks. Fat balls almost always used to be sold within plastic mesh netting, something that was occasionally responsible for the death of a feeding bird, either caught by its foot or by its barbed tongue. Although a substantial number of netted fat balls are still sold (and purchased) within the UK market, there has been a welcome move towards un-netted fat balls.

Suet may also be presented in a pelletised form, something that is popular with Starlings, or in or around objects such as plastic sticks or coconut shells – popular with Blue Tits and Great Tits. Suet products often contain additional material, perhaps added to increase the range of species that will take it, to broaden its nutritional composition or to secure a marketing advantage. These include seeds, nuts, berries and insect protein (typically mealworms). Suet products may also contain ash in variable quantities, added in order to bind the material together and to reduce the chances of the product breaking down. Again, the Bird Care Standards Association has a set of rules relating to this type of product, though these are geared more towards the source of the fat, rather than the composition of the product or content of additional material. The standards are as follows:

i) suet should be derived from animals that have received ante- and post-mortem examination by veterinary officers and found to be fit for human consumption;

ii) all suet should be processed at a fully licensed and approved EU or US abattoir, and

iii) any suet products that are blended with peanuts, either in whole or granular form, must use peanuts that contain a nil detectable aflatoxin level.

Live foods

The term ‘live foods’ refers to the provision of insects, typically mealworms and wax worms, which are bred specifically for this and the wider pet food market. These may be provided alive or, more commonly, in a desiccated form. Mealworms are not worms but larvae of darkling beetles belonging to the genus Tenebrio. Within the UK it is the larvae of the Yellow Mealworm Tenebrio molitor that are most commonly used as food for wild and aviary birds, captive reptiles and amphibians. The smaller sized larvae of the Dark Mealworm Tenebrio obscura are sometimes used. As their name suggests, mealworms have a long association with human beings, occurring as pests of grain and other cereal products. Mealworms are fairly easy to rear, both at home and commercially, and a number of companies are now involved in the large-scale farming of these beetles, producing insect protein not just for wild birds and the pets already mentioned but also as a contribution to cat and dog food products and, looking to the future, human foods (Grau et al., 2017).

FIG 21. Mealworms are often very popular with Starlings, Robins and Blackbirds, with individual birds quickly learning to exploit them where they are offered. (John Harding)

Wild birds seem to prefer live mealworms, presumably because they attract attention by their movements, but probably because they are far closer in their composition to wild caught food. Mealworms have a high protein (13–22 per cent) and fat (9–20 per cent) content and are also a source of polyunsaturated fatty acids, essential amino acids and zinc. Perhaps surprisingly, they have a significantly higher nutritional value than either beef or chicken.

Other foods

Bread appears to be a popular food with householders, perhaps because it provides an opportunity for them to direct unwanted crusts or stale slices towards other creatures perceived to be in need of sustenance. Its nutritional value is, in the context of garden birds, unclear. Bread was by far the commonest food provided by householders who responded to a questionnaire survey carried out in Cardiff in the 1980s (Cowie & Hinsley, 1988a), with c. 90 per cent of respondents provisioning bread in both winter and summer. Galbraith et al. (2014) found bread to be the most commonly provisioned food in their study, estimating that the equivalent of more than five million loaves was put out by New Zealand residents annually.

FIG 22. A small number of UK householders put out meat scraps or even whole carcasses for visiting Red Kites. (Jill Pakenham)

Apart from the small number of householders nationally who provide meat for visiting Red Kites Milvus milvus, only a small proportion of the kitchen scraps fed to wild birds here in the UK include meat. The situation is very different in Australia where the provision of meat is fairly typical. This reflects clear differences between the main groups of birds visiting garden feeding stations here in the UK (seed-eating species) and in Australia (omnivorous species, such as Laughing Kookabura Dacelo novaeguineae, Australian Magpie Gymnorhina tibicen and Grey Butcherbird Cracticus torquatus). As we’ll see later in this chapter, the provision of meat has potential implications for wild bird health.

In parts of North America, sugar solution is used to attract and feed hummingbirds. Typically made from one-part cane sugar to four-parts water, the solution is seen as a supplementary food rather than a replacement for the sugars that these tiny birds would normally secure from flowers. While those hummingbirds tested appear to be more strongly influenced by the position of a food source rather than its colour, many feeders are red in colour because this is perceived as increasing their attractiveness to hummingbirds. Once a food source has been identified, colour may act as an important discriminator stimulus and it may be that individuals learn to associate this colour with these new nectaring opportunities (Miller & Miller, 1971). The increasing use of hummingbird feeders has been linked to the changes in the range of Anna’s Hummingbird Calypte anna, a species that now overwinters and presumably breeds at more northerly latitudes than it did a few decades ago. Examination of Project FeederWatch data (see Chapter 6 for more on this project) demonstrates that more participants in the large-scale study now offer sugar solution in hummingbird feeders than once did, though it is unclear whether the increase in feeder provision has driven the range expansion of this hummingbird species, or the range expansion has led to more people putting out hummingbird feeders (Greig et al., 2017).

THE NATURAL FOODS AVAILABLE WITHIN GARDENS

Invertebrates

Many of the birds making use of garden feeding stations also feed on invertebrates; for some garden birds, such as Wren, it is the insects and spiders found within the garden that are the food of choice. Because species like Wren usually ignore the food provided at bird tables and in hanging feeders, their presence in the garden can be easily overlooked. We don’t know a great deal about how the features present within gardens and across the wider urban environment shape the community of invertebrates that is found there. While there are some obvious relationships, others are masked by the complexity of the many microhabitats present and the influence of components within the wider landscape. Work carried out as part of the Biodiversity in Urban Gardens project (BUGS) underlines the difficulties in seeking to identify relationships between invertebrate abundance and particular features (Smith et al., 2006a).

The plant species growing within gardens will have a shaping influence on many of the invertebrate species present, not least because the plants provide many of the feeding opportunities that such invertebrates require. A typical garden flora will almost certainly comprise a mixture of native and non-native species, and the latter may prove unsuitable for insects because of their chemical composition or structural characteristics. Things may not be as bad as you might assume, however, since the chemistry of individual plant species tends to be fairly similar within a family and many of the non-native plants introduced into UK gardens belong to families that also contain a native component. The BUGS project revealed that of the 1,166 plant species recorded in 61 Sheffield gardens, 70 per cent were non-native; however, at the family level, just 36 per cent of the families were alien in origin (Smith et al., 2006b).

FIG 23. While some hoverfly species avoid being predated by mimicking less palatable species, other garden-visiting flies may be taken by Spotted Flycatchers and other bird species. (Mike Toms)

Despite this, there is evidence from work carried out in the suburban landscapes of southeastern Pennsylvania, US, that native planting can be better for both invertebrates and the birds that feed on them (Burghardt et al., 2008). Karin Burghardt and colleagues looked at a number of avian and lepidopteran community measures across paired suburban properties, one property in each pair being planted with entirely native plants and the other with a conventional suburban mix of natives and non-natives. The native-planted properties were found to support significantly more caterpillars and range of caterpillar species and had a significantly greater abundance, species richness and diversity of birds; they also had more breeding pairs of native bird species.

The structure of plants can be an issue where the cultivars presented in garden centres and plant catalogues are divergent from native forms. For example, the ‘double cultivars’ of certain flower forms, which prove popular with gardeners because of their extended flowering season and novel appearance, are less suitable for nectar-feeding insects and they also set less seed. If flower selection and form reduces opportunities for invertebrates within the garden, then it will also reduce opportunities for insect-eating birds. One consequence of this can be seen in the generally lower productivity of tit species within the urban environment. Great Tits and Blue Tits feed their young on small caterpillars and appear to struggle to find enough of these in many urban and suburban gardens (Cowie & Hinsley, 1987).

FIG 24. The larvae of craneflies and other soil-dwelling invertebrates taken from garden lawns are important for Starlings and other species. (Jill Pakenham)

It is also important to recognise the widespread use of insecticides and related compounds in gardens, since these have the potential to both reduce prey availability and to enter the food chain, where problems may then occur. More widely within the built environment there is the risk from pollution, such as with heavy metals (see Chapter 4), which can also impact on the availability of invertebrates and alter the composition of the prey communities available to foraging birds (Eeva et al., 2005).

Fruits, seeds and nectar

Garden plants can also provide food for visiting birds more directly, through their fruits and seeds; in fact, many plants rely on birds to act as dispersal agents for their seeds, offering a nutritious fleshy fruit to attract the bird to take the seed. For example, the natural foods taken by garden-wintering Blackcaps include the berries of Cotoneaster, Honeysuckle Lonicera, Holly Ilex aquifolium, Mistletoe Viscum album and Sea-buckthorn Hippophae rhamnoides. Blackcaps have also been reported feeding on the seeds of Daisy Bush Brachyglottis greyi (Hardy, 1978). The different fruits and seeds become available at different times of the year, though predominantly from October through to January, and this can alter the shape of the bird community visiting gardens. The movement into gardens of wintering Redwing, Fieldfare and Waxwing may reflect the availability of favoured berries in gardens and their scarcity elsewhere. It has been noted, for example, that the timing of hedgerow cutting within the UK’s arable landscapes may remove large quantities of the berry standing crop from the wider landscape, leaving gardens as an important resource (Croxton & Sparks, 2004).

Some of these berries are favoured over others and in some cases a long fruiting season, such as in Holly, can suggest that the berry isn’t particularly favoured by visiting birds, only being eaten once other options are no longer available. In addition to the differences seen between different plant species, the nutritional characteristics of individual fruits may also vary with season. In many berries, the water content of the pulp decreases as the season progresses, while the average lipid content increases. Berry colour may be used as a signal, alerting berry-eating birds to the reward on offer, and there is evidence that birds may select fruits of a particular colour because of their nutritional value. Some bird species appear to select fruits with a high anthocyanin content; anthocyanin is an antioxidant and berries rich in this pigment tend to be black in colour or ultraviolet reflecting. Work on Blackcaps indicates that individuals actively select for anthocyanins in their diet and that they use fruit colour as an honest signal of anthocyanin rewards when foraging (Schaefer et al., 2007).

The consequences of the variation seen in fruits leads to species-specific preferences within the bird species that feed upon them. A series of studies by David and Barbara Snow has highlighted some of these preferences. Mistle Thrush, for example, was found to favour sloes over haws, while the preference was reversed in both Redwing and Fieldfare. Song Thrush Turdus philomelos showed a clear preference for Yew Taxus baccata, Elder Sambucus nigra and Guelder Rose Viburnum opulus, and the apparent avoidance of rosehips, while Blackbird was found to be fairly catholic in its tastes (Snow & Snow, 1988). Blackcaps make use of smaller berries in gardens, such as those of Cotoneaster conspicua (Fitzpatrick, 1996a), reflecting their almost entirely frugivorous diet (when fruit is available) within the Mediterranean wintering area (Jordano & Herrera, 1981), although it is interesting to note the importance of supplementary foods presented at garden feeding stations highlighted by Plummer et al. (2015) and explored later in this chapter.

FIG 25. Berry-producing garden shrubs can be very popular with Mistle Thrushes and other visiting thrushes, together with Blackcaps and Waxwings. (John Harding)

The presence of non-native berry-producing shrubs in gardens, while potentially a valuable food resource for visiting birds, brings with it a possible conservation issue. Since birds are the main dispersal agent for the seeds held within berries, they are a potential route by which non-native species may become more widely established within the wider countryside (Greenberg & Walter, 2010).

It is not just the berries that birds seek; some, such as Greenfinch, are after the seeds themselves. This can prompt plants to incorporate toxic compounds into the seed coat or its lining. Although it is the absence within the wider countryside of the seeds of larger shrubs and trees that can drive birds to garden feeding stations (see later in this chapter), it is important to note that some of our garden birds specialise in the seeds of smaller plants. The Goldfinch, for example, specialises in seeds of the Compositae family, particularly the thistles and dandelions and it is this habit that probably first brought them into gardens to feed on ornamental thistles, teasel, lavender and cornflower (Glue, 1996). Maddock (1988) reports how, from the winter of 1983/84, a few Goldfinches would visit a suburban Oxfordshire garden to feed on lavender and dry teasel heads. Their interest was maintained by brushing the teasel heads with Niger. Over the following winter, up to two dozen Goldfinches again visited the teasel head before turning to a mix of Niger, canary seed and millet provided at a garden feeding station. Important berry- and seed-producing plants for birds are shown in Table 4.

TABLE 4. Suitable plants for providing berries and seeds for garden birds. Adapted from Toms et al., (2008).

On occasion, garden birds have been reported stealing nectar from flowering plants, the latter typically exotic species whose flowers show features used to attract birds as pollinators (Búrquez, 1989; Proctor et al., 1996). Reports from within the UK have included Blue Tit – feeding from Crown Imperial Fritillaria imperialis (Thompson et al., 1996); Blackcap – feeding from Mahonia (Harrup, 1998); and Blackcap feeding from Kniphofia. In addition, the behaviour is widely recognised in a number of the warbler species migrating through the Mediterranean region (Cecere et al., 2011). Nectar feeding has also been recorded for a number of European plants (see the review by Ford, 1985), including those in the genera Rhamnus, Ferula, Acer, Crataegus, Ribes and Salix.

Nectar feeding is thought to be a widespread behaviour in UK Blue Tits, with the species recorded feeding from a variety of flowers across 33 counties within the UK (Fitzpatrick, 1994). There has also been a small amount of more detailed work, examining the contribution that nectar taken from Flowering Currant Ribes sanguineum makes to Blue Tit spring diet (Fitzpatrick, 1994). This revealed that although the nectar was not the preferred food – it was used most when peanuts were unavailable because of competition from other birds – it was a highly profitable food source, contributing up to 50 per cent of the average daily metabolic rates of the Blue Tits studied. The use of nectar sources by suburban populations of predominantly nectar-feeding birds has been examined in Australia, with a view to understanding the contribution that native and exotic shrubs make towards the available nectar resource (French et al., 2005).

FOOD SELECTION BY GARDEN BIRDS

The contribution that food provisioned at garden feeding stations makes to the energetic and nutritional requirements of garden birds varies between seasons and between species. It also varies between the different types of food provided, with some higher in fat or protein content than others, and with differences in the amount of time required to ‘handle’ and process the food. It is difficult to study the feeding preferences of birds out in the field, particularly in a garden setting where individual birds can find alternative feeding opportunities nearby. A small amount of work has been done here in the UK with, for example, Greenwood & Clarke (1991) examining the selection of black-striped sunflower seeds and peanuts – peanuts were the preferred food taken.

One of the biggest studies to have been carried out was that undertaken by Geis (1980) in the United States. The study, carried out between November 1977 and July 1979, made use of a network of volunteers, observing and recording feeding preferences at specially designed feeding stations, capturing details of 179,000 feeding visits. Geis adopted a standard approach, whereby the food types being tested were compared against two ‘standard’ foods commonly provided by householders: black-striped sunflower seed and white proso millet. The results of the work underlined a general pattern of preferences across the bird species visiting and also highlighted the individual preferences of particular species. American Goldfinch, for example, favoured hulled sunflower seeds over Niger, with Niger favoured over oil-type sunflower seeds, black-striped sunflower seeds and white proso millet. In contrast, House Sparrows favoured white proso millet but would feed on almost anything, with the exception of flax and rape seed. Selection of Niger by fine-billed finches is something that has been documented in other studies (Horn et al., 2014).

FIG 26. Large-billed species like House Sparrow are more likely to take larger seeds from a seed mix, the smallest seeds being difficult for them to handle. (John Harding)

The abilities of wild birds to select foodstuffs based on their nutritional characteristics is, as we have just seen in relation to fruits and berries, evident from observational studies; it is also evident from experimental work, though this is more limited. Work on Common Myna Acridotheres tristis has, for example, revealed that urban populations display a strong preference for food with a high protein content, favouring this over foods with high lipid and high carbohydrate contents in field-based trials (Machovsky-Capuska et al., 2016). This work suggested that these urban Common Mynas were protein limited, something reinforced by the way in which the birds competed for access to this food.

Two reportedly individually identifiable Blue Tits, visiting a bird feeder in a suburban Belfast garden during winter, were estimated to have obtained 25.36 per cent and 44.47 per cent of their daily energy requirements from peanuts (Fitzpatrick, 1995); comparable figures from the same study for two Great Tits were 16.41 per cent and 22.2 per cent. Although this study doesn’t adequately deal with questions over how the birds could be recognised as individuals from their plumage characteristics, the work has been used to suggest that these suburban individuals were obtaining a significant component of their daily energy requirement from the food supplied. Better-documented studies, this time on provisioned Black-capped Chickadees Poecile atricapillus in Alaska, found that individuals were obtaining up to 29 per cent of their daily energy requirements from the food supplied (Brittingham & Temple, 1992a; Karasov et al., 1992).

Selection for particular food items might also be shaped by how that food is presented. Garden bird species that would feed within trees and shrubs when foraging in the wider countryside – such as the tits – were some of the first to take advantage of seeds and peanuts presented in hanging feeders and mesh cages. The strong feet of these species enable them to grip hold of small perches or mesh, something that ground-feeding species like Robin, Dunnock and Blackbird would find too challenging. These latter species would be more likely to feed from a bird table or to take food from the ground. Changes in feeder design, most notably in the shape and size of feeder perches, can exert a big influence on which birds are then able to feed. The move from a straight perch to an ‘o’ shaped perch seems to have aided Robins to feed from hanging feeders. The advice to those feeding garden birds is to feed a range of foods in a number of different ways, providing opportunities for the different species and their favoured means and locations for taking food.

FIG 27. Like Robin and Wren, the Dunnock prefers to feed on the ground but it will venture onto bird tables and, occasionally, onto hanging feeders. (John Harding)

HOW AND WHEN BIRDS USE GARDEN FEEDERS

As we will see in the following section, the use of garden feeding stations is influenced by food availability over wider areas, from that available in the nearby countryside to that available many hundreds of miles away in other countries. Before we turn to look at the use of gardens in this wider context, we will start by looking at how the location of feeding opportunities within a garden can influence its use. The foraging behaviour of small birds is influenced by the conflicting demands of needing to feed but also to avoid predators; during the breeding season, the need to maintain a territory and attract a mate can, for many individuals, also be added to these demands (Lima & Dill, 1990). These considerations are likely to influence key decisions about where and when to feed, and we know, for example, that the pattern of arrivals and departures throughout the day at garden feeding stations is shaped by the balance of risk between starvation and predation.

Juvenile Blue Tits and Great Tits are known to be subordinate to older individuals and have been shown to make greater use of bird feeders located further from cover (and with a higher risk of Sparrowhawk Accipiter nisus predation) than adult birds. Hinsley et al. (1995) reported that 40 per cent of the tits they trapped at a feeder located close to cover were adults, with this figure dropping to 17 per cent at a feeder located further away. Another garden-based study found a similar pattern, this time looking at a number of garden bird species (Cowie & Simons, 1991). Cowie and Simons found that total food consumption declined with distance from cover in their experimental study using feeders positioned at different distances from vegetation. The House Sparrows in their study only used the feeder closest to cover, with Blue Tit only slightly more willing to feed further out into the garden. Only Greenfinch seemed comfortable feeding on the more exposed feeders; this species tended to arrive to feed in sizeable flocks and it seems likely that this flocking behaviour increased the chances that a potential predator would be spotted early enough for the birds to make their escape. Interestingly, the proportion of time that individual birds spent manipulating food items decreased in favour of increased vigilance when birds were feeding further from cover, again underlining the need to balance the energetic and nutritional returns against the risk of predation. Krebs (1980), looking at handling times in Great Tits, found that food-deprived individuals demonstrated shorter food-handling times than satiated individuals, presumably because they spent less time scanning for predators in order to take on board as much food as possible. A study by Suhonen (1993) has also demonstrated that foraging site selection in tits – this time mixed flocks of Crested Tit Lophophanes cristatus and Willow Tit Poecile montana – is influenced by a combination of the dominance relationships present and the perceived risk of predation by Pygmy Owls Glaucidium passerinum.

Most small birds will rush to the available cover if a predator is detected nearby; at some point they must then make a decision to leave the safety that the cover provides in order to resume feeding. Several pieces of work demonstrate that this return to feeding is influenced by the age and status of the individuals involved. De Laet (1985), Hegner (1985) and Hogstad (1988) found that dominant Great Tits, Blue Tits and Willow Tits (respectively) typically would not leave the available cover until subordinate individuals had first done so. Some individuals will make use of the available cover more directly, for example by carrying food items from feeders into nearby cover for consumption. Research has demonstrated that the chances of a bird doing this increase as the distance to cover decreases, the size of the food item increases or if the individual is less dominant than other individuals that are also using the feeding stations. In the UK, Coal Tits will often remove food from feeding stations to consume elsewhere, presumably because they are some way down the pecking order.

FIG 28. The small size of the Coal Tit, and its low ranking within the garden bird dominance hierarchy, may be one reason why this species often takes a seed from a garden feeding station to eat it in a nearby bush or shrub. It may also cache seed for use at a later date. (John Harding)

The balance of energetic demands, predation risk and competition from other individuals also shapes the time at which birds feed over the course of a day. We are all familiar with the dawn peak in feeding activity seen in garden birds here in the UK. However, this is not the only peak in activity seen at garden feeding stations, since many observers have reported a second peak towards late morning and a third just before dusk. Fitzpatrick (1997) examined these temporal patterns in feeder use by studying the birds visiting feeders in a Belfast garden. Her work, carried out during the winter months, revealed the patterns of activity at garden feeders for House Sparrow, Blue Tit and Great Tit. Although there were some differences between the three species, the broad patterns underlined the three peaks reported elsewhere.

The use of garden feeders may also be influenced by which other species are present, perhaps mediated through dominance hierarchies (see Chapter 6), or because of individual behaviour. The use of camera traps and PIT tags (which allow the automated identification of individual birds) has enabled researchers to look at some of the factors that may modify the ways in which feeding stations are accessed. Galbraith et al. (2017b), for example, have been able to establish if individual birds are consistent in their use of supplementary food over time or whether use might vary seasonally. Working in New Zealand, the researchers found that although House Sparrows were numerous at monitored feeding stations, individuals were highly variable in their feeder use. In contrast, feeding station use by visiting Spotted Doves Streptopelia chinensis tended to be based around a core group of individuals who were highly consistent in their behaviour. Work carried out in the UK (Crates et al., 2016; Jack, 2016) has also reported variation between both species and individuals in their use of feeding stations.

THE USE OF GARDEN FEEDING STATIONS AND GARDEN TYPE

The extent to which garden feeding stations are used is shaped first and foremost by where the garden is located; put up a peanut feeder in a garden in the Scottish Highlands and you might just attract Crested Tit; place the same feeder in a garden in suburban Surrey and you won’t, but you might attract Ring-necked Parakeet. In addition to geography, the nature of the surrounding habitat will play its part. A rural garden in the east of England, surrounded by farmland, might receive visiting Yellowhammer and Reed Bunting Emberiza schoeniclus late in the winter when farmland seed supplies are very low, while one located next to a conifer plantation will be visited by Siskin and Coal Tit. We might therefore expect some general patterns to arise, indicating the likely garden bird community in gardens of different types and located within different landscapes. This is something that we have been able to examine by using data from BTO Garden BirdWatch (see Chapter 6 for more on this project). This work looked at the probability of occurrence in Garden BirdWatch gardens for 40 species (Chamberlain et al., 2004a). There were 26 species that showed a significant association with the habitat character outside of the garden and in most cases the probability of occurrence was higher in gardens located within rural habitats. For several species the probability of occurrence was very low in all but the most rural habitats (Pied Wagtail, Rook, Siskin – winter only – and Yellowhammer). Interestingly, there were some species that were more likely to occur in gardens located within more urbanised habitats, including: Black-headed Gull (medium and large gardens), Feral Pigeon, Magpie (large gardens), Starling and House Sparrow. Several species showed obvious peaks in their occurrence at intermediate scores, suggesting an association with suburban gardens; these included Coal Tit, Long-tailed Tit Aegithalos caudatus, Nuthatch, Treecreeper Certhia familiaris, Jay Garrulus glandarius, Magpie (medium gardens) and Bullfinch Pyrrhula pyrrhula.

FIG 29. Black-headed Gulls are occasional visitors to garden feeding stations, rarely seen in summer when they have their characteristic chocolate brown heads, but sometimes forced to turn to gardens during periods of poor winter weather. (John Harding)

THE USE OF GARDEN FEEDERS IN RELATION TO WIDER AVAILABILITY OF FOOD

As we have just touched on, the use of garden feeders by seed-eating birds is likely to be influenced by the availability of favoured seeds within the wider environment. This is likely to be particularly relevant in relation to tree seeds, since many tree species produce very large numbers of seeds in some years but not others – a process known as masting. In mast years, the very large volume of seeds produced is thought to saturate seed predators with an over-abundance of food, leaving some seeds to germinate untouched. Since the seed crop produced the following year is typically very much smaller, the populations of seed predators are prevented from growing sharply, leaving the balance in favour of the trees when they next come to produce a bumper crop. A number of studies have shown that the population size and breeding performance of various bird species is linked to these masting events. But can such masting events influence the extent to which seed-eating birds make use of garden feeding stations?

This is something that has been investigated by using the garden bird datasets collected by the BTO through the Garden Bird Feeding Survey (for Beech Fagus sylvatica) and Garden BirdWatch (for Sitka Spruce Picea sitchensis). Chamberlain et al. (2007a) found that the probability of occurrence at garden feeding stations for seven beechmast-eating species (Great Spotted Woodpecker, Woodpigeon, Great Tit, Coal Tit, Nuthatch, Jay and Chaffinch) was significantly lower in those years when a masting event occurred. A similar pattern was found for Siskin and Coal Tit in relation to Sitka Spruce, a species known to produce mast crops at intervals of three to five years (McKenzie et al., 2007). The seed and cone crops produced by these trees are synchronised, within a species, over very large areas, which is why such patterns can be seen very clearly in the BTO datasets. As we will see in the final chapter of this book, Sitka Spruce has been of particular importance to Coal Tit and Siskin populations within the UK, with its small seeds (relative to those of other conifers) proving particularly valuable. The availability of conifer seed to small birds may also be shaped by weather conditions, since it is known that conifer cones open to release their seed on dry days but remain firmly closed when it is wet (Harris, 1969). This may explain the apparent increase in bird-feeder use witnessed in my own garden on damp days during late winter, when Siskin numbers generally peak within gardens.

FIG 30. Coal Tits make greater use of garden feeding stations in those winters when the Sitka Spruce seed crop is poor. Data reproduced from BTO Garden BirdWatch with permission.

There is also evidence of a movement into (largely) rural gardens during the late winter period, with farmland buntings and finches showing their annual peak in garden use at this time according to data from BTO Garden BirdWatch (see Figure 31). The late winter period is known to be a difficult time for seed-eating birds using farmland habitats, the loss of overwinter stubbles leaving a ‘hungry gap’ in food availability (Siriwardena et al., 2008), something that may prompt the birds to aggregate in patches of habitat where seeds can still be found (Gillings et al., 2008) or to move out of farmland and into other habitats (Gillings & Beaven, 2004). Examination of data from the Winter Farmland Bird Survey (Gillings & Beaven, 2004), alongside that from BTO Garden BirdWatch for the same weeks, is suggestive of a movement out of farmland and into gardens for Goldfinch and potentially other species (Figure 32). Further supporting evidence comes from the long-running Garden Bird Feeding Survey. This shows an increase in the use of gardens by Reed Bunting and Yellowhammer during the period of greatest agricultural intensification (Chamberlain et al., 2003; 2005); although not supported by formal statistical tests, it could reflect increased use of garden foods because of a decline in seed availability within wider farming landscapes.

FIG 31. The use of garden feeding stations by Yellowhammers peaks during the second half of the winter, a period during which farmland seed supplies are at their lowest. Data reproduced from BTO Garden BirdWatch with permission.

Work carried out in France supports the value of rural gardens for farmland birds during the winter months. Using data from the French Garden Birdwatch scheme, Pierret & Jiguet (2018) looked at garden use by farmland bird species along a gradient denoting agricultural intensification. Garden feeders located within intensively cultivated landscapes were found to attract more birds, with this relationship strongest for truly farmland species. The pattern became more pronounced as the winter progressed, suggesting that as farmland seed supplies become depleted, so farmland birds increasingly turn to gardens.

One scarce farmland bird for which a number of rural gardens may be of particular importance within the UK is the Cirl Bunting Emberiza cirlus, whose small Devonshire population makes regular use of garden feeding stations in the south of the county during the winter months. Of course, to fully understand the role that gardens play in the ecology of bird communities breeding within farmland, we need to be able to track their individual movements, something that is becoming increasingly possible through the use of emerging technologies, such as PIT tags. The pull of garden feeding stations has wider implications for those involved in survey work, particularly during the winter months. Ask any volunteer who has participated in bird atlasing during the winter months, and found themselves in an area of open country, and they will tell you how it is the rural gardens and their feeding stations that are the place to look for small birds. This effect has been studied experimentally through work carried out in Maine, US, where researchers were testing a bird survey technique widely used in the breeding season and involving an observer making a series of stops along a linear and driven route. At each stop the observer would carry out a ‘point count’ noting the bird species recorded during a fixed time interval before moving on to the next stop. By examining the densities of birds recorded at stops with and without a feeding station, the researchers found that for five species densities were higher at stops with feeding stations. As well as this, presence of the food appeared to influence the birds’ habitat selection, pulling them into edge habitats.

FIG 32. Data from two BTO surveys suggest a movement into gardens by Goldfinches as the winter progresses, something that could be studied in greater detail through the use of ringing or tagging technologies. Data reproduced from BTO with permission.

IS THE PROVISION OF SUPPLEMENTARY FOOD FOR WILD BIRDS A GOOD THING?

Little work has been done to examine differences in the potential costs or benefits of different types of food – at its simplest, high-quality or low-quality foods. We are not yet in a position to be able to determine whether it is better to encourage people to feed wild birds, regardless of the type of food that they provide, because it fosters a sense of connection with the natural world, or discourage the provision of feeding altogether if we cannot provide foods that are entirely suitable for the species that we are trying to help. We provide supplementary food and garden for wildlife more generally because we see these activities as being beneficial for wildlife. However, while these resource subsidies and habitat modifications may provide opportunities for birds and other wildlife within our increasingly urbanised landscapes, they may also fundamentally alter ecological processes and community structure (Shochat, 2004). Understanding these potential impacts requires new research.

Later in this chapter we will look in more detail at some of the costs and benefits for birds of feeding on food provided at garden feeding stations (or experimentally in other habitats). One study, however, is worth mentioning here since it nicely summarises some of these costs and benefits. The study in question was carried out over three years and examined how bird feeding impacts bird health – something that involved looking at body condition, stress, antioxidant levels, nutritional condition, immune function and levels of disease – for wild bird communities in two forested sites, one where food was provisioned and one where it wasn’t (Wilcoxen et al., 2015). Wilcoxen’s study revealed some clear and beneficial effects; birds at the fed site had significantly larger fat stores, greater antioxidant capacity and reduced stress. In addition, fed birds also showed better body condition and better nutritional condition, though not in every year. However, fed birds were more likely to be suffering from disease. Ten months after the feeding experiment had ended, the researchers repeated the sampling carried out at the two sites; this time the differences had gone, adding further supporting evidence that the differences had been due to the provision of food. Feeding does, it seems, deliver some health benefits, but these come with a certain level of cost.

DOES PROVIDING FOOD ENCOURAGE DEPENDENCY?

One of the most common criticisms of the practice of providing food for wild birds is that it causes dependency, prompting questions as to what happens to birds if feeding stops, and are individuals that use feeders regularly able to then recognise natural foods or have the skills needed to exploit them? Little work has been done on the question of dependency, in part a reflection of the often quoted work done by Brittingham & Temple (1992b) – see below – and in part a reflection of knowledge gained on how small birds track different feeding opportunities.

Brittingham and Temple recognised that two different types of dependency may occur. Over the short term, birds visiting a feeder may come to expect that particular location to provide food; if the feeder is left empty or is removed, then the bird may suffer. Over the longer term, those individuals that are heavy users of feeders may lose the ability to recognise naturally occurring food items, lose the skills to handle them and suffer from reduced survival rates. Working on two resident populations of Black-capped Chickadee, Brittingham and Temple tested whether the population that had utilised feeders in the past experienced lower survival rates than a population that had never been exposed to them during a winter when feeders were not available to either population. The researchers found no difference between the average monthly survival rates of the two populations and concluded that there was no evidence for dependency in this species.

FIG 33. The question of whether wild birds become dependent on garden feeding stations has been largely answered by scientific studies; these indicate that small birds are aware of other feeding opportunities and will quickly turn to these if food is no longer available at a given site. (John Harding)

The question of whether the sudden cessation of food provision can have a negative effect on individual birds has not been tested but is not thought to be a particular issue. The thinking that supports this view comes from the work on chickadees, which demonstrates that individual birds track a number of food patches at the same time, sampling these no matter how abundant food is at any one patch (Brittingham & Temple, 1992b). For a chickadee living in the northern part of North America – and being one of the smallest bird species to remain so far north during the winter months – tracking a range of food patches has clear survival benefits; if conditions change and one patch is no longer available, individuals already know where other patches are located. The sudden loss of a feeding opportunity might be more of an issue for other species, but one would expect the urban environment and its gardens to provide multiple feeding opportunities within a short distance of each other. This is something that has been examined, to a small degree, through the work of Patrick Thompson, who looked at the effect of interrupting regular food provision in gardens on the numbers and behaviour of House Sparrows and Greenfinches (Thompson, 1989). Thompson’s work revealed that the daily pattern of feeding station use was unaffected by the number of neighbouring gardens providing food. However, the numbers of individuals using the experimental garden feeding station was dependent on food provision in neighbouring gardens. When the number of neighbouring gardens providing food was reduced, there was an increase in activity within the experimental garden, where feeding had continued.

THE VALUE OF GARDEN FEEDING STATIONS FOR WILD BIRDS

Many of the individuals providing food for birds at their garden feeding stations during the winter months do so because they believe that it will help the birds through a difficult period of the year. The provision of food might improve the survival rates of fed individuals, but to what degree. Work on Black-capped Chickadees in North America has revealed that supplementary feeding can make a significant difference to overwinter survival. The fed chickadees studied by Brittingham & Temple (1988a) were significantly heavier than unfed birds and, while this difference in weight was very small (equivalent to just 1 per cent of body weight and representing two hours of foraging time), it made a big difference to their longer-term survival rates. Fed individuals had a 70 per cent survival rate through to the end of the winter and beyond, compared with just 30 per cent in unfed birds. Work on Willow Tits and Crested Tits, carried out in coniferous woodland near Gothenburg in Sweden by Jansson et al. (1981), also found that the provision of additional food resource improved winter survival significantly. In this case, the three researchers noted that the provisioned food helped to reduce predation risk by allowing individuals to spend more time in far more protected places than they would frequent under natural food conditions.

FOOD PROVISION AND MEASURES OF REPRODUCTIVE SUCCESS

Various studies have demonstrated that increasing the availability of food resources through the provision of supplementary food can advance the onset of breeding, prolong the period over which breeding takes place and increase both the number of young raised per breeding attempt and the number of breeding attempts made. Work on Florida Scrub-jays Aphelocoma coerulescens (Fleischer et al., 2003; Schoech et al., 2004) has, for example, revealed that populations with access to supplementary food can start breeding up to 17 days earlier than populations without such access. Similar findings have been revealed for Australian Magpie (Rollinson & Jones, 2002).

The timing of breeding

The work on Florida Scrub-jays reveals both the differences that supplementary food provision can make to the timing of breeding and some of the mechanisms behind it. Suburban scrub-jay populations have access to supplemented food in the form of peanuts, cracked corn, birdseed, bread and pet food, with up to 30 per cent of their diet made up of such items (Fleischer et al., 2003). This food forms a predictable resource, which is in contrast to the unpredictable nature of resources experienced by wildland populations. Predictability appears to be important and probably explains why Fleischer et al. (2003) found their suburban scrub-jay populations to be consistent in the timing of their breeding across years, while wildland populations varied. Having a predictable food source in the form of supplemented food may well explain why suburban scrub-jay populations spend less time foraging and more time perching than wildland birds – scrub-jays use a sentinel system, with perching birds alerting other individuals to the presence of potential predators. Suburban scrub-jays also handled more food items per hour, suggesting that their foraging was more efficient. Interestingly, while food consumption rates were not found to vary between the two habitats, those individuals that were the most efficient foragers were also the earliest breeders.

So what does the work on scrub-jays tell us about the effects of supplementary feeding on the timing of breeding? If, as the work of Fleischer et al. (2003) suggests, foraging efficiency increases through access to garden feeding stations, the predictability of this significant food resource may provide the birds with a suitable cue for the appropriate timing of breeding. Of course, the supplementary food might influence the timing of breeding in other ways, perhaps by altering female body condition or behaviour. These two possibilities are known as the ‘anticipation hypothesis’, where females use food availability as a cue for the onset of breeding, and the ‘constraint hypothesis’ – where food availability constrains the female’s ability to produce eggs.

Earlier work by the same research group (Schoech & Bowman, 2001) revealed few differences in female body condition prior to breeding between suburban and wildland populations, suggesting instead that the rate of food intake or time spent foraging – both of which are shaped by food predictability – were more important. Later work on Florida Scrub-jays has attempted to look in greater detail at some of the underlying mechanisms, focussing on protein in the food taken by the birds and also monitoring the levels of key reproductive hormones. Protein is thought to be of particular importance, with evidence suggesting that wildland scrub-jay populations are protein limited – they have lower levels of plasma protein than suburban birds (Schoech & Bowman, 2003). Wildland scrub-jays feed on acorns, the tannic acids in which have been shown to reduce the ability to assimilate proteins. Schoech & Bowman provided their study populations with supplementary food of two different types; one was high in fat and high in protein (‘HFHP’), and one was high in fat but low in protein (‘HFLP’). Scrub-jays in both treatment groups bred earlier than those in an unfed control group, but birds in the HFLP group bred later than those in the HFHP group.

The researchers then looked at testosterone levels in male scrub-jays, estradiol levels in females, and corticosterone levels in both sexes. The HFHP males had higher testosterone levels than either the HFLP or control group males, but no difference was found in estradiol levels in the females. Estradiol mediates maturation of the ovarian follicles and also appears to stimulate nest building and solicitation behaviour, so the researchers had hypothesised that there would be differences in this key hormone between groups. That the high-protein diet influenced a key male hormone rather than a key female hormone could suggest that male scrub-jays play a larger role in determining when reproduction commences than the females. The corticosterone findings were particularly interesting because they revealed that the levels of this ‘stress’-related hormone were lower in suburban populations of the scrub-jays than in either the HFHP, HFHP or control group – though HFHP birds had lower corticosterone levels than HFLP or control group birds. This suggests that the predictability of food resources may influence corticosterone levels and that corticosterone may play a role in the timing of reproduction. This work highlights the complex pathways by which something as apparently simple as the provision of additional food resources may have on the timing of breeding in birds.

Work on species found breeding in UK gardens has generally identified a similar pattern, with the provision of supplementary food leading to earlier laying in Blue Tits in Northern Ireland (Robb et al., 2008a). Robb’s Blue Tits laid an average of 2.5 days earlier than unfed birds. However, Plummer et al. (2013), working on Blue Tits, found that laying dates did not differ significantly between their fed and unfed groups. It is worth just noting that most of these studies have been carried out in woodland habitats rather than in gardens, something that reflects the very great difficulty in running controlled experiments within an urban setting (see Chapter 6). Elsewhere, the laying dates of Willow Tit and Crested Tit were advanced by 2–5 and 5–8 days respectively when provided with sunflower seed and a tallow-based mix containing soy protein, wheat germ, sunflower seed, vitamins and minerals (Brömssen & Jansson, 1980). In a review of 59 studies, Robb et al. (2008b) found that 58 per cent reported significantly earlier laying dates in supplemented populations. Some studies, but not others, suggest that supplementary food may shorten incubation periods (Nilsson & Smith, 1988; Nager et al., 1997, Harrison et al., 2010). The Harrison et al. (2010) study revealed that supplemented Great Tits reduced their incubation period in each of the three study years, while Blue Tits did so in two of the three years.

FIG 34. The presence of supplementary food at garden feeding stations can advance the timing of breeding in species like Blue Tit, something that has been proved experimentally here in the UK. (Jill Pakenham)

Clutch size, egg and offspring quality and fledging success

The formation of an egg is not only costly in terms of energy but also in terms of nutrients, such as protein. If females are limited by resources as they go into the egg laying period, then it follows that the provision of suitable resources may lead to an increase in clutch size. However, the evidence for this is mixed, with some authors finding an effect (e.g. Soler & Soler, 1996) but not others (Brömssen & Jansson, 1980; Svensson & Nilsson, 1995). In one case, the provision of supplementary food appeared to reduce clutch size significantly in Blue Tits and Great Tits breeding in a Worcestershire woodland (Harrison et al., 2010). The Harrison study revealed an interesting mix of results; in addition to the significant reduction in clutch size and the shortened incubation periods noted above, food supplementation from several weeks pre-laying through to hatching appeared to reduce hatching success in Blue Tits, but not Great Tits. Robb et al.’s review, mentioned earlier, found that in 44 studies that dealt with food supplementation and both laying dates and clutch size, 12 reported increases in both and 16 reported increases in clutch size alone (Robb et al., 2008b).

FIG 35. Supplementary food has been shown to influence the number and quality of offspring produced by birds like Great Tit. (Mike Toms)

The provision of supplementary food, just prior to and during egg laying, has been shown to result in increased deposition of carotenoids within the yolk of an egg, leading to increased hatching success (Møller et al., 2008) and, subsequently, higher rates of adult survival (McGraw et al., 2005). Chicks hatched from eggs with high levels of antioxidants have been shown to have lower rates of mortality in the days following hatching (Royle et al., 2011), so micronutrients stored by the female bird and derived from supplementary food, could deliver benefits to the resulting chicks. However, the findings of various studies show some differences. Supplementary feeding during the winter months – this time with fat – has been shown to affect investment in egg production, resulting in a smaller relative yolk mass in larger eggs and reduced yolk carotenoid concentration in early breeding Blue Tits (Plummer et al., 2013); these negative effects were absent in Blue Tits fed with fat plus vitamin E (see below). Egg size increases were reported in 38 per cent of the studies examined by Robb et al. (2008b). Returning to Florida Scrub-jays, Jim Reynolds (Reynolds et al., 2003) found that females fed with a high-fat high-protein diet produced heavier third eggs, containing more water and more protein than the eggs of unsupplemented individuals.

Of course, one of the key figures when discussing breeding success is the number of chicks that fledge from the nest; while this is obviously influenced by some of the factors we have just discussed – such as the number of eggs – it is worth looking at whether supplementary feeding influences this figure. Working on a Blue Tit population in Northern Ireland, Gillian Robb and colleagues found that supplementary feeding significantly affected the numbers of chicks that fledged, such that individuals for which peanuts had been provided during the winter months fledged almost one extra chick per nest compared to those without access to the food (Robb et al., 2008a). Although Brömsson & Jansson (1980) did not find any effects of food supplementation on clutch size or the number of fledglings in Willow Tit or Crested Tit, they did find that Crested Tit nestlings were significantly heavier within the fed study site and that there was a tendency to produce second clutches more often in the experimental area.

One area where concern has been raised about the practice of feeding garden birds during the breeding season is in relation to nestling health and well-being. It has been suggested that garden birds might feed their nestlings on artificial foods provided at garden feeding stations in preference to the natural, predominantly invertebrate-based, foods that they actually need. This suggests that garden birds are either lazy or unable to identify foods as being unsuitable for their nestlings. However, given what we know about food selection behaviour in wild birds, and which has been presented elsewhere in this chapter, both of these suggestions seem unlikely. Cowie & Hinsley (1988a; 1988b) examined nestling diet in a population of suburban Blue Tits and Great Tits, using nest cameras to identify the food provided by parent birds and how this was delivered. The cameras revealed that, on average, 15 per cent of the nestling diet of each of the two species was made up of artificial foods, notably bread, fat and peanuts. The amount provided varied between pairs, ranging from 6 per cent to 34 per cent in Great Tit and from 6 to 28 per cent in Blue Tit. Despite this, the researchers failed to find any relationship between the amount of artificial food provided and measures of nestling health. That adult garden birds are able to make appropriate decisions about the food they provide to their chicks can be seen from observations that if additional insect food is made available to nesting tits, or if their brood size is artificially reduced by removing some of the nestlings, the parent birds respond by dropping artificial foodstuffs from their diet and increasing the numbers of invertebrates provisioned.

FIG 36. Adult birds appear to be able to identify appropriate foods for their nestlings and fledglings, with most species feeding their young on invertebrate prey while they are still in the nest, and making use of bird table fare as the young develop towards independence. (John Harding)

Supplementary food, such as that presented to birds within the garden environment, may have consequences for other measures of breeding success, and indeed for other aspects of the breeding cycle. Studies have revealed that nesting adults, provided with additional food, will often reduce foraging effort, something that can provide time for other activities – such as predator vigilance (Arcese & Smith, 1988). It can also have some rather surprising effects. Conservationists, working to secure a future for the Kakapo Strigops habroptilus through supplementary feeding of the worryingly small female population, discovered to their horror that the quality of food being provided led to a male-biased sex ratio in the resulting chicks. It is thought that this came about because the female is the heterogametic sex – it has two different sex chromosomes – and can in some instances determine the sex ratio of offspring at the point of fertilisation. A change to the food quality being provided saw a more balanced sex ratio restored (Robertson et al., 2006).

Diet and nutrition have also been linked to important traits associated with reproduction, including mate selection. Mate selection is often based around traits that indicate the quality of a potential mate. Such traits, which include plumage colouration and song complexity, are often nutritionally or energetically costly to maintain.

THE HEALTH EFFECTS OF PROVIDING FOOD

Until recently, very little work had been done on the nutritional composition of the foods being provided at garden feeding stations; even now we are only just beginning to explore the health and other impacts of such provisioning. We know that the proteins and lipids found in naturally occurring foods play an important role in the control of reproduction, underlining that we need to establish how supplemented foods differ from what birds would be eating normally and what, if any, effects such differences have on the health and behaviour of individual birds. We also know that the provision of supplementary food can support birds as they replace lost feathers, something tested experimentally by Grubb & Cimprich (1990).

FIG 37. Although the black bib of male House Sparrows has an important function, signalling status, it is not as sensitive to nutritional conditions as one might expect. (John Harding)

A poor diet can have implications for the health of a wild bird, perhaps increasing susceptibility to disease or resulting in poor-quality plumage or other abnormalities. In geese and birds of prey, nutritional imbalance has been suggested as the primary cause of ‘angel wing’ disorder, while the effects of malnutrition on plumage characteristics have been seen in Brown-headed Cowbirds Molothrus ater (McGraw et al., 2002). Interestingly, McGraw’s work, which looked at both Brown-headed Cowbird and House Sparrow, found that important signalling plumage – the melanin-based black bib of male House Sparrows and the brown hood of the cowbirds – was not affected by nutritional stress. That such melanin-based ornaments are less sensitive to nutritional conditions during moult suggests these may signal hormonal status and/or competitive ability rather than represent an accurate signal of health.

The provision of meat at garden feeding stations, which is widely practised in Australia but not the UK, could have implications for the health of those birds feeding upon it. It has been demonstrated that birds like Australian Magpie, taking meat from garden feeding stations, have elevated levels of plasma cholesterol and fatty acids (Ishigame et al., 2006). A secondary concern with meat is that it has an unbalanced calcium to phosphorous ratio – calcium absorption can be hindered by high phosphorus levels – which can lead to a range of skeletal or physiological problems.

The reliance on foods provided at garden feeding stations may lead to a dietary imbalance, directly affecting an individual bird, and/or impairing its reproductive potential during a subsequent breeding season. Where events in one season influence an individual’s performance in a later season, this is known as a ‘carry-over effect’. Such effects have received increasing attention over the last few years, particularly in relation to supplementary feeding. Another potential effect of food provision at garden feeding stations is that it may alter population structure, perhaps by enabling low quality individuals to recruit into the breeding population. The phenotypic quality of individuals is strongly influenced by oxidative stress, with this oxidative stress reflecting the imbalance between the harmful reactive oxygen species (known as ROSs) produced as by-products of metabolism and the body’s antioxidant defence system (Selman et al., 2012).

Carry-over effects are often talked about in terms of energetics, with macronutrients – such as fats – thought to be of particular importance. However, small birds, such as those visiting garden feeding stations in the UK, lack the capacity to store large amounts of fat, meaning they have to rely on daily food intake to meet the energetic demands imposed by reproduction. Because of this, it might be that micronutrients – such as antioxidants – are more important; these may be stored in the liver and any subcutaneous fat to deliver reserves that can be utilised during a subsequent breeding season or at other times when they are needed (Metzger & Bairlein, 2011).

FIG 38. Adult Great Spotted Woodpeckers bring their chicks to garden feeding stations soon after they have left the nest. Presumably knowledge of a reliable food source will help the young woodpeckers as they become independent. (Jill Pakenham)

Work by Kate Plummer and colleagues has sought to establish the extent to which supplementary feeding produces the individual- and population-level effects we have just described (Plummer et al., 2018). Plummer used different woodland populations of Blue Tits to compare the effects of providing fat, and fat plus vitamin E (an antioxidant), with a control population of unfed birds. Feeding was only carried out during the winter months, ending at least a month before the tits began egg laying. Provisioning with fat and vitamin E improved the survival, recruitment and breeding condition of birds that had been in significantly poorer condition prior to feeding; provisioning with fat only was found to have a detrimental impact on breeding birds. Because birds that had been supplied with fat and vitamin E were found to have lower levels of carotenoids in their breast feathers – lower levels of carotenoids being indicative of lower quality individuals – Plummer was able to conclude that supplementing with vitamin E and fat in winter had altered the survival and recruitment prospects of these lower quality individuals. It appears, therefore, that the provision of supplementary foods during the winter months can alter both the structure of the breeding population and the condition of individual breeding birds. Such effects can also have consequences beyond the breeding season where, for example, food provision alters levels of oxidative damage; in Plummer’s study, individuals with higher blood plasma concentrations of malondialdehyde (indicative of oxidative damage) produced offspring that were structurally smaller and suffered from reduced fledging success.

The importance of antioxidants, like vitamin E, can also be seen from Plummer’s work on yolk mass, mentioned earlier. While Plummer et al. (2013) found that winter provisioning with fat subsequently impaired an individual’s ability to acquire, assimilate and/or mobilise key resources for yolk formation, this was not the case where vitamin E was also included in the food presented. A high-fat diet, such as that potentially obtained from the food provided at garden feeding stations, may well increase the requirement for antioxidants in order to combat the greater levels of oxidative damage associated with a diet rich in fats.

FOOD PROVISION, POPULATION SIZE AND COMMUNITY STRUCTURE

Plummer’s work shows that the effects of food provision can potentially pass from one generation to the next, and alter the composition of breeding populations in subsequent seasons, but what about its impact at a wider community level? Work carried out in the Netherlands has revealed how the provision of supplementary food can lead to an increase in local population size, and how the scale of the effect can change in relation to the availability of wider food resources (van Balen, 1980). Research by van Balen monitored two populations of Great Tits, breeding in similar woodlands just 7 km apart; before introducing a regime of winter feeding, the two populations were similar and had been fluctuating more or less in parallel over several years. Following the introduction of feeding, the number of breeding pairs in the fed woodland increased to become 40 per cent higher than that in the unfed woodland. Not only this, but the scale of the difference varied according to the size of the beechmast crop; in poor beechmast years, the fed population was almost double that of the unfed population the following breeding season; following years with a good mast crop, there was little difference between the numbers of Great Tits breeding in the two woods. A similar finding was noted by Källander (1981), working in Sweden, whose first year of study (1969–70) coincided with a particularly severe winter. Despite this, Great Tits in a wood provisioned with sunflower seeds increased in number, while those in an unfed wood declined. The following year, in which there was a good crop of beechmast, saw populations in both woods increase.

If the provision of food at garden feeding stations leads to an increase in the numbers of birds locally, then we might expect to see a relationship between feeder density and bird numbers across individual towns and cities. There is some evidence of this from the work carried out in Sheffield (Fuller et al., 2008; 2012), with the density of three of the seven urban-adapted species studied shown to be positively related to the density of bird feeders. House Sparrow showed the strongest pattern, with 57 per cent of the variation in House Sparrow densities explained by the density of bird feeders; the other two species were Blackbird and Starling. Interestingly, given the frequency with which Blue Tits and Great Tits are seen at garden feeding stations, it was surprising that they did not show such a relationship, yet Blackbird did.

Some of the strongest evidence for community-level effects comes from work carried out in New Zealand, where most of the native bird species able to persist within urban areas are either predominantly insectivorous (e.g. Grey Warbler Geryone cigata), frugivorous (e.g. New Zealand Pigeon Hemiphaga novaeseelandiae) or nectarivorous (e.g. Tūī Prosthemadera novaeseelandiae). In contrast, many of the introduced species are either granviorous (e.g. House Sparrow and Spotted Dove) or adaptable enough to take bird table fare. Much of the food provided at garden feeding stations in New Zealand would appear to favour the introduced species, something confirmed by Galbraith et al. (2015) through a feeding experiment. The introduction of feeding altered the community structure, with significant increases seen in the abundance of House Sparrow and Spotted Dove, and to a lesser degree in Starling and Song Thrush. The introduction of feeding was also shown to have a negative effect on the abundance of the native Grey Warbler. Once the feeding experiment ended, virtually all of the observed changes in community structure disappeared. Observational work on these birds (Galbraith et al., 2017b) revealed that House Sparrow and Spotted Dove dominated the feeding stations.

Supplementary feeding in the Sydney area, Australia, together with the establishment of both native and non-native fruiting shrubs in gardens, is thought to have been a significant factor in the major changes seen over the past 100 years in the diversity and abundance of parrots (Burgin & Saunders, 2007). The provision of food at garden feeding stations is thought to have aided colonisation and range expansion of the Collared Dove within the UK. Within North America, the northwards range expansions of House Finch, Tufted Titmouse Baeolophus bicolor and Northern Cardinal have also been attributed to the food available at garden feeding stations during the winter months.

FIG 39. The presence of garden feeding stations is thought to have aided the successful colonisation of the UK by Collared Doves. (John Harding)

INFLUENCE OF SUPPLEMENTARY FOOD ON NEST PREDATION

We have already seen how the provision of supplementary food can alter the timing of breeding and community structure. A special case of the latter relates to the ways in which the provision of supplementary foods can influence relationships between predators and their prey. This is of particular relevance when discussing urban and suburban gardens because anthropogenic foods have become a defining characteristic of urbanised landscapes (Warren et al., 2006). The nature of interactions between breeding birds and their predators may be especially influenced by the provision of such supplementary foods because both songbirds and their generalist predators readily exploit these resources, something that can lead to them occupying the same urban sites. BTO work, in the form of a meta-analysis led by Dan Chamberlain, failed to reveal any strong underlying pattern to the effects of urbanisation – and the associated anthropogenic food sources – on the relationship between predators and levels of nest predation (Chamberlain et al., 2009a), but there have been some useful field experiments on the subject.

FIG 40. There is little evidence that the presence of bird feeders and their use by Grey Squirrels leads to increased levels of nest predation in the local area, but such an effect has been seen for other predators. (John Harding)

Supplementary food may reduce levels of nest predation by providing an alternative food supply for would-be nest predators; alternatively, it may increase levels of nest predation by attracting predators or elevating their population densities. Jennifer Malpass and her colleagues investigated how the presence of bird feeders affected predator abundance and nest survival of American Robin Turdus migratorius and Northern Cardinal across seven neighbourhoods in Ohio, US (Malpass et al., 2017). Malpass found that the relative abundance of both Brown-headed Cowbird and American Crow Corvus brachyrhynchos – known nest predators of the two study species – was positively associated with the number of bird feeders present within a neighbourhood. No similar relationship was found for the other nest predators studied, which included Grey Squirrel Sciurus carolinensis, Blue Jay Cyanocitta cristata and Common Grackle Quiscalus quiscula. While the relative abundance of American Crow and Brown-headed Cowbird was greater in areas with bird feeders, there was no consistent relationship between the numbers of bird feeders and predation of the songbird nests.

DISEASE AND FOOD PROVISION

The presence of supplementary food at garden feeding stations may contribute positively to bird populations by increasing the availability of food during those periods when natural food supplies are in short supply. However, as will be explored in Chapter 4, the provision of food at bird tables and in hanging feeders may also facilitate the spread of disease. The relationships between food provision and disease occurrence can be more complex than they first appear. It is not simply that garden feeding stations attract large numbers of birds, and through this lead to increased opportunities for disease transmission; in some instances the presence of the food can act to ameliorate the impacts of disease. This is something that has been studied in North America, in relation to the outbreak of the bacterial disease mycoplasmal conjunctivitis in North American House Finches (Fischer & Miller, 2015).

Mycoplasmal conjunctivitis impairs vision, making it difficult for affected individuals to find and locate food. If such individuals have access to abundant and predictable food resources, this may afford them with the time and energy needed to recover. Interestingly, there is also some limited evidence that individuals infected at feeding stations may develop less severe symptoms than those infected elsewhere, something that may lead to a faster recovery. Fischer & Miller (2015) used information from the FHWAR (Fishing, Hunting and Wildlife-associated Recreation) census to examine how food provision influenced House Finch populations before and after the emergence of mycoplasmal conjunctivitis. The two researchers found that the availability of bird food appeared to have a positive effect on House Finch populations, with House Finch density prior to the disease outbreak strongly related to the density of US citizens providing food for birds. After the disease arrived, reducing House Finch numbers substantially, the relationship between feeder density and finch numbers remained.

A comparison was then made between pre- and post-disease emergence populations, the researchers seeking to determine whether finch populations declined more strongly where feeder densities were high – which is what you might expect if high densities of feeders lead to high rates of disease transmission and, through this, to a correspondingly greater level of population decline. The results of the study revealed a positive relationship between changes in finch densities and changes in feeder densities between the two periods, indicating that higher densities of feeders actually resulted in lower rates of population decline. This adds weight to the work of Dhondt et al. (2007), suggesting that food provision might act as a ‘crutch’ for sick birds and improve their chances of survival and recovery. Other factors also need to be considered, not least that House Finches may spread mycoplasmal conjunctivitis to other species, including House Sparrow and American Goldfinch.

FOOD PROVISION AND BEHAVIOUR

As we have already discovered, food resources are central to many different aspects of a bird’s life, so it is little wonder then that the provision of food can alter behaviour. The area where we might expect to see clear evidence for changes in behaviour is in relation to territory, since territory provides a means of partitioning and defending resources, including food. Black-capped Chickadees show a degree of territoriality during the winter months, defending small foraging patches from intruders. However, as Wilson (2001) discovered, such territoriality breaks down when a significant food source is added to a foraging patch, the resident flock (typically the resident mated pair and six to ten first winter birds) unable to defend the new resource against the large numbers of other chickadees wanting to make use of it.

FIG 41. When supplementary food is available, both Blue Tits and Great Tits are less likely to join the mobile foraging flocks that are a feature of the early winter in woodland habitats in the UK. (Jill Pakenham)

A feature of the tit species that are familiar to garden feeding stations here in the UK is their willingness to form mixed-species flocks during the autumn and winter months, a behaviour that also occurs in other tit species elsewhere in the world. This behaviour is thought to improve an individual’s chances of finding food during those periods when food is scarce and encounter rates low. You might, therefore, expect to see the provision of supplementary food exert an influence on the tendency to form or join mixed-species flocks. This is exactly what Thomas Grubb found, working on tits in deciduous woodland in Oxfordshire (Grubb, 1987). Grubb was interested in whether mixed-species flocking was driven by predation risk, by food availability or by both. His results suggested that food was the primary driver; Blue Tits and Great Tits without access to supplementary food flocked with other species while foraging more often than was the case when supplementary food was available. Interestingly, Long-tailed Tits – which ignored the artificial food – foraged in mixed-species flocks regardless of whether or not provisioned food was available. Work in Japan on Varied Tit Sittiparus varius underlines the behavioural flexibility in whether or not individuals join mixed-species flocks (Kubuta & Nakamura, 2000), revealing that individuals participate in mixed-species flocks to obtain the short-term benefits of increased foraging efficiency but, independent of food provision, they also obtain long-term benefits from the stability of their pair bonds and strong site fidelity.

The studies just mentioned are relevant to the discussion of how the winter provision of food at garden feeding stations may alter the behaviour of the birds that visit. However, garden feeding may also change behaviour during the breeding season, something that may be of particular importance. As we shall see in our examination of the breeding ecology of garden birds (Chapter 3), the dawn chorus provides both a means to demonstrate ownership of a breeding territory and a mechanism to advertise your status as a mate. Research suggests that singing at dusk and, particularly, dawn may provide an honest signal of the energetic status of the singing male. If this is the case, then we might expect the availability of food in garden feeders to shape the performance of those males with access to it. Experimental work on Blackbirds supports this, with Cuthill & MacDonald (1990) finding that food-supplemented males sang significantly more than unsupplemented males. This difference was largely the result of supplemented males initiating song earlier and having higher peak rates of song delivery. Could this mean that male Blackbirds with access to a reliable food source at garden feeding stations are more likely to attract a mate than those nesting in other habitats where perhaps food is more limiting? A similar piece of work, this time on Great Tits, casts some uncertainty on whether the relationship is as simple as it appears from the work done on Blackbirds. Katja Saggese and her collaborators provided male Great Tits with a continuous food supply over two weeks and then compared their singing activity with a group of unfed males (Saggese et al., 2011). In contrast to Cuthill & MacDonald’s findings, the food supplemented males started their dawn singing later than the control males, an effect that still continued two weeks after the provision of food had ended. The researchers were unsure of the reasons for what they had observed, but they felt that it could have been due to the presence of predators, attracted to the feeding stations, or something about the quality of the food itself.

FIG 42. Male Blackbirds with access to supplementary food sing for longer than those without access to such food, suggesting that the food provided at garden feeding stations might help local birds in defence of their territories and mate attraction. (Mike Toms)

FOOD PROVISION AND MOVEMENTS

Birds tend to reduce the size of the area over which they forage when provisioned with supplementary food, something that can lead to a reduction in the size of breeding territory and even a change in mating system in the case of Dunnock (Davies & Lundberg, 1984). As we will see in a moment in relation to Blackcaps, the provision of food in gardens can have a significant impact on movement and wintering behaviour.

Although not garden-related, the scale of feeding impacts can be seen from work on the White Stork Ciconia ciconia, a species that was wholly migratory in Europe but which has established resident populations across parts of Iberia largely in response to the year-round food available at landfill sites (Gilbert et al., 2016). Landfills and food discarded by people have also played a role in the expansion of urban-breeding Lesser Black-backed Gulls Larus fuscus here in the UK (Coulson & Coulson, 2008), a species that now winters here in large numbers, when it formerly wintered in southern Europe and north and west Africa.

FOOD PROVISION AND THE EVOLUTION OF NEW BEHAVIOUR – WINTERING BLACKCAPS

The Blackcap is a common summer visitor to much of Britain, wintering in southern Europe and south into North Africa. Although there are occasional records, the Blackcap was rarely encountered in Britain during the winter months 60 years ago (Stafford, 1956). However, since the 1950s we have seen a rapid and substantial increase in wintering records, as information from both the Garden Bird Feeding Survey and Bird Atlas 2007–11 show (Balmer et al., 2013). Data from BTO-led bird atlases show that the Blackcap’s wintering range in the UK has expanded by 77 per cent over the last 30 years. The increase in wintering numbers has come about because Blackcaps breeding in southern Germany and Austria have increasingly migrated in a northwesterly direction to Britain for the winter, rather than in a southwesterly direction to traditional wintering areas located in southern Spain (Helbig et al., 1994). This new migration strategy has been shown to be genetically encoded (Berthold et al., 1992), and is maintained via reproductive isolation and assortative mating, linked to fitness benefits on the breeding grounds (Bearhop et al., 2005).

FIG 43. The use of gardens by Blackcaps peaks during the winter months. Data reproduced from BTO Garden BirdWatch with permission.

Early observations of increasing numbers of wintering Blackcaps coincided with the introduction of commercial bird foods (Callahan, 2014), suggesting that supplementary feeding at garden feeding stations might be applying a selection pressure for the evolution of this new migratory route. Interestingly, while the Blackcaps wintering in southern Spain are predominantly frugivorous in their dietary choices, those wintering in Britain are known to use a wide variety of supplementary food types (Tellería et al., 2013; Plummer et al., 2015). The availability of supplementary food is unlikely to be the sole driver of evolutionary change, however, and it is likely that a changing climate has also had some influence over the adoption of this new migration route and new wintering area. Winter conditions in Britain, and more widely across the Northern Hemisphere, are becoming milder, something that has enabled a number of species to shift their wintering range northwards (IPCC, 2013).

Unravelling these different drivers requires access to long-term datasets on food provision, Blackcap occurrence and climatic factors. Fortunately, BTO researchers have been able to use data from BTO’s weekly Garden BirdWatch survey to explore the question of whether food provision has played a part in the evolution of a new migratory strategy in this central European Blackcap population (Plummer et al., 2015). Earlier work using the BTO Garden BirdWatch dataset has revealed that Blackcaps are strongly associated with suburban gardens during the winter months (Chamberlain et al., 2004a) which makes this dataset ideal for examining the interactions between food provision and Blackcap wintering behaviour. Examination of the foods provided in Garden BirdWatch gardens and their use by wintering Blackcaps revealed a strong preference by the birds for sunflower hearts and fat-based products. By calculating an annual measure of the proportion of winter weeks in which these foodstuffs were provided at each of 3,806 sites – each of which had a minimum of at least 16 weekly submissions in a minimum of at least nine winters – it has been possible to explore the relationship between Blackcap occurrence and food provision and how this has changed over time.

Wintering Blackcaps showed regional variation in their use of Garden BirdWatch gardens, with greater occupancy of sites in the south and west (see Figure 43), where wintering conditions are milder. Occupancy rates were influenced by both the provision of supplementary food and winter temperature. Birds were recorded more often in gardens that provided food more frequently, and showed a preference for sites that had a warmer local climate during the winter months. One of the most interesting findings was that Blackcap occurrence has become more strongly associated with the provision of fat products and sunflower hearts over time, suggesting that the birds are adapting their feeding habits to exploit human-provisioned foods. This supports the theory of Berthold & Terrill (1988) that the Blackcap’s new migration strategy is likely to have evolved in response to increased supplementary feeding activities in gardens in Britain.

The results of the Plummer study also reveal that gardens are used less during milder winters, which – together with the relationship found with local climate – supports the hypothesis that an improving winter climate is likely to have enabled the Blackcap to increase its wintering range into Britain. In addition, the growing use of provisioned food adds further support to the work of Rolshausen et al. (2009), who have found that Blackcaps wintering in Britain have relatively narrower and longer beaks than those wintering in Spain. Such differences suggest that these British migrants have adapted to a more generalist diet than their Spanish counterparts, adding phenotypical divergence to the genetic divergence already documented between these two populations (Berthold et al., 1992). Rolshausen et al. also found differences in wing morphology, bill colour and plumage colour between the two populations.

WATER AND GARDEN BIRDS

Garden birds require water, both to drink and in many cases for bathing – though see Chapter 5. This water may be present in the form of a pond, puddle or bird bath and all can be well used by visiting birds. Figures from national studies suggest that 10 per cent of UK dwellings have a pond associated with them, though not all of these will be suitable for birds. Some may be netted to protect precious fish, while others may be steep-sided and inaccessible. Those with a shallow end or with branches that dip into the water are likely to be used most often, the birds able to perch within or close to the water when drinking or bathing. Just over half of the respondents to a questionnaire survey carried out by Cowie & Hinsley (1988a) in suburban Cardiff reported providing water for visiting birds. The provision of water can influence the community of birds using a garden, with seed-eating species seemingly preferring sites at which they can both feed and drink. Water may also shape behaviour – for example, by providing some garden Blackbirds with the opportunity to develop the habit of catching and eating small newts and the tadpoles of Common Frog Rana temporaria – and play a role in disease transmission. Despite this, its importance has received little research interest and there is the potential here for much new information to be gained.

FIG 44. Water is important for garden birds and the presence of a pond or other water source can prove very attractive to visiting birds. (Jill Pakenham)

GARDEN BIRD FOOD AND OTHER WILDLIFE

The food available at garden feeding stations isn’t just used by garden birds; it may be taken by other creatures or, in some instances, germinate and lead to the establishment of non-native flora. Many of the other beneficiaries of garden feeding are mammals, with mice, voles, rats and squirrels the most commonly reported feeders here in the UK. While some of these creatures are tolerated, or even encouraged, some are viewed as a pest. Common Rat Rattus norvegicus and Grey Squirrel are the two most commonly seen as a negative consequence of garden feeding. The presence of Grey Squirrels at a garden feeding station may prevent birds from taking food, something determined experimentally by Hugh Hanmer and colleagues at the University of Reading (Hanmer et al., 2018).

FIG 45. Some garden feeding station visitors are less welcome than others. (John Harding)

Efforts to deter squirrels from garden feeding stations tend to be based around either the physical adaptation of feeding devices or treatment of the food itself. Two other options – providing squirrels with their own feeder some distance from the feeding stations, and lethal control – are not widely practised. Treatment of peanuts and other foods with capsaicin – the pungent component that is responsible for the sensation one gets when eating chilli peppers – has proved a useful deterrent in trials carried out in the US (Curtis et al., 2000). Treatment of sunflower seeds significantly reduced both the amount of seed taken by Grey Squirrels and the amount of time that they spent feeding, but did not affect the amount of seed taken by feeding birds. Capsaicin binds to pain receptors in mammals and has a number of other effects that result in many mammals finding the substance repellent. Although at least some bird species can taste capsaicin, it does not appear to be harmful or repellent to them. There is, however, one other finding worth mentioning here, which is that Grey Squirrels can learn to open treated sunflower seeds, avoiding the treated husk to feed on the heart (Fitzgerald et al., 1995). Fortunately, a company in the US has developed a way to treat both the husk and the heart within.

A perhaps unforeseen consequence for wider biodiversity on the provision of food at garden feeding stations is that the presence of increased numbers of insectivorous or omnivorous birds can lead to increased levels of predation of garden-dwelling invertebrates. Investigating the possible effects of wild bird feeding on the size and survivorship of Pea Aphid Acyrthosiphon psium colonies in suburban gardens, Melanie Orros and Mark Fellowes found that the presence of bird feeders had a significant impact. Both the size and survivorship of aphid colonies was significantly reduced in gardens where a feeding station was present (Orros & Fellowes, 2012). The same researchers have also found that supplementary feeding of garden birds indirectly affects ground beetle populations (Orros et al., 2015a), and work in the US suggests a similar impact on arthropods overwintering on bark (Martinson & Flaspohler, 2003).

CONCLUDING REMARKS

A key take-home message about bird feeding is that there is still a lot for us to learn; it is no longer appropriate to merely follow the simplistic view that feeding garden birds is largely beneficial, and that any deleterious aspects are outweighed by the benefits gained. Some authors have even suggested that bird feeders are ecological traps, perhaps tempting birds to initiate breeding attempts earlier in the year than is beneficial and leading to a mismatch between the food demands of the resulting chicks and the availability of the peak invertebrate resources on which they depend. The scrub-jays breeding in suburban habitats with access to supplementary food may well breed earlier but they can find themselves out of synch with the natural food items that they need for their growing nestlings, something that could potentially lead to decreased rather than increased breeding success (Schoech & Bowman, 2001).

The provision of supplementary foods, many of which would not be taken by these species within the wider environment, has complex effects on individual birds and their broader populations. The extent to which food provision is considered to be deleterious or beneficial will, in part, depend on whether you are examining its effects at the level of the individual bird or the wider population. Feeding can change the structure of populations, something that may have profound consequences for the size of broader populations, both of the species under study and, potentially, those of the other species with which it competes or interacts. The effects of supplementary food may also differ depending upon external factors, perhaps having the greatest impact when times are tough or in territories low in resources or occupied by inexperienced birds (Robb et al., 2008a). This was certainly the case for Arcese & Smith (1988) working on a high-density population of Song Sparrow Melospiza melodia. Arcese & Smith found particularly large effects of supplementary feeding, presumably because the control birds – those without access to additional food – were under heavy competitive pressure, suppressing clutch size and other measures of breeding success. They found, for example, that fed birds laid 18 days earlier, on average, than control birds and were also more synchronous. One of the challenges to unravelling these effects is the difficulty in studying wild birds, of following individuals throughout their entire life cycle and in dealing with populations that may move between habitats and over significant distances.

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