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William Collins

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Source ISBN 9780007308323

Ebook Edition © JANUARY 2019 ISBN: 9780007403691

Version: 2019-01-09

DEDICATION

TO MY PARENTS

CONTENTS

Cover

Title Page

Copyright

Dedication

Chapter 1: Men and Birds

Chapter 2: Ecological Considerations

Chapter 3: Some Predators and Their Prey

Chapter 4: Birds and Forests

Chapter 5: Birds on the Farm: The Community

Chapter 6: Birds on the Farm: The Animal Feeders

Chapter 7: Birds on the Farm: The Seed-Eaters

Chapter 8: Birds on the Farm: The Grazers

Chapter 9: Bird Problems in Horticulture

Chapter 10: Birds and the Fisherman

Chapter 11: Birds and Industrial Man

Chapter 12: Wild Life Management

Tables

Footnotes

Bibliography

Index

About the Publisher

CHAPTER 1

MEN AND BIRDS

WE can only speculate on the impact of the stone-age hunters of Europe on birds: they must have had some effect at least on those species which were easy to catch. Certainly the larger mammals seem to have felt the pressure of man the hunter as long ago as the Pleistocene. Using molar tooth remains, Soergel has calculated the ages of Pleistocene elephants found at sites where either Heidelberg or Neanderthal man was present and at comparable sites where no human remains were found. In each case the age structure of the elephant population is much lower where man abided, suggesting that he was cropping the populations sufficiently heavily to increase the need for a high rate of replacement by young animals. Some indication of the importance of birds to these ancient hunters may be gleaned from the cave drawings at Lascaux, France. Fisher and Peterson give good reason for believing that they depict a fauna existing in a warm inter-glacial, 100,000 years ago. Flamingo, spoonbill, geese, crane, eagle and raven are all shown, together with a dead hunter wearing a bird mask and sporting a hunting stick adorned with birds.

The earliest indications of man’s interest in cereal culture date back in the Near East to about 8,000 BC. Remains of sickles, pestle and mortar have been found in mesolithic caves of Palestine, and arable farming had reached an advanced stage in the Near East by 3000 BC. This later neolithic culture did not spread to Britain until around 2000 BC and throughout the early and middle bronze ages men were still very much hunters rather than pastoralists, living a semi-nomadic life. Settlement brought man a new relationship with birds, because instead of loose contacts during hunting expeditions, conditions now existed for confining wild creatures and breeding them in captivity, leading to selective breeding and domestication. It is probably no coincidence, as the late F. E. Zeuner points out, that all the important domesticated birds are seed-eaters (pigeons and fowl), grazers or vegetable-eaters (geese and ducks), which could have gained an early association with man through their croprobbing habits. Hence, the first use of birds was as a direct food substitute, their value as egg-producers and for pleasure and sport was not exploited until later. Nonetheless, as early as the Iron Age, in the Hallstatt culture which flourished on the Austrian shores of Lake Constance, there is evidence in the form of a crude carving that man had established a ritualistic link with what was probably a goose. The goose-herd survives as an important element in the rural economy of the Balkans today.

The ancient Greeks and Egyptians had domesticated grey-lag geese, and according to the Odyssey, Penelope owned twenty. Aesop’s fable of the goose laying golden eggs symbolises the wealth enjoyed by these early farmers, by their counterparts the swan-owners of medieval England and the present-day turkey farmers. Furthermore, just as the modern broiler or battery industry is concerned with the most efficient conversion of vegetable food into protein, a fifth dynasty relief depicts captive geese being forcibly stuffed with food – perhaps these Egyptians were already enjoying pâté de foie gras. In Britain, domestic fowl were doubtless kept before the arrival of the Romans, but little information exists until 1578 when Bishop Leslie describes how the birds were caught in nets when moulting, to be pinioned and tamed; at this time the grey-lag was a common breeder in the English fens.

Evidence of the domestication of rock doves goes back to 4500 BC in ancient Iraq, and it is possible that neolithic man had learned how to breed the species in captivity. Early inhabitants of Britain farmed the doo-caves, erecting extra ledges to facilitate the collection of the young, which were then used immediately or reared for breeding. The Norman lords had their dove-cotes, and the practice flourished in rural communities. Domestic fowl, which were probably kept in India as early as 3200 BC, were also associated with religious and sacrificial functions. The evolution of both the rock dove and fowl (and as will be mentioned below the house sparrow too) likely occurred in close association with the emergence of man, making them in a sense pre-adapted to domestication. Indeed, it is conceivable that these species would not have evolved in the absence of man the pastoralist. Recent studies by Collias and Saichuae of the jungle fowl Gallus gallus, the ancestor of the domestic fowl, in Thailand and Malaya, show that it depends on the man-altered habitat produced as a consequence of the cut, slash and burn type of shifting native husbandry. The bird thrives in the secondary scrub eventually produced when man moves elsewhere, and it seems to be poorly adapted to virgin forest. Furthermore, the behavioural patterns of these jungle fowl are virtually identical with those exhibited by domestic strains so that years of selective breeding have achieved relatively little modification in the species-characteristic behaviour.

A painting on the tomb of Haremhab at Thebes in the days of Thothmes IV (1420–11 BC) shows how men kept pelicans in enclosures and collected their eggs for food. Iron Age Glastonbury was less sophisticated, although pelicans and various other birds were caught and eaten by the local inhabitants. We have no idea why the pelican (Dalmatian Pelican) became extinct in Britain between then and the Dark Ages, and if we blame hunting man we have to explain why another great delicacy – the crane – managed to survive until about 1590. Bearing in mind the considerable changes in bird distribution which resulted from the improvement in climate from the late nineteenth century to about 1950, we may hazard a guess that climatic fluctuations also caused important faunal changes in these early years. Pelicans were still nesting in western Europe in the estuaries of the Scheldt, Rhine and Elbe in Pliny’s time. The Dalmatian Pelican belongs to a group of birds which formerly lived on the edge of the Sarmatic inland sea which in Pleistocene times covered much of central Europe, and most of the typical members of this fauna are now relic species, (marbled duck, red-crested pochard, Mediterranean black-headed gull). The doom of these species could well have resulted mainly from the increasing desiccation which followed the wet Atlantic period; although there have been many ups and downs since, the process continues and the remaining strongholds of these birds are vanishing, as the lakes of the Asiatic steppes dry up.

One of these climatic fluctuations led, around 500 BC, to a much wetter period and probably gave an indirect boost to agriculture: Trow-Smith suggests that the resultant growth of scrub on the lighter cultivated soils forced the Celtic immigrants of the late Bronze Age to attend to the heavier lands, and stimulated the invention of the first early plough, the aratrum. Thus the irregular plots of neolithic farmers were replaced by straight furrows and the rectangular Celtic fields. Later, Belgic immigrants in about 75 BC brought the heavy plough caruca which resulted in the long Belgic strip cultivation, the precursor of the medieval system, and opened up land too intractable for the prehistoric plough. The Roman occupation extended the scope of agriculture but did little to improve techniques and it seems unlikely that these peoples did much to affect significantly our forest wild life. The population at this time was probably around a quarter million and very much concentrated in south-east England and East Anglia. The Romans, however, with their dykes and causeways did start the settlement of the Lincolnshire and Cambridgeshire fens. Land development was hindered by the arrival of the Anglo-Saxons, although they continued the drainage of marshes and used them as sheep pasturage. The process was in full swing between 1150 and 1300 and eventually led to the loss of the larger fenland species. The crane disappeared in about 1590, though the spoonbill hung on until 1667 on the Orwell at Trimley, Suffolk. Those other wetland birds, the bittern, Savi’s warbler and black tern, remained until the early nineteenth century (they have now returned following intensive conservation efforts).

The Anglo-Saxons and their medieval descendants were free to catch what birds they liked for food and their main problem was the best means to adopt. Some idea of these methods can be gleaned from the Colloquy of Ælfric, a tenth-century educational dialogue between master and pupil quoted by Gurney.

Magister: What say you, Auceps? How do you beguile birds?

Auceps: I beguile birds in many ways; sometimes with nets, sometimes with snares, sometimes with bird lime, sometimes with a call, sometimes with a bow, sometimes with a decoy.

With William the Conqueror came many of the inconsistencies which still dictate the relationships between birds and men. William introduced the special hunting forests where only the privileged could take the better game animals, and the harsh, unjust forest laws to enforce his ideals, foreshadowing the Game Act of 1831. With William we see the beginnings of those illogical attitudes which defined sport as the prerogative of gentlemen, as distinct from such plebeian pursuits as shooting sitting birds, which though much more humane, were associated with the illicit movements of the poacher or serf. Sport in Norman days included hawking, first introduced to England about AD 860, stag-hunting, hare-hunting and greyhound coursing; and the idea developed of game as animals worth eating and thus to be preserved. Deprived of hunting rights, the husbandman had to find other means for amusement, and cock-fighting provided one outlet. In ages when petty pilfering carried the death penalty and the sanctity of human life counted for little, this must have seemed an innocuous enough sport. Sir Gervase Markham (1638) gives a good account of cocking: ‘then you shall take his wings and spreading them forth by the length of the first feather of his rising wing clip the rest slipe wise with sharp points, that in his rising he may therewith endanger the eye of his adversary: then with a sharp knife you shall scape smooth and sharpen his spures.’ Cock tournaments did not become illegal until 1849, and even today occasional legal proceedings are reported against some mains in north-east England. Quite apart from the appeal of betting, cock-fighting attracted a big following of people roused by sado-masochistic impulses – just as today many watch wrestling and boxing matches who can hardly be all that interested in the arts of self defence. Even at the beginning of the nineteenth century, fashionable ladies could foregather at Hurlingham to watch the gentlemen shoot pigeons released from cages; today trap shooting is harmless because clay discs replace a bewildered bird.

Inevitably there was pressure to find alternative meat supplies, to supplement the vagaries of hunting and to guarantee winter meat, and domesticated and semi-tame birds helped satisfy this need. But from William’s time a host of rules and regulations restricted the number of persons able to enjoy such privileges. Richard II passed a statute forbidding artificers, labourers and persons not having lands of the value of 40s. a year from keeping greyhounds, or using ferrets for taking deer, hares or conies and other gentlemen’s game. Edward IV enacted that, except for the sons of the King, only freeholders of land above a certain value could mark swans; swans on land below this value could be seized for the King.

Most north temperate birds have their breeding season proximately controlled by seasonal changes in daylength, and the deposition of post-nuptial migratory fat and other physiological events also have a photoperiodic basis. This was discovered empirically long ago and led to the practice of mewing – putting birds in the mews usually reserved for hawks, and then subjecting them to artificial light regimes. By being given extra light, muiten birds in Holland could be brought into breeding condition and made to sing in autumn, thereby serving as excellent decoys for the bird catchers trying to attract passing migrants. In Britain, the earlier process of keeping birds in the dark to fatten them was known, and to save on mew space it proved quicker simply to sew up the eyes of cranes and swans: the modern poultry farmer is doing nothing new in principle in regulating the light regime for his battery hens.

The Japanese and Chinese had learned in the fifth century AD that cormorants could be trained to catch fish for their masters – a fascinating use for which these birds are still employed in the East. Knowledge of the practice reached Europe and James I and Charles I both employed a Master of the Cormorants, who, at the royal bidding, travelled widely to display the skill of his charges.

Stone Age man had doubtless learned to take advantage of birds that had regular and predictable habits which made them easier to catch. The flightless condition assumed by wildfowl after breeding must early have attracted attention. Suitable ponds were being turned into decoys by the reign of King John (1199–1266) and were equipped with netted pipes into which the birds were driven, especially flappers in July. The more sophisticated Dutch technique, whereby dogs were employed to appear from behind a series of screens to arouse the curiosity of the ducks and so entice them up the pipes, was not introduced to Britain until the sixteenth century.

It was soon realised that certain dogs were more effective than others; those most resembling a fox in colour and shape seemed particularly useful, presumably because ducks have evolved responses to stimuli originating from natural predators because these have biological significance. The last commercial duck decoy at Nacton in Suffolk has only just ceased operating for profit and has been acquired by the Wildfowl Trust as a centre for the ringing and scientific study of wild duck. The regular migration of wood-pigeons through the valleys of the Pyrenees has enabled the development of a local trapping enterprise by the Basque bird catchers. Long nets are strung across the valleys into which the pigeons are decoyed by men stationed in towers sited along the valleys. These men throw white bats resembling table tennis rackets under the passing flocks, and the white flash stimulates a following response in the birds, causing them to fly lower and lower and finally into the nets.

The role of birds in culture up until Norman times followed a primitive urge to increase the fertility and ease of capture of prey, and was based to a large extent on superstition, fable and omen; the drumming of woodpeckers caused them to be accredited with rainmaking abilities and black-coloured birds were associated with the devil. The Norman love of jousting and tournament combined with twelfth- and thirteenth-century religious influences to produce the heyday of heraldry – an extension of mystical symbolism, both in a military form with such subjects as the eagle and owl, and with the Christian dove and pelican. Bird art in Britain was also much in the hands of the ecclesiastics, and as Tudor points out, certain birds still featured mostly in religious symbolism, as they had in ancient Egyptian civilisations: the hawk was the emblem of Horus, the Sacred Ibis that of Throth, and they were much used for decorating illuminated manuscripts, psalters and breviaries. Between the Middle Ages and the late Baroque, and particularly in the Florentine schools of art, small birds occur as accessory symbols in hundreds of devotional paintings, over three-quarters being goldfinches. Gold wings and the bird’s association with thistles (crown of thorns) made it a symbol of the resurrection and of the soul, but it has also featured as an augur of disease, particularly plague. Friedmann, who tells us this, notes that in nearly all cases the artist has depicted a docile bird; only Michelangelo (in a marble relief now in Burlington House) showed it in a realistic, albeit symbolic attitude fluttering and scaring the infant Christ. The freer use of birds in art came with the Renaissance – but that is outside the scope of this book. Readers interested in the role of birds in music, painting and literature are recommended to read the excellent pen picture provided by Fisher (1966) or specialist works, a few of which are mentioned in the bibliography.

Our knowledge of ornithology in the fourteenth and fifteenth centuries remains extremely scanty, though it is unlikely that man was doing much harm to wild life, especially after the ravages of the plague had reduced the population by nearly half. In 1348, when the Black Death started, the priors of the Monastery of Durham were finding rooks and crows sufficiently numerous on their manors to require thinning and they were enjoying young rooks in season. James I later passed an Act in Scotland in 1424 requiring them to be kept in check. Under-population and under-privilege for most country dwellers, and the decline of the monasteries and their dissolution were associated with agricultural stagnation; in 1500 there were about three sheep to each one of 2½–3 million people. Yet in the mid-sixteenth century emerged the farmers of foresight, who set the pattern of agriculture for years to come. Sir A. Fitzherbert’s Book of Husbandry published in 1534 shows that his knowledge of land drainage and preparation, and of the culture of corn, could set him on a par with the modern barley barons. Both he and Thomas Tusser were among the earliest advocates of enclosure, both emphasising the wastefulness of the old open field system which prevented the development of winter corn growing. Tusser (1573) writes:

The flocks of the lords of the soil,

Do yearly the winter corn wrong,

The same in a manner they spoil,

With feeding so low and so long

And therefore that champion field,

Doth seldom good winter corn yield.

It may be that this early enclosure was creating the right conditions for the proliferation of the corvids. Henry VIII passed an Act which having specifically stated that choughs (jackdaws), crows and rooks had increased and were injuring corn, directed that all persons possessing land were to destroy them. Parishes of at least ten households had to provide nets and facilities for catching the adults and meet annually to agree how best to destroy all the young. Much of the content of this Act was revived in another passed by Elizabeth I, in which provision was made for destroying ‘noyfull fowls and vermin’, including the corvids, buzzard, kite, osprey, harrier, woodpecker, kingfisher, shag, cormorant, bullfinch; and authority was given to the churchwardens to pay a bonus for destruction of these birds, ranging from one to four pence. The money came from taxing the landowners. Henry, or his advisers, clearly had the wits to realise that bonus schemes can be abused, because it was expressly forbidden to make payment for these birds if taken in any park, warren or ground employed for the maintenance of any game of conies, or for any ‘stares’ taken in dove-houses or kites and ravens killed in and around towns; but we do not know how these rules were enforced. A Government subsidy of one shilling and later two shillings was being paid on the tails of grey squirrels until stopped in 1958, and there were those enlightened gentlemen who trapped the animals, cut off their tails and released the creatures in hope that the progeny would provide further remuneration.

The era of the country house dawned in the sixteenth century and ornamental estates and parks blossomed in the seventeenth, and it became increasingly fashionable to keep ornamental birds. Aviculture was not new, the Romans having long before indulged their love of keeping exotic animals in captivity, one result being the introduction of the pheasant Phasianus colchicus to Britain. Increasing travel and interest in science led in turn to more attention being paid to wild life. Charles II had his pelicans and other birds in St. James’s Park, and the climate of opinion in which he founded the Royal Society was appropriate to experimental science, and led to the introduction of various exotic plants and animals, including the red-legged partridge (unsuccessful introduction attempted in 1673) and Canada goose. The rise of the squirearchy was also marked by the appearance of various game laws in the Restoration period, forcing the farmers and yeomen to make unpopular sacrifices in order that the squire could shoot, just as the King by means of the forest laws had supplanted all classes in preceding ages. The cross-bow, firing metal bolts, had been the important weapon until this time; but now the shot-gun replaced hawking as the gentleman’s means of relaxation. It became more common during Charles II’s reign, though not without much controversy, to shoot pheasants in flight instead of simply stalking perched birds or walking them up with dogs. Birds were caught for food with cross-bow, lime, snares, traps and nets, and nets were even used in sport. With the development of the shot-gun, it was gradually agreed that only certain species should be recognised as suitable for sport and be classified as game, and grouse and black cock were allotted to this category. The breech loading gun brought more controversy, just as double-barrelled guns and American repeaters were to do later, but it meant that birds could be driven over the guns and grouse shooting could emerge in about 1855, ensuring that from then on the nation’s parliamentary business finished in time for the glorious 12th August. Actually in 1915, a bill was passed in the Lords authorising the shooting of grouse on 5th August, so that those for whom the war gave less spare time could be ensured their full share of enjoyment. It is to the credit of the Commons that they rejected the bill, amidst shouts of ‘we want to shoot Germans not grouse’. So much have the priorities of men and birds become confused.

The arbitrary rules which grew up in connection with shooting are of interest and importance because they still govern much of our present-day approach to pest control. I have a good friend with a lifetime’s experience of both country sporting traditions and of pest control who was appalled at the suggestion that pigeons should be shot when sitting on their nests. He preferred to flush the bird first before shooting, in order to give it a sporting chance, despite the fact that my suggestion would give much greater prospects of a quick and humane kill. Game preservation and shooting have brought out some of man’s worst attitudes to wild life. In the early days of the big country estates, poachers and gamekeepers were often engaged in open warfare; if hideous traps could be set for vermin it presumably seemed logical for man-traps to be set for poachers. Those days left a legacy of slyness, suspicion and conservatism which has been slow to disappear. The nineteenth century witnessed the worst excesses, with the ruthless slaughter of any predatory bird or mammal, and nothing was allowed to stand in the way of prodigious bags. William Cobbett in Rural Rides (1830) was contemptuous of these so-called sportsmen – especially those who claimed big bags. ‘A professed shot is almost always a disagreeable brother sportsman. He must have in the first place a head rather of the emptiest to pride himself upon so poor a talent.’ In 1903 the Game Rearers Annual stated: ‘The hooded crows are most difficult to destroy and unless poison is used (which, by the way, is illegal) they cannot be successfully coped with. Where poison is used it is generally placed in the eyes of a dead sheep, which usually provides a fatal lure to the hoodies.’ So much for the early Bird Protection Laws. Even today it is proving a slow uphill struggle to educate the more recalcitrant gamekeepers into accepting that such actions do not automatically improve game prospects, but there has been much progress.

Increasing demands for shooting at first led to the augmentation of pheasant stocks by hand rearing, a technique which has grown enormously in the last fifty years owing to a break up of large estates, and to the formation of shooting syndicates and the attraction of city business men, helped by improvements in transport and more leisure. This in turn has led to the scientific study of game-birds and the elevation of game biology to a highly respected place in the wider field of wild life conservation and management, and was heralded by the grouse inquiry (Lovat 1911) already mentioned in the preface. Following the growth of ecology, the Imperial Chemical Industries, Game Research Station (later Eley Game Advisory Station) was formed in 1933. Its ultimate aim was of course to improve prospects for selling cartridges, but under the direction of A. D. Middleton (himself a vital force in the growth of ecology at Oxford) it has set the standards for field ecology in a much wider sense, and made possible the formation of the progressive Game Research Association in 1960, which has subsequently become the Game Conservancy since 1969. In the meantime, the study of grouse was recommenced when the Nature Conservancy inaugurated its Unit of Grouse and Moorland Ecology in 1959 at Aberdeen University. One hopes that the study and amenity of game will become even more integrated within the broader concepts of wild life management in Britain.

But we have moved on too far in time if we are to appreciate what man has done to the land as a habitat for birds. The rate of tree felling increased as the Tudor squires enclosed waste, common and woods, a process that accelerated after 1660 when the Government relinquished its opposition to private agreements negotiated by the landlords. All the same the pattern of the countryside as we see it today had only emerged in south-east England. Even by the eighteenth century there was abundant wilderness, about a quarter of the country being heath and moor, and much of the remainder was medieval with open fields of meadow and arable land adjoining gorse-covered common pasture; by 1760 less than a quarter million acres had been enclosed. One can picture the goose-girl tending her flock idly watching the goldfinches on thistle clumps or stonechats singing from some furze. Bustards were still stalking the chalk wolds of eastern England, white-tailed eagles were nesting on the Isle of Wight.

The big rural transformation came between 1750 and the General Enclosure Act of 1845, most of the open fields going in the sixty years of Farmer George III. Most of the earlier enclosures had been of common, waste and woodland, expensive to achieve but less disturbing for the community in that the old peasant system could still be based on self-sufficiency. The enclosures of the eighteenth and nineteenth centuries were based on economic necessity and were a national policy that brought much suffering and ill-will. In less than a century, they transformed peasant England into modern England. There were many protagonists. Pioneers of modern farming like Arthur Young, who did much to disseminate the latest results of experiments and supported all the agencies like farmers’ clubs, ploughing matches and sparrow clubs which helped to diffuse ideas, claimed that enclosure brought more employment in fence mending and turnip husbandry, even though sheep were lost. Young was enthusiastic but he was inflexible and lacked judgement and others were less enchanted:

The fault is great in man or woman,

Who steals a goose from off a common,

But what can plead that man’s excuse,

Who steals a common from a goose.

(The Tickler Magazine, 1 February 1821.)

John Clare in ‘The Village Minstrel’ sums up some of the changes that must have grieved the ornithologist:

There once were days, the woodman knows it well,

Where shades e’en echoed with the singing thrush;

There once were hours, the ploughman’s tale can tell,

When morning’s beauty wore its earliest blush,

How woodlarks carol’d from each stumpy bush;

Lubin himself has mark’d the soar and sing:

The thorns are gone, the wood-lark’s song is hush’d,

Spring more resembles winter now than spring,

The shades are banish’d all – the birds have too to wing.

The population of England and Wales slowly increased from about 4 million in 1603 to 5½ million in 1714, held in check by a high death-rate. The explosion occurred with the Industrial Revolution, from 9 million in 1801 to 18 million in 1851. For a while the rate of increase was held down by a high urban mortality, through epidemics of cholera and other diseases of overcrowding. Improved hygiene in the latter half of the nineteenth century removed this check and the rate of natural increase was only halted in the 1920s (it fell from 10.4 in 1921 to 3.9 in 1931) as a direct consequence of contraception but by now there were 40 million people in England and Wales, reaching 52 million in 1959.

These demographic statistics serve two functions here. They illustrate for the human population some of the principles to be discussed in the next chapter in relation to birds. In particular, although the birth-rate was high in the seventeenth century (as it was earlier) the population was relatively stable due to a high reciprocal death-rate. Population size increased in response to improved environmental resources during the Industrial and Agrarian Revolutions. The data also emphasise the pressures that man had come to inflict on the land and, by inference, on its wild life. In the nineteenth century, bustard, bittern, avocet, ruff, black tern and roseate tern vanished, and large birds of prey like the goshawk became rare. Loss or fragmentation of their habitat was not the sole reason because some have since returned, and man’s greed must be blamed to a large extent: there were those to whom the rarity of a bird made its capture imperative.

Those people living near large colonies of nesting birds have for long been able to harvest them for food. Fisher and Peterson have related how ancient sea-fowling communities in Greenland, Iceland, the Faroes and St Kilda had evolved a rational level of exploitation of the seabird colonies of gannets, fulmars and auks, ensuring that sufficient eggs or young birds were left to prevent declines in the future harvest. By trial and error they have found that about half the auks’ eggs can be collected, as these species lay repeats, but fulmars do not and are better left alone to hatch their young. In a valuable review dealing with the exploitation of the eggs of wild birds throughout the world, Cott has emphasised how the factors governing the utilisation of wild birds’ eggs are accessibility, palatability and availability. The first two are rarely limiting, but the size and concentration of the potential crop is important. Cott found that with the exception of the eggs of certain boobies, cormorants and pelicans, which are rank and fishy to a cultivated palate, there is a broad correlation between palatability, size and colonial grouping. For reasons which will become clear in Chapter 2, most adult birds and their eggs can withstand considerable cropping without the replacement potential being adversely affected, and, as with all wild food resources, the object is to achieve the highest annual cropping rate without detriment to the maintenance of a sustained yield. In the late nineteenth century this level was exceeded in species after species, with the same thoughtless greed with which the Victorians exploited other natural resources and the colonies. In 1884, 130,000 guillemot eggs were collected from Bempton (at the time, tons of eggs were sent to Leeds where the albumen was used in the manufacture of patent leather); in 1840, 44,000 black-headed gull eggs were taken from Scoulton Mere, and 89,600 puffin’s eggs were taken from St Kilda in 1876.

The adornment of their ladies with feathers was another excuse for slaughtering wild birds. Quite apart from a scandalous trade in ostrich and egret plumes, a host of seabirds were massacred within the British Isles. Kittiwake wings were in demand for the millinery trade and a large industry existed at Clovelly in Devon and elsewhere; 9,000 are supposed to have been killed on Lundy in one fortnight. Coulson (1963) gives many other examples and relates how the species was also shot for sport and food, resulting in a decline from which it did not recover until the early decades of the twentieth century.

It is against the background of serious over-exploitation that the increase of a very wide range of seabirds in the twentieth century must be viewed and several examples will appear in later chapters – the great crested grebe (see here), kittiwake (see here), oystercatcher (see here). The eider perhaps should also be mentioned. One hundred years ago it was confined to the islands on the west coast of Scotland, and at one mainland site in East Lothian and in the Farne Islands. From this very restricted range it had spread to a wider range in coastal Scotland and to the Shetland and Orkney Islands by about 1890, and it became common by 1922. The bird has always been subject to intensive cropping both for its eggs and its down. While its down could be collected from the nest after the eggs have hatched, in the majority of cases the eggs and down have been lifted together, repeat layings often being harvested as well. On the Farne Islands the eider has always received a measure of protection owing to its association with St Cuthbert, but as long ago as 1397 the Bursar’s roll of the Monastery of Durham, which contained the Shrine of Cuthbert, mentions the use of eider-down for stuffing and cushions. Several colonies in Scotland and the Farne Islands were reduced during the 1939–45 war when the birds were collected for food, but these have recovered. In addition, Tavener has documented a marked post-war rise in numbers of non-breeders round the British coast, associated with an increase in the Dutch population, and also increases in other parts of Europe and North America, all apparently resulting from protection. There has, however, been little southward extension of the bird’s breeding range in Britain in the past 20 years. More recent studies by Dr H. Milne on the Sands of Forvie Nature Reserve, Aberdeenshire, showed that local numbers increased from about 3,000 birds in 1961–3 to around 5,000 during 1964–7 and that most of this increase resulted from a particularly good breeding season in 1963. Thus home-production rather than immigration apparently accounts for eider increases in Britain at least.

The excesses of the late nineteenth century roused the passions of a few men, and probably more women of suffragette spirit, and their campaigning led to the first Seabirds Preservation Act of 1869, hopelessly inadequate in its conception but a step in the right direction, to be followed by the Protection Acts of 1880–96. This same climate of moral indignation also led to the formation of the Society (later Royal Society) for the Protection of Birds in 1891. The Society went from strength to strength in the vanguard of the more enlightened attitude to birds characteristic of the start of the present century, doing immense good for ornithology, albeit sometimes for the wrong reasons. Today, the R.S.P.B. stands in the forefront of a scientific and imaginative approach to bird conservation.

With the outbreak of the First World War a new and widespread appreciation of birds was apparent. Birds were still used, but in an atmosphere of greater affection and regard. Canaries are about fifteen times more sensitive to poisonous gases than man, and they were accordingly kept in cages in the trenches to give advance warning of a gas attack, just as coal miners had used them in the mines. Soldiers enjoyed their companionship, and singing birds also were extensively used in ambulance trains. As pigeons had relayed the conquest of Gaul to Rome and had brought the first news of Napoleon’s defeat at Waterloo to England, so they were put to extensive use in the war, old converted London buses being used as mobile pigeon lofts. In the Second World War, pigeons were again used extensively; for example, the underground movement in France employed them to send back messages to England. German gunners tried to shoot these birds down as they crossed the Brittany cliffs, and in Britain the authorities attempted to exterminate our south coast peregrines, for fear that they too might be successful in intercepting some vital message. The following appeared in The Times on 19 August, 1943:

A pigeon, released by a bomber crew from their rubber dinghy, has recently been responsible for their rescue in the Mediterranean. … Realizing that something had gone wrong when there was no response to their first S.O.S., they had released their carrier pigeon from its container. As soon as the message it carried had been deciphered, an air-sea rescue launch put out, and the airmen were safely rescued.

In contrast with a report for 26 September, 1969:

‘In what was called “the craziest strike of all”, 44 men went on strike for half a day at the giant pressed steel Fisher Body plant in Swindon because of low-flying pigeons. The dispute was the latest of a series of labour stoppages which is costing Britain’s car industry the loss of millions of pounds in exports. The workers who walked out were those who were caught in the crossfire of the “feeders”, men who have been scattering bread crumbs and other bits of food to encourage the pigeons, and the “whangers”, other workers who have been throwing nuts and bolts and other missiles to scare the birds away. “We were fed up with either being hit by nuts and bolts aimed by the whangers, or strafed by the pigeons diving for food,” a press operator said. Meanwhile, a works management committee has asked the “whangers” to stop throwing missiles and the “feeders” to eat their sandwiches themselves.’

Some of the major developments in the relationships between men and birds since these years have been very briefly mentioned in the preface, but much more could be added for which there is no space. It would be pertinent to follow the growth of pet-keeping and other manifestations of an increasing public interest in living things. Similarly, we might examine the post-war boom in bird watching and the significance of such discoveries as the number of rare birds visiting sewage farms; the stampede to such sites was like an ornithological ‘gold rush’ once the initial discoveries at Nottingham were disclosed (Staton 1943). Has this widespread interest in wild life come too late?

The main hazards to birds today arise from the increasing pollution of the environment with the waste resulting from the sheer numbers of man; ironically man has become an indirect threat to the survival of wild life just when he seems to have learned to appreciate it. Mellanby has recently dealt with the pollution problem so admirably that I shall not attempt more than a passing reference here. But this should not detract from the severity of problems about which new facts emerge almost daily. For instance, we are still treading extremely cautiously so far as certain persistent agricultural chemicals are concerned. When the threat to wild life from organochlorine insecticides became really apparent in 1960–1, there was all manner of special pleading; agriculturalists claimed that food production must be the over-riding concern, manufacturers of chemicals naturally enough belittled the hazards to birds, bird protectionists made exaggerated claims, and Rachel Carson did the public at large a service with her deliberately biased book. In the years that have followed the initial hysteria, good sense has prevailed. Research has got under way and is producing facts where before there was only conjecture. When bird watchers first pointed out that birds of prey were breaking and eating their eggs and that this seemed to be associated with the decline in numbers associated with toxic chemicals, their suggestion was greeted with scepticism by many people. Now, a careful study by Ratcliffe has suggested some answers, for he has demonstrated that there has been a decline in egg-shell thickness in the peregrine, sparrowhawk and golden eagle since about 1950, and this explains why eggs are broken more readily and are subsequently eaten by the parent birds. Thus, of 109 peregrine eyries examined between 1904–50 there were only three instances of egg breakage, compared with 47 in 168 eyries observed between 1951–66. Egg-breakage, decrease in egg-shell weight, status of the breeding population and exposure to persistent pesticides are correlated, and it would be unreasonable to suppose that there is no causal connection, even if the exact factors and mechanisms involved remain to be elucidated. Calcium metabolism in birds is controlled by oestrogen and parathyroid hormone, and there is evidence that pp D.D.T. will interfere with oestrogen-based mechanisms in the Bengalese finch (Jefferies 1967).

Pollution of the environment by persistent, poisonous chemicals, is the most obvious problem, firstly because small residues can be accumulated in food chains, to give lethal dosages to the top predators, and secondly because they may produce unsuspected side-effects. But pollution by detergents, oil, smoke and other waste from man also present grave problems. Oil spillage at sea, either accidental or resulting from the purposeful jettisoning, is a serious hazard to seabirds against which the International Committee for Bird Preservation and other bodies have long campaigned, to a large extent successfully in the sense that the problem is recognised internationally. Bourne (1968), in a valuable review of the subject, mentions that as long ago as 1907 the largest seven-masted schooner built, the Thomas W. Lawson, was wrecked on the Isles of Scilly on her maiden voyage. The release of her entire cargo of ‘two million gallons’ of crude oil caused a vast slaughter of local seabirds, particularly puffins. In those days Annet is supposed to have supported about 100,000 puffins, whereas to-day only about 100 remain. All the colonies in the Western Approaches have been similarly reduced (as Parslow 1967 has shown) and it seems likely that oil pollution has been a major cause.

The loss at sea of oil-carrying vessels during the 1914–18 war resulted in a large increase in the numbers of oiled seabirds. This led the Royal Society for the Protection of Birds to publish figures in 1921 which played a large part in the introduction of the ‘Oil in Navigable Waters Act’ soon after. In the Second World War most tankers carried petroleum spirits, and the destruction of shipping presented less hazard. But this situation has changed as the needs of a modern industrialised world have led to an enormous expansion in oil traffic at sea; nowadays crude oil is carried to refinement plants near the destination in giant tankers. Constant pollution arises from ships washing-out at sea after a voyage and purposely releasing oil, while occasional accidents can have widescale repercussions. Thus, during the night of 18 September, 1966, the German tanker Seestern allowed 1,700 tons of crude diesel oil to escape into the Medway Estuary on a flood tide, polluting 8,000 acres of saltings. Probably about 5,000 birds died immediately. Certainly, 936 black-headed gulls, 927 great black-backed gulls, 184 dunlin, 165 herring gulls, 135 redshank, 98 common gulls, 90 oystercatchers, 65 curlew and various other birds (including an American pectoral sandpiper) were picked up. In this case the number of birds using the area had declined, by 20–100% depending on the species, in the following winter, but has since recovered so possibly no permanent damage has been done, yet for years an area could be denuded of suitable plant and animal food for birds and other wildlife. Only a small oil slick of about 87 tons hit the Tay estuary, the most important wintering ground for eiders, in March 1968. Fortunately, most of the birds had left, but between 7–26% of the national eider population was wiped out: the happy state of affairs pertaining to eiders and described above could soon be reversed. These modern incidents compare with R.S.P.B. estimates made for the 1940s and 50s that between 50,000–250,000 birds were being killed per year in home waters.

On 18 March, 1967, the Torrey Canyon ran on to the Seven Stones Reef and in all about 60,000 tons of crude oil were lost into the sea. At least 10,000 birds were collected, and many more must have died unbeknown, of which 9 out of 10 were guillemots. In the aftermath we now know (see special review by Bourne 1970, and Parslow 1967) that guillemots at some breeding colonies in Scilly and the north Cornish coast have been considerably reduced; in Cornwall, but not Scilly, fewer shags and herring gulls were breeding in 1966; kittiwake numbers at a colony within the worst polluted area were reduced in 1966. Only a few razorbills and apparently no puffins suffered in Britain, though these last are difficult to census. This was not the case in the Sept Iles in Britanny, the most important seabird colony in France, where careful protection had allowed a recovery in seabird numbers following the persecutions of the last century. While aerial species such as the gannet had escaped damage, such divers as the shag had suffered markedly and the auks very severely: counts before and after the incident show that the number of pairs of guillemots fell from 270 to 50, of razorbills from 450 to 50, and of puffins from 2,500 to 400. This is the first incident where really detailed knowledge has been available, making a fairly comprehensive ecological survey possible so that wildlife interests are given more than passing regard. The public has been aroused at the prospects of ruined beaches and the ‘overriding concern of the Government throughout has been to preserve the coasts from oil pollution and to adopt a course most likely to achieve this end’. Unfortunately the enormous quantities of detergents used for this purpose have done vastly more immediate and probably long-term harm to intertidal organisms than the oil itself. Whatever the pros and cons of the whole sad story, it illustrates the dilemma man finds himself in today. In some fields his technology has progressed far too quickly, while in others it has lagged, so that when accidents occur he is too often forced to resort to ill-conceived panic measures. Since this book has been in press there has been another major wreck of auks, this time in the Irish Sea in August 1969. At first attributed to gales, it seems that many birds came ashore under conditions of not particularly unfavourable weather. Analyses show the bodies to contain high levels of polychlorinated biphenyls, agents used in the paints and plastics industry. This new chemical hazard, unrealised when Mellanby prepared his book, highlights the complexity of the interaction of man and wildlife and the need for drastic measures if man is to avoid becoming the ultimate victim of this extensive environmental pollution, which wildlife is indicating.

CHAPTER 2

ECOLOGICAL CONSIDERATIONS

A THOROUGH insight into the relationships between birds and humans demands some understanding of population ecology, a knowledge of how animal numbers fluctuate and change through births and deaths and of the factors which determine these processes. Populations have dynamic properties and these cannot be neglected by people concerned with wild life management, whether as game preservers, conservationists, pest controllers or farmers. This chapter attempts to set out some of the basic principles with pertinent examples, but these same principles will emerge in later sections in various guises. Our knowledge of the subject was first collated and clearly enumerated by Dr D. Lack (1954) in a stimulating book The Natural Regulation of Animal Numbers since when more field studies have been made by various workers, which Lack (1966) has summarised in his Population Studies of Birds. Both books should be consulted by the reader who is really interested in this subject. In this account I have drawn on examples which, wherever possible, have direct relevance to economic ornithology.

Farmers tend to the pessimistic belief that all problem birds become more abundant every year. Yet careful counts of most such species living in stable environments usually show there to be no clear tendency towards a steady increase, or even decrease, though numbers may fluctuate from year to year. For instance, many farmers believe that the wood-pigeon has increased drastically during this century to become more of a pest, though in conditions of stable agriculture this is unlikely to be true. In fact, there is evidence of a decline since the early 1960s associated with a reduction in the acreage of winter clover-leys and pastures. Certainly the species has moved into newly developed marginal land; places like the east Suffolk heathlands, which have been ploughed and claimed for agriculture since the Second World War, have been colonised by wood-pigeons. Fig. 1 gives some indication of how wood-pigeon numbers have varied on a Scottish estate near Dundee since 1887, and it is evident that fluctuations have occurred within narrow limits, with no evidence at all for a sustained rise or fall. In contrast, Fig. 1 also shows how the closely related stock dove has increased over the same period; this species first colonised in Scotland in 1866, reached Fife in 1878 and increased dramatically in Scotland in the next ten years. Every year each pair of wood-pigeons rears on average just over two young and it is evident that if all these survived to breed in their turn, population size would increase exponentially. That this does not normally occur indicates that some form of regulatory mechanism must be operating. Furthermore, this regulation must be density-dependent, that is, it must become proportionately more effective at high population densities and proportionately less effective at low ones. If the regulatory factor (s) operated without regard to density, it is evident that population size could fluctuate widely without reference to a particular level – to the constant mean represented by the dotted line in the figure.

FIG. 1. Annual number of wood-pigeons and stock doves shot on an estate near Dundee. The data refer to an area which remained virtually unchanged during the period under review, and nearly all the birds were shot by one man who maintained a reasonably constant shooting pressure. They, therefore, probably provide a fair index of the total population. The autumn of 1909 was a disastrous one for the harvest owing to gales and rain from August until October so that much corn remained uncut. This resulted in an influx of wood-pigeons and the appearance of stock doves in large numbers. (Data by courtesy of Dr J. Berry.)

FIG. 2. Logarithmic increase of the collared dove in Britain. (Data from Hudson 1965).

Sometimes bird numbers do in fact increase geometrically, as when a species moves into a previously untenanted region where there is scope for it to live; in biological terms where a vacant niche exists (see below). In this way the collared dove dispersed dramatically across Europe to reach Britain in 1952. It had spread to south-east Europe from northern India by the sixteenth century but had then remained static in its European outpost until 1930. It has subsequently spread north-west across Europe, reaching Jutland in 1950 and Britain in 1955; an expansion 1,000 miles across Europe in twenty-four years. Why this spread was so long delayed is not clear, though it is most feasible, as Mayr has suggested, that a genetical mutation occurred which suddenly rendered the species less restricted in its needs. (We can imagine a bird, though not necessarily the collared dove, to be restricted by a temperature tolerance which a single sudden genetic mutation could remedy.) Throughout Britain and Europe a vacant niche existed for a small dove living close to man; it is even possible that the decline in popularity of the dove-cote pigeon created this niche. Whatever the explanation, a high survival rate among collared doves has evidently been possible, and their potential capacity for geometric increase has been realised: see Fig. 2, which relates to Britain. The Syrian woodpecker may be on the brink of a similar explosion. It is considered to be a recently evolved species (post-glacial) which replaces the great spotted woodpecker in south-east Asia. It spread to Bulgaria to breed in 1890, to Hungary in 1949 and to Vienna in 1951. A significant feature of the bird’s ecology in Europe is its confinement to cultivated areas which do not suit the great spotted woodpecker. If it proves to be better adapted to this man-made niche we can anticipate a continuing advance across Europe. Agricultural development in the Balkans may well bring other surprising range extensions.

Accepting* that populations are controlled in a density-dependent manner the next question to consider is the nature of these regulatory processes. Fundamentally, either the birth-rate or death-rate must be the factor of change. Wynne-Edwards and his supporters have argued that the reproductive rate is of much importance and that those animals with a high expectation of survival, such as many seabirds, have evolved low reproductive rates and vice versa. They have also claimed that a host of conventional behaviours have evolved as a means of regulating numbers. For instance, Wynne-Edwards regards the eating of eggs and young, practised by many raptors and also storks, as a device to limit their reproductive output; deferred maturity (gulls do not breed until three or four years old), territory formation, and various other behaviours are similarly regarded in this light. These views form part of his more general thesis that animal numbers in undisturbed habitats are at an optimum density and that maintaining this optimum has selective advantages. Special cases of this theme have attracted various supporters. E. M. Nicholson (1955) is one and he ascribes population control to density-dependent movements, rather than to mortality, while Lidiker (1962) goes further in seeing emigration as a mechanism enabling populations to have densities below the optimum carrying capacity of their habitat. Hence, Wynne-Edwards regards behaviours such as those listed above as homeostatic (self-regulatory) mechanisms, being induced by the population rather than by the environment.

Wynne-Edwards also drew parallels between bird and human populations, claiming that infanticide, taboos and other methods of reproductive restraint practised by so-called primitive societies were extensions of these same deep-rooted animal behaviours. In so doing he resuscitated some very early ideas of Carr-Saunders (published in 1922) which the well-known demographer did not subsequently repeat. Indeed, Mary Douglas, in criticising the idea of an optimum population, points out that there are many examples of human under-population. Considering certain primitive societies in detail, she makes it clear that they represent highly evolved and complex groups about which generalisations are meaningless. For instance, the Netsilik Eskimos live in a harsh environment where males suffer a very heavy mortality in hunting, and female infanticide is practised primarily to maintain an even sex balance. The Ndembu, a Lunda tribe in Zambia, grow cassava as their staple crop and live at a density of 3–5 people per square mile, whereas cassava could support a density of 18 people per square mile. The reason for the discrepancy is that the tribe passionately love hunting and move their villages to where game is available so that they never reach a stage where they are up against the ceiling imposed by their basic resources. As Douglas says: ‘they live for the oysters and champagne of life not the bread and butter.’ On the other hand, the Rendille are a tribe of camel herders in Kenya and live on the meat and milk of their sheep and camel herds. An optimum number of people is needed to maintain the camel herd, and when smallpox reduced man-power, stock had to be lost. In more normal conditions a balance of man-power is achieved by emigration, monogamy (herds are not divided but go only to the eldest son), while a measure of infanticide is practised (all boys born on Wednesdays).

In general terms, homeostatic population control in human groups depends on limited social advantages (the enjoyment of hunting by the Ndembu; education, motor cars and social prestige in western Europe) and not on any relationship to resources per se, as is the case with infra human species. Douglas could well be right in not attributing the increase in the Irish population between 1780–1840 to the adoption of potatoes as a staple diet, but rather to the ruination of Irish society by penal laws and English trade tariffs. Similarly she argues that the miseries of enclosures and the Poor Laws resulted in the population explosion which led to the Industrial Revolution, rather than that increased resources stimulated an increase in population. If true, this is quite different from the way in which animal populations are determined.

Animal populations cannot be directly compared even with the supposedly most primitive of human ones. Moreover, unequipped with any cultural tradition and means of communication, some special mechanism would have to be found to account for what amounts to altruistic behaviour. The Darwinian viewpoint is that if two animals (of the same or different species) are in competition, the one able to maintain the highest reproductive output must succeed at the expense of the other, all else being equal. This is the meaning of natural selection, which amounts to individual selection. What then would prevent the genes of parents which practise restraint from being at first swamped and then lost in competition with less socially inclined individuals? Wynne-Edwards surmounted this problem of explaining how individuals could acquire genes which cause them to behave in socially advantageous ways, sometimes at their own expense, by invoking the concept of group selection. In other words, in many situations survival of the group is more important than survival of the individual. Without entering into detail it must be said that the genetical basis for group selection is very restricted, and it can only be shown to be feasible in small isolated populations (birds do not satisfy the requirements of isolation as envisaged by geneticists) or in those, such as the social insects, where numerous genetically identical individuals are produced from one female, for which the term kin-selection is more appropriate. It seems to me that all the examples given by Wynne-Edwards can be answered (or will be when knowledge accumulates) more satisfactorily by individual selection and that to introduce group selection is unnecessary. This applies to two situations which I have studied closely – the peck order in birds and stress disease. I myself, therefore, reject the concept of an optimum population in this sense, together with the view that animals impose their own control over population increase. I have discussed the subject at some length because it defines my approach to the subject of applied ornithology. So far as reproduction is concerned I fully support Lack’s thesis that the reproductive rate has evolved as the highest possible; selective disadvantages follow from the production of more or fewer young than the optimum number. Disadvantages which accrue from overproduction include reduced survival chances for the offspring because they are undernourished, or impairment of the parents’ health; underproduction leads to a failure in intra-specific competition with more fecund individuals.

As most birds produce a large excess of young, the total post-breeding population increases considerably, two-fold in the wood-pigeon, up to six-fold in the great tit, but by less than half in the fulmar. Various mortality factors, including disease, predation and starvation, now remove surplus individuals so that a balance with environmental resources is achieved, and a pattern of sharp fluctuations within each twelve-month period is superimposed on more subtle changes in the breeding population from one year to another. Lack has argued, and again I agree with his views, that the food supply is usually, but not invariably, the most important factor affecting these annual fluctuations in birds. While food could ultimately be the most important factor in all cases, other agencies, for instance predators, may hold numbers below the level which would be imposed by food shortage. The most important fact to appreciate from the viewpoint of economic ornithology is that causes of death are effective until the population is in balance with the environment, and in the absence of one such factor another will take its place. Conversely mortality factors are not usually additive – in the sense that two together do not decrease numbers more than one alone. The degree of stability seen in a population will, therefore, depend primarily on that of the environment and not on any essential characteristic of the population. By environment we not only mean the food supply but include all the other components, biotic and physical, which may interact to cause competition and mortality, directly or indirectly.

Blank, Southwood and Cross have recently shown in a neat but semi-mathematical way how the various causes of mortality contribute to the regulation of a partridge population on a Hampshire estate which was studied from 1949–59. I shall illustrate less elegantly certain aspects by using as an example another partridge population living on a Norfolk estate, for which details of annual fluctuations in numbers have been given by Middleton and Huband (Table 1). Following breeding, numbers increase over tenfold but there immediately follows a period during which it is mostly the young which are lost, so that from a post-breeding average of nearly ten chicks to each adult the ratio falls to only 1.5 per adult by August, most of this chick loss occurring in the first few weeks of life. Jenkins (1961) had earlier claimed that this heavy loss of young, which is particularly heavy in cool, wet and windy summer weather, was the main variable determining the number of partridges later available for shooting. This was confirmed by Blank et al., who emphasised chick loss as the major contributor towards the total mortality occurring each year in partridges, and as the most important determinant of the September ratio of old to young. They found that about half of the variation in total mortality was due to fluctuations in chick loss, but that about half this chick mortality was unrelated to the size of the population at hatching. This means that the survival of chicks was partly responsible for regulating the autumn population (in the sense that autumn numbers were largely governed by how many young survived). However, because this survival was only partly determined by population size there was a margin of production which caused autumn numbers to fluctuate partly independently of population density. Thus 35% of the year-to-year fluctuation in the September population resulted from variations in chick mortality. In further studies Southwood and Gross were able to show that 94% of the variation in breeding success (they used the ratio of old to young birds in September as a measure of breeding success, this also being a measure of chick survival as explained above) could be accounted for by variations in general insect abundance in cereal and forage crops in June. They measured insect abundance by the use of suction traps and showed that the insects sampled were in the main those eaten by partridges, judged by analysing the crop contents of chicks.

The number of partridges finally surviving to breed in March depends on mortality factors operating in the winter, shooting being the most important. Shooting is contrived to operate in a density-dependent manner with proportionately more birds being shot when numbers are high, but this is, of course, an artificial situation which masks the effect of natural regulatory agents. These last are likely to reside in the nature of the environment, the amount and type of cover which in turn influence territory size and may result in surplus birds emigrating. They are considered in greater detail below (see here) as they are involved as factors determining long-term changes in population size as distinct from annual fluctuations. To summarise, variations in chick survival dependent on arthropod food supplies are responsible for the marked ups and downs of partridge numbers from year to year. Density-related variations in chick survival, together with density-dependent winter losses, are responsible for keeping the spring breeding population within relatively narrow bounds from one year to another. Nevertheless, there has been a general long-term decline in this level which we shall consider below. It is to be noted that pre-hatching factors that influence the viability of eggs (the ability of the female to lay down yolk reserves could depend on spring food supplies and influence the viability of any eggs she laid), the hatching success of eggs, or any other cause of mortality, were not related to population size nor to variations in total mortality.

For the figures given in Table 1 it can be ascertained that the number of adults breeding in any one year was not at all related to the number breeding in the next year. In other words, a small breeding population could be followed by an increase or decrease in the following season and vice versa. But, as Fig 3a. shows, the percentage change in breeding population from one year to the next was positively correlated with the autumn ratio of young/old, this in turn depending on the survival of young during the summer. In Norfolk this post-breeding chick loss was not correlated with the size of postbreeding population. Thus for two quite separate populations of the grey partridge the major cause of changes in numbers from one breeding season to the next has been the death-rate of young in the summer months; when this has been low, breeding numbers have tended to increase. Annual differences in reproductive output have not contributed to the changes. In the Hampshire study the summer loss was dependent on the total partridge density, and this supplied the necessary regulation to keep fluctuations within relatively narrow limits. In addition, as we shall find, both populations experience density-dependent losses in winter, but the absence of any density related loss in summer among the Norfolk birds could be associated with the long-term decline this population is experiencing. (Fig. 26, see here.) However, this last suggestion needs corroboration.

FIG. 3a. Percentage change in numbers of grey partridges between successive breeding seasons (abscissa) related to the corresponding autumn ratio of number of juveniles per adult (vertical scale). The correlation coefficient is statistically highly significant with r₁₁ = 0.822.

3b. Percentage change in numbers of red-legged partridges between successive breeding seasons (abscissa), related to the corresponding autumn ratio of number of juveniles per adult (vertical scale). The correlation coefficient is not significant with r₁₁ = 0.367. (Data derived from Table 1, from Middleton & Huband 1966).

Those mortality factors which, like chick loss, affect the size of the actual population, have been termed ‘key factors’ because they provide the key to predicting future population size, and are responsible for the year-to-year fluctuations in numbers. Perrins’s work on the great tit and my own studies of the wood-pigeon had earlier demonstrated that, in these two species, the major factor influencing changes in numbers from one year to the next is the survival rate of young after the breeding season, not the reproductive output itself nor the adult loss. Juvenile survival has been proved to depend on the food supply in the case of the wood-pigeon, and there is good reason to believe that it is also involved in the case of the great tit. However, for both these species, and unlike the Hampshire partridges, juvenile mortality is not seen to be density-dependent (at least not with the data so far available) and so some other cause of death could be responsible for regulating the populations, in the strict sense of the term. Lack suspected, for the great tit, that winter losses in relation to food supply may provide the critical density-dependent mortality, but emphasised the difficulty of proving this point. The problem is that food stocks and bird numbers vary considerably in different years. It may happen that if food stocks are high, bird numbers can be already low or high so that in neither case is any compensatory mortality required. It is difficult to isolate such effects, because in field studies it is not feasible to examine a variable number of the animals in relation to the same food supply each year. Failure to obtain the required data does not in this case invalidate the theory. It should be mentioned that a population will be regulated even though most of the deaths which occur are not related to density, provided that a small density-related adjustment is eventually applied. For instance, 6o%-8o% of losses could be suffered quite at random provided that after these had occurred there was a small loss, say in the order of under 10%, which was related to density. Indeed, this is the more usual situation.

On the Norfolk estate under discussion, red-legged partridges have increased over roughly the same period that the grey have declined (see Fig. 26 and Chapter 7, p. 176). It is interesting that, in contrast to the case of the grey partridge, the autumn ratio of young to old has not been the factor which determines the subsequent spring population of the red-legged partridge (Fig. 3b). There were some years when more red-legs were shot in the autumn and winter on the estate than were actually known to be there at the beginning of the shooting season, and numbers sometimes were higher in spring than in the autumn. Clearly, something has been happening which has enabled immigrant red-legs to move into the area from surrounding farms or marginal land (see here). This suggestion highlights another facet of population control which must be considered, namely the role of immigration and emigration. In the case of the wood-pigeon, juvenile birds which are surplus to the carrying capacity of the area in the autumn do not necessarily die but may emigrate to other areas – to marginal habitats or even to France. A proportion survive to return in the spring. Although winter numbers may be directly related to the food supply – for the death-rate depends on how many surplus birds exist in relation to this food supply (in a poor year for food there is not necessarily any mortality if the population is already in balance with food stocks) – the number in spring will depend not only on local survival but also on how many of the emigrants survive to return. These complications do not invalidate the contention that in any area at the worst season numbers are regulated by the food supply (for the rest of the year there may be more food than birds to eat it, especially if breeding cannot be accomplished quickly enough), but they add to the difficulty of demonstrating clear-cut relationships, and are sometimes introduced in ill-conceived arguments as evidence against the theory.

Fig. 4 shows the autumn population, in decreasing order of size, of the grey partridge on the Norfolk estate against the percentage of birds dying in winter, either as a result of shooting or through natural causes. Similar data for the red-leg are also detailed, these being plotted against the appropriate years for grey and not in their own size order. The figure demonstrates in the case of the grey partridge that mortality due to shooting and other factors combined is positively correlated with the size of the autumn population, that is, it is density-dependent. The percentage of birds shot is even more strongly correlated with the numbers available for shooting and is also correlated with the total mortality. In contrast, natural mortality is not correlated with autumn numbers nor with shooting loss. That death by shooting should be adjusted to the autumn population is to be expected, since the people involved determine their activities according to the prospects; in years when autumn numbers are small, with a low young to old ratio, shooting is voluntarily abolished or curtailed. This attitude is mistaken for the simple reason that, in spite of shooting, additional animals have in any case to be lost to ensure stability in the spring, and if none are shot more disappear through other causes. This is well shown in Fig. 4 for the year 1954 when little or no shooting occurred and the level of natural mortality was raised. As a result the death-rate was at virtually the same level as it was in 1952 and 1955, when extensive shooting took place. Although enough animals are shot to ensure a density-dependent controlling effect on the winter population, this is only so because shooting obscures other causes of death. Because shooting may actually account for deaths it is not the reason why these occur: they would also occur in the absence of shooting. Jenkins reached this same conclusion for partridges he studied on Lord Rank’s estate at Micheldever in Hampshire, and he and his colleagues (Jenkins, Watson and Miller) have more recently found that the same applies in the case of the grouse – the details are shown graphically in Fig. 5. The practical conclusion to be drawn from these studies is that people could as a rule enjoy more intensive shooting without detriment to game stocks – though this conclusion caused scepticism among many shooting men. It is ironical, therefore, that my colleagues and I found exactly the same principle to apply to a pest bird, the wood-pigeon. The number of these shot during organised battues in February was always less than the number which had to be lost to bring about stability in relation to clover stocks. We concluded that winter shooting was not controlling the population, nor in the circumstances did it reduce crop damage. Again this conclusion caused scepticism among many shooting men who could not appreciate that causes of death are compensatory, not additive. Only if more birds are shot than will be lost in any case can shooting become a controlling factor, in the sense that it will reduce numbers to lower levels in the next season.

FIG. 4. Relationship between the autumn population of the grey partridge (top hatched histogram) and red-legged partridge (third histogram from top, hatched) with the percentage of these populations lost between autumn and the following spring represented in the histograms below each. The percentage of birds lost is given by the open columns, and the percentage of these birds which were shot by the solid black. Loss due to other causes, is therefore, the difference between black and open parts of the columns.

[For the grey partridge the total mortality in winter is correlated with the autumn population being heavier in years of higher autumn numbers r₁₁ =0.827. The percentage shot each winter is correlated with the autumn population r₁₁ =0.909. The percentage shot is not correlated with the number lost for other reasons r₁₁ = —0.420.

For the red-legged partridge the total mortality in winter is less correlated with the autumn population of red-legs, r₁₁ =0.583, and more correlated with the autumn population of grey birds r₁₁ =0.747. The number of red-legs shot bears no relation to the autumn population of red-legs but is related to the autumn population of the grey, r₁₁ =0.789.]

The reason for some of the apparently anomalous results with the red-leg in which a higher proportion of birds was sometimes shot than was present in autumn is that the birds were moving into the area. Thus in 1952 the autumn population was 214, 259 birds were shot yet the spring population was 120, i.e. 121% of the autumn population was shot, there was a 77% loss not due to shooting (theoretically the difference between the total loss from autumn to spring=44%; with the loss due to shooting subtracted = — 77%, and this has to be shown by drawing the column below the base line. (Data derived from Table 1, from Middleton & Huband 1966).

The number of red-legged partridges shot on the Norfolk estate has borne only a slight correlation with the autumn numbers (Fig. 4). As the red-leg was much rarer than the grey, any decision on the numbers to be shot was made relative to the latter species, or more accurately to the total numbers of both species, rather than to the autumn population of the red-leg. For this reason the percentage of red-legs shot was fairly strongly correlated with autumn numbers of grey birds, while the total winter mortality (shooting plus natural) was slightly less strongly correlated. This illustrates how the amount of predation (in this case shooting) suffered by a species may be determined by the availability of similar prey. In such circumstances, the situation may arise where undesirably large numbers are lost by accident, even resulting in the extinction of a species.

Still considering Fig. 4, it is possible to imagine that this represented a species where the shooting had been done for pest control and not for sport, and that it was supported by a bounty. It becomes evident that a bonus scheme, unless it actually results in more animals being killed than would die in any case, would in this case prove a complete waste of money as a means of controlling numbers. It could only be justified if the animals concerned were killed before they caused damage. While the undesirability of a direct subsidy is fairly evident, there are often cases where grant-aid is paid in an indirect manner which obscures its futility. Variations in kill dependent on population size, and hence variations in subsidy, would be anticipated in different seasons – yet it usually happens that the amount claimed for a subsidy stays fairly constant. This is so in the case of the amount paid for wood-pigeon shooting. This suggests that only an arbitrary cull is being achieved; arbitrary in the sense that people now do roughly the same amount of shooting each year, and claim a fairly constant and acceptable level of support. It is extremely unlikely that this reflects realistically the variable level of crop damage caused by pigeons.

It is seen that most of the annual fluctuations which occur in the numbers of any bird depend primarily on juvenile rather than adult survival. Most adult birds die not through starvation but by accident – by predation, occasional disease, and pure accidents such as flying into a telegraph wire. In general, big birds are less prone to accident; they are less likely to be caught by a predator and so they tend to have lower death-rates, but there are many exceptions. Established adults must have already experienced a season of food shortage, which they have successfully survived in competition with other individuals. It is unlikely that they will suffer in subsequent years, unless a particularly lean season occurs. In other words, food shortage may only seriously affect an established adult in one year out of many (a hard winter is one example of this). Moreover, in many cases most of the adults which die by accident do so outside the season of normal food shortage; adult starlings, for example (see here), are at greater risk of death during the breeding season, when they are busily occupied with minding their young and are more often caught unawares by predators. If a population remains stable (as in Fig. 1) but produces a large excess of young, it follows that a large number of these must die. This juvenile mortality should be seen primarily as a consequence of the young birds’ competition with the adults, whose greater experience nearly always enables them to survive better than their inexperienced offspring. Indeed, the number of young which will survive depends on how many adults are lost to make room for them, the final adjustment occurring at the worst season of the year for whatever factor is limiting adult population size. This can be food without it appearing obvious. Thus, after breeding, there exists a big excess of young although there may still be enough food to support all individuals. Accidental deaths will occur throughout this time, but it is likely that young will be most severely affected through inexperience. Eventually, and it may be gradually or suddenly, the season of minimum food supplies will arrive. If by this time there are already too many adults, then virtually all the young will now be lost as well as a few adults. If some catastrophe has occurred and no adults are available, then larger numbers of young will survive to restore the former balance between total numbers and environmental resources. But these two extreme situations will occur only rarely. The above account is slightly over-simplified, as in reality adults themselves do suffer a little from competition with their young, and the process is not completely one-sided. Removal of juveniles increases survival prospects for the adults. Furthermore, it will be appreciated that several factors influencing bird numbers may act simultaneously, so that adjustments are continuous – this is why animal populations are called dynamic.

FIG. 5. The top of the columns represents the total number of grouse in different autumns on study areas in Scotland on low ground (left) or high ground (right). The number of these birds which were shot is indicated by the solid areas, the number lost through other causes by open columns and the number of grouse alive in the spring by hatching. The data show that more birds must die than are actually shot. On high ground in 1961 there were more grouse in spring and autumn, as a result of immigration. (Data from Jenkins, Watson & Miller 1963).

Our studies of the wood-pigeon provide an example of some of these processes in operation. In 1959 the post-breeding population comprised 171 birds per 100 acres with 1.3 juveniles to every adult. In 1963 there were only 101 birds per 100 acres and 1.5 juveniles to each adult. The clover food supply, at the worst time of the succeeding winters of 1960 and 1964 was near enough the same and so the population was reduced to 34 and 35 birds per 100 acres respectively. But in the 1959–60 season the competition needed to bring about balance (171 down to 34) was clearly much greater than in 1963–4 (101 down to 33). The effect on the juveniles was striking. By the February of 1960 there were only 0.1 young to every adult against 1.1 in 1964. Hence, in both years total numbers reached the same level by winter, but juveniles suffered a 96% loss in 1959–60 against 74% in 1963–4. Adult loss in the first year was 59%, and 58% in the second season. These figures do of course illustrate a density-dependent loss of young. The term mortality has not been used, because some birds were lost through the emigration of both young and old, though it amounts to mortality so far as the carrying capacity of the land was involved in mid-winter. The true annual death-rate of adults was lower than the figures quoted.

This example shows how the age structure of a population may be altered without any change in its ultimate size. The red grouse provides another illustration of this effect, achieved by deliberate killing. It has already been noted that grouse numbers in spring are unaffected by shooting. Yet Jenkins and his team found that over the autumn the death-rates of adults and young were equal (at around 70% per annum), in sharp contrast to all other birds so far studied. This was because so many birds were shot that deaths from natural causes were not fully obvious; presumably shooting is not selective of either age group. If enough animals (old and young combined) are shot natural competition between adults and juveniles can be reduced. In an unshot population of white-tailed ptarmigan in Canada (Choate 1963) the juveniles did suffer a much higher death-rate than the adults. Again, young wood-pigeons are easier to shoot than adults until mid-winter, by which time they seem to have learnt to avoid men with guns and from February onwards are no more easily shot than old birds. But they still do less well than adults when competing for food (and at breeding sites later on) and for this reason suffer a higher death-rate. Clearly, the amount of winter shooting can alter the age structure of the population without affecting final numbers in summer.

As mentioned above, before any artificial killing can result in a reduction of population size, the total number of animals killed must exceed the rate of deaths from natural causes. In addition, as increasing numbers are killed artificially, natural mortality factors cease to operate and must be replaced by artificial ones if compensatory changes are to be avoided. Inability to reach the necessary threshold means failure so far as artificial control is concerned. If numbers need to be kept down it is best, all else being equal, to defer artificial killing to the season when natural factors have taken their toll. If such population control cannot be achieved artificially and numbers cannot be held below a natural optimum, it is vital to show that any cropping does prevent damage; it is of course feasible that artificial killing will remove animals before they would normally die, and so protects crops. But every case has to be taken on its own merits, and examples of wise and foolish applications will be found in later chapters.

The level around which a population fluctuates may be subjected to long-term changes not resulting from the key factors so far considered. Blank et al. have demonstrated that the number of breeding grey partridges (which have shown evidence of a general decline during the present century) is determined by the nature of the habitat. The favoured sites for nesting are an incomplete hedge, that is, a group of separate bushes often patchily arranged on a grassy bank, or a wide grass track. Jenkins (1961) also showed that a lack of winter cover (cereal crops as distinct from tall grass provide little cover) results in the formation of larger territories, because the males can see rivals at greater distances and this causes the breeding population to be lower; under these circumstances surplus birds move on to marginal habitats. It is conceivable that changes in land usage and farming techniques have harmed the partridge by causing the population to fluctuate around a lower level.

The well-documented decline of the corncrake seems to have resulted from farm mechanisation, which has eliminated the old hand cutting of hay and enabled the harvest of silage to take place earlier, to the detriment of nesting corncrakes. Once generally distributed throughout the British Isles, the species disappeared from East Anglia before 1900, from east midland and southern England by about 1914, and from Wales, northern England and east Scotland by about 1939. It remains common only in Ireland, parts of western Scotland and the Hebrides, Orkneys and Shetland – areas where the old methods of hay production to a large extent remain unchanged.

Britain has experienced several periods of radically altered climate which must have considerably affected bird distribution, particularly of those species at the edge of their range in northern Europe. Since the mid-nineteenth century changes in air circulation have caused a warming of the atmosphere particularly noticeable in northern areas; in central England, the decadal mean temperature of the summer months had risen about 2° C between 1900 and 1950, while a similar increase occurred in Finland and elsewhere. The temperature increases were particularly noticeable further north and at first were limited to the winter months (a 9° C increase in Spitsbergen), causing arctic ice to melt and polar seas to become warmer, but leading by the 1880s to increased temperatures during the northern springs, and to a summer increase in the 1920s.

Since about 1950 there has been a reverse trend in weather conditions which can be expected to bring about another southern displacement of the avi-fauna. Another complication is that the increase in mean temperatures has, in the maritime countries like Britain, resulted in distinctly wetter and cloudier summers. This is doubtless the reason why certain birds which depend on large flying insects have declined in recent years. Red-backed shrikes still produce more than enough offspring to ensure their increase, given the right conditions, but the bird has markedly declined in Britain and other parts of north-west Europe. Destruction of their habitat has sometimes been held to explain the decrease but this is by no means always the case, many areas now untenanted by shrikes appear to be unchanged, certainly in several Suffolk and Surrey localities that I know personally. Peakall, who has documented the decline, believes that a changing climate provides the explanation as flying insects become scarce on grey cloudy days. This was brought home to me very clearly when photographing red-backed shrikes on a Surrey heath in 1967 (see Pl. 1). During a six-hour session in a fully accepted hide when it was dull and overcast, the adults were clearly finding it difficult to get food. Even though they had large hungry young, each parent was visiting the nest about once every hour, bringing mostly ground beetles, until eventually they found a nest of young birds, and for a while flew back and forth with the nestlings. The following day was bright and sunny and the adults were feeding the young every five minutes or so, bringing dragonflies, butterflies, bees and lizards – in fact all the creatures which depend on sunshine to become active.

Other birds which depend on large flying insects, and which are at the edge of their distribution in north-west Europe and Britain, seem to be suffering a similar south-east contraction in range. Monk has shown that in 1850 the wryneck was very common in south-east England and the midlands and was scarce though regular north to the borders and west in Wales. By 1954, only 365 pairs could be accounted for and the total dropped to about 205 in 1958, since when it has fallen further. Moreover, well over three-quarters of the British breeding population is now confined to Kent. According to a survey carried out in 1957 and 1958 by Stafford, the nightjar is much more widely distributed in Britain, and although most common in southern England, it is frequent in the north of England and Wales though only irregular and local in Scotland. As it feeds on night-flying insects it has presumably been less adversely affected by cloudy summers. Even so, the survey showed that a decline has occurred during this century – though increased disturbance may be an important factor with this large and quiet-seeking species.

The stonechat, a bird of gorse commons, seems to have declined for a different reason. It feeds on large insects but catches them on fairly open ground, using a convenient bush as a vantage point. The ground and low herbage dwelling invertebrates on which it feeds can be found even in winter, so it remains a resident, like that exceptional Sylvid warbler, the Dartford warbler, which locally shares a very similar habitat. Magee, in a study of the stonechat in 1961, obtained breeding records from only twenty-three counties in England and seven in Wales, mostly only small numbers being involved. Only nine English, four Scottish and three Welsh counties had more than twenty pairs, whereas at the turn of the century the species bred in every English county. In 1961, 264 pairs were recorded in Pembrokeshire, 262 in Hampshire, 150 in Glamorgan, 104 in Cornwall, 96 in Devon and 51 in Dorset, after which Surrey with 39 pairs had the next highest English or Welsh county total. Magee pointed out that counties like Cornwall and Pembrokeshire have long stretches of coastline and still have extensive bracken and gorse headlands providing suitable conditions for the species. Otherwise poor gorse-covered commons or alternatively heath moor with numerous bushes have been largely lost as a habitat in Britain; the only extensive areas are to be found in those counties where tolerably high numbers still occur. Another complication is that severe winters hit stonechats very hard and it may take several years for numbers to recover. In this connection the coastal counties are probably less seriously affected, and Magee gives evidence that following any hard winter, recolonisation is usually first noticed in coastal areas from which birds then spread to inland habitats. In recent decades the predominantly mild winters between 1917 and 1939 allowed a fairly extensive re-occupation of inland habitats after the drastic reductions of the 1916–17 hard winter (41 consecutive night frosts at Hampstead) only to be followed by extensive reductions again with the hard winters of 1939–40 and subsequently. Hard winters are not regulatory factors in the accepted sense. Blanket snow cover or extended frosts affect wide areas irrespective of the number of animals present which must all suffer in a density-independent manner. A measure of density-dependence may be imposed if the animals are able to compete for restricted pockets of food; but this is a special case.

In south-west Europe, Cetti’s warbler is one of a small group of resident warblers which are similarly sensitive to hard winters and this applies to the resident Dartford warbler which is on the edge of its range in southern Britain. Cetti’s warbler spread north in France during the 1940s and 1950s with the period of mild winters and these also enabled a large proportion of Dartford warblers to survive in winter and then spread to other suitable habitats; a similar population increase and irruption to new areas occurred in the bearded tit for the same reason. The Dartford warbler formerly extended from Suffolk and Kent to Cornwall, but fragmentation of suitable heathland at the turn of the century has virtually restricted it to the New Forest, this being the only large enough area of suitable habitat, within its range, which can provide stability. Tubbs has demonstrated how during open winters the population can build up to occupy other heathland in Surrey and north Hampshire, where too small a population exists to withstand bad winters and where it has been exterminated two or three times since the 1940s – the winter of 1947 proving particularly disastrous. Tubbs has rightly emphasised the importance of maintaining the New Forest as a reservoir for the species. Here its numbers increased from around 80 pairs in 1955 to 382 pairs in 1961. Then came the snow of 1962 which exterminated the species in Surrey (the birds were trapped while roosting in tall heather by an overnight blanketing fall of snow) and left only 60 pairs in Hampshire. The following hard winter of 1962–3 virtually exterminated even the New Forest population, as well as causing a big retreat to the south of Cetti’s warbler in France.

Apart from changes in range caused by weather, several species have recently increased in response to addition to available habitats, which are often man-made. In inland Eurasia the little ringed plover replaces the ringed plover and nests on the sand and pebbly shores of slow-moving rivers or inland lakes, predominantly near fresh water. A pair nested at the Tring reservoirs, Hertfordshire, in 1938-the first nesting record in Britain. No further nests were found until 1944, when two pairs nested in another part of Hertfordshire, and one pair in Middlesex; the increase since then is shown in Fig. 6. By 1962 there were approximately 157 in Britain, nearly two-thirds being concentrated in the counties south of the Welland-Severn line and none extending further north than Yorkshire or west of Gloucestershire or Cheshire. Parrinder, who has documented these changes, points out that gravel pits provide over three-quarters of the nesting sites of little ringed plovers; sewage farms, reservoirs, brick pits and such places comprise the remainder. Most gravel-pits are centred in the areas where the species already occurs, except that a surplus of potential sites appears to exist in Lancashire, Northumberland and Durham and parts of Wales and these sites may next be occupied. Parrinder emphasises how gravel and sand production for building and road construction have increased with the post-war building boom and, as much of the material is derived from new workings, these have also increased. There can be little doubt that the creation of a new environment has favoured the species. It is interesting that our ringed plover has not been able to colonise these places, particularly as a local inland breeding population occurs in the East Anglian Breckland and the bird also breeds inland on some of the east Suffolk heathlands and in parts of northeast Scotland, for example in the Abernethy Forest.

FIG. 6. Increase in number of pairs of little ringed plovers summering in Britain. (Data from Parrinder 1964).

FIG. 7. Changing status of black redstart in Britain showing increase in number of territory holding males during the 1939–45 war with a decline following the final clear-up of war damaged sites after 1950. The solid line gives figures for the whole of Great Britain, while the dotted line is the contribution made by the City of London and Dover combined. (Data from Fitter 1965).

The black redstart was originally a bird of the warm montane regions of the southern Palaearctic, like the rock thrush and crag martin, but spread northwards after the last glaciation. It has adapted itself to a man-made environment, using buildings for nesting sites in lieu of cliff faces, and in parts of Germany it replaces the robin as the familiar follower of man. Its northward spread was still in progress across Germany during the last hundred years, and it only reached Jutland in the second half of the nineteenth century, and Scandinavia in the early 1940s. There is no record of it in Britain before 1819, though it became a regular passage migrant on the south and east coasts during the middle of the nineteenth century. Sporadic nesting attempts followed, in Durham in 1845, Sussex in 1909 and then a slow build-up from 1923 onwards. Three pairs nested at the Palace of Engineering in Wembley, Middlesex from 1926–41 but altogether less than half a dozen pairs were breeding in Britain up to the mid 1930s after which a big increase occurred, detailed in Fig. 7. The ability of the species to establish itself in this way has followed the sudden availability of nesting sites and feeding grounds, in the form of war-time bombed sites, particularly in London and Dover. Rebuilding and the post-war clean-up account for the subsequent decline in numbers.

There are also examples of bird numbers reduced through direct persecution, especially when the victim is fairly rare. The great crested grebe was certainly widely distributed in suitable places in Britain in the early nineteenth century, but by the middle years of the century a big demand arose for its breast feathers to make ‘grebe furs’ for a fashionable home market, and the slaughter began in 1857. By 1860, the species was reduced to 42 known pairs and was only saved by the sanctuary afforded by private estates, while further help came with the Bird Protection Acts of 1870–1880. Nevertheless, some increase was under way by 1880, before protection could have been very effective, and Harrison and Hollom (1932), who record these early changes, consider that human persecution came at the start of a period of long-term cyclical increase. By 1931, there were around 1,150 breeding pairs (with non-breeders, about 2,650 adults) in England and Wales and about another 80 pairs in Scotland. A sample census by Hollom (1959) showed that about the same number of adult grebes existed in Britain in the 1940s, but that an increase then began. When Prestt and Mills undertook a census in 1965 there were approximately 4,500 adults in Britain. This increase seems to have been favoured by man’s activities in creating numerous new reservoirs and gravel pits, just as the little ringed plover benefited. The 70% increase of the population in about twenty years can be compared with an increase of 84% in sand and gravel production between 1948 and 1957. That an increase followed the creation of new habitats also indicates that saturation had previously been attained and that the bird was regulated in the sense already discussed.

For a species to extend its range and take advantage of newly developed habitats it would be helpful for it to possess some kind of exploratory behaviour rather than rely on chance movements. It is becoming clear that immediately after breeding, many species, which normally migrate south, first indulge in northerly flights. The large-scale ringing of sand martins has shown that birds breeding at a colony in the south of England may move north to have their second brood, and juveniles marked in southern England have been found again in roosts in the north in the same season. Wood-pigeons also display northerly flights in September and October, before adopting a southerly orientation later in the autumn. Collared doves ringed in Europe as nestlings have moved north to Britain in the same autumn, and numbers of serins have turned up in south-west England in recent autumns. These movements seem adaptive in that young individuals which are surplus to the needs of the area in which they are born will stand more chance of finding new places to settle if they first explore north. The same principle applies to those birds of southeast and east Europe which might be expected to move north-west or west. I suspect that this factor may account for big arrivals of redbreasted flycatchers, woodchat shrikes, barred warblers, melodious and icterine warblers – all predominantly juvenile – into Britain in September 1958 and in subsequent years. Williamson showed that red-breasted flycatchers and icterine warblers arrived in Britain in clear anticyclonic weather with light winds, and as both migrate south-east to Asia, their movement several hundreds of miles off-course is remarkable. The explanation that they drifted in with down winds seems unlikely, and instead I wonder whether the existence of anticyclonic weather facilitated a normal adaptation after breeding in the form of a deliberate dispersal north-west, a process possibly truncated in years of less favourable weather.

Man has done so much in a passive way to alter the avifauna of Europe that it seems reasonable to take active steps to reintroduce lost species. Any reservations that this would be unnatural, should be tempered by the thought that the environment we have created is in any case artificial. Probably more pleasure than harm has been derived from the reintroduction of the capercaillie. It would seem laudable to follow up a recent suggestion and attempt the reintroduction of the bustard to parts of the East Anglian Breckland, and to encourage black terns to stay and breed. It is quite a different matter to introduce alien species to a new country, especially without sound biological knowledge. In Britain, some of these introductions, red-legged partridge, various pheasants, little owl, Canada and Egyptian goose and Mandarin duck have on balance improved our bird-life, but the same could not be said of the introduction of the house-sparrow and starling to Australia and North America.

Perhaps a more interesting question to ask here is why the majority of introduced species are unsuccessful. This is part of the much bigger question of what factors determine faunal diversity and enable some habitats to support more species than others. The concept of a niche, which refers to the animal’s place in the biotic environment and its relations to food and predators, should now be widely appreciated. It is a fundamental tenet of ecology that no two species can occupy the same niche in any one habitat, because both cannot be equally well adapted. R. and J. MacArthur (1961) examined numerous habitats at different latitudes for their plant species composition and foliage profile and ‘species diversity’. They found this last to be a more useful measure than the actual number of species because their calculations allowed at a habitat containing 50 of species A and 50 of B to rank a higher diversity than one with 99 of A and 1 of B. (The latter tends to be the farmland situation, the former that of tropical forest.) It turned out that neither the variety of plant species nor the latitude affected the amount of species diversity which instead depended entirely on the variation in foliage height, probably because birds mostly respond to different configurations of vegetation in different layers. This means that habitats of the same structural profile have the same diversity of bird species. In any area, a bird might either feed on all food of a suitable size within a narrowly defined habitat or, alternatively, be selective of food but collect it throughout a wider range of habitats. In other words, birds could partition their food or their habitat. The former has occurred because feeding specialisation brings the greatest advantages and has been favoured by natural selection. Partitioning the habitat would necessitate birds moving from one suitable micro-habitat (say a species of tree) to another, and it would depend on the pattern of the total habitat how much time would be wasted in the process. But adaptation to a comparatively broad habitat structure, for instance, to arboreal or ground feeding, must in turn impose physical limitations which restrict the diversity of feeding adaptations; in practice, bill size and shape is about all that can be much modified to suit the collection of different foods.

From the viewpoint of zoo-geography, the Palaearctic has existed as an entirety for sufficient time to ensure that most niches are filled by highly efficient species. Furthermore, man and birds have lived side by side since Neolithic times, so that the new habitats created by agriculture and man’s other activities have been occupied by the species best suited to them. The same is not true of Australia and New Zealand, which were cut off from the main centres of evolution at an earlier stage, one result being that primitive marsupial mammals were not replaced by the better adapted placental mammals. Birds are less insular, however, and the native avifauna of Australia seems to be the best fitted to occupy the niches available. Thus of at least 24 bird species deliberately introduced into Australia in the past, only 12 have become established. It is significant that only the blackbird has managed to invade native forest, the remainder existing in areas of recent agricultural development or urbanisation. But introduced birds like the feral pigeon, starling and house sparrow are better equipped to occupy the man-made niches than the native fauna, simply because these are species which have already been selected to occupy a man-made environment. New Zealand has an impoverished avifauna compared with Australia on which it has depended for colonisation, and the process is still incomplete. In consequence, fewer niches are saturated in New Zealand, so more exotic species have been successful. Of 130 species originally introduced, 24 have become established, although apart from the blackbird, chaffinch and redpoll which appear to be filling unexploited niches, most are again restricted to man-made habitats. Hawaii, which is even more isolated, and not saturated by a wide diversity of species, must have even more vacant niches, for, according to Elton (1958), of 94 birds introduced 53 have become established, some deep in the native forest.

According to Middleton, the European goldfinch has been successful in Australia and New Zealand only in the man-made agricultural areas, to which none of the native Australian Ploceid finches were adapted. In contrast, the European goldfinch has not been a successful bird in North America because it has virtually the same ecological requirements as the fitter endemic American goldfinch. Only one small colony of European goldfinches became established near New York, though these have since vanished when their habitat was destroyed for building purposes. Again the European house-sparrow has been highly successful in Australia, over roughly the same range as the goldfinch, whereas the introduced greenfinch is more restricted as it has rather more conservative ecotone requirements. It is interesting that this reflects a trend occurring today in Britain; the greenfinch is declining with the loss of hedgerows and woodland edges, while the goldfinch and linnet are increasing.

To return to New Zealand, it is noticeable that the birds which have become pests in agricultural areas, apart from being introduced species as one would expect from the comments above, present the same kinds of problems as they do in Britain. I am grateful to Dr P. C. Bull for allowing me to give details. As we shall see, skylarks (see here) are locally troublesome in Britain to young seedling crops such as lettuce. Near Hastings, N.Z., they and house sparrows have together been responsible for damaging asparagus and other seedlings. Both blackbird and song thrush and also the starling, resort to orchards in the dry season after breeding and cause considerable damage to all kinds of fruit, ripening pears, cherries and grapes. Redpolls do considerable damage to apricot blossom in their search for insects, and blossom searching is a habit which is increasing in Britain (see here). Locally in Britain, linnets peck out the seeds from strawberries (see here), while in N.Z. goldfinches do the same.

Various attempts were made to introduce the rook into N.Z. from 1762 onwards but only 35, liberated near Christchurch in 1873, seem to have thrived. The species occurs in five localities on the yellow-grey earths in the east of the county, generally where cereal growing occurs. Rooks at first increased very slowly but there was a rapid increase between 1935 and 1950, and a final levelling off with the density of birds in their favoured areas becoming virtually the same as that in Britain (around 16 nests per square mile). At Christ-church, the population increased from 1,000 birds in one rookery in 1925 to 7–10,000 in 1947 (13 rookeries), since when the numbers have remained roughly constant with 19 rookeries in use. Until 1926, the rookeries were in eucalypts, probably the favourite tree, but following a disease epidemic which killed these trees the birds changed to pines. Bull points out that the rate of increase of rooks has been slower than that of other introduced passerines and attributes this to their gregarious nesting habits and their need for group stimulation, and to an early shortage of suitable habitats. A feature of N.Z. rookeries is their very large size compared with British ones (rookeries of over 1,000 nests are quite common), and the traditional return to the same nesting sites may partly explain the slowness of expansion. (The large size of rookeries, and difficulties in getting sufficient food locally may also explain why the birds seem to lay, on average, smaller clutches in N.Z.; 3.4 eggs against 4 + in Britain, (see here), though more data are needed to establish the point.) Frequently, only when man actively disturbed these large rookeries did they become fragmented in surrounding areas, often with a rapid increase in total bird numbers in the district. As in Britain, rooks uproot seedling peas and corn, take ripening peas (and pumpkins) and maize and are also partial to walnuts.

The number of closely related birds which can live in the same habitat without competing for food depends to a large measure on the degree of stability within the environment. Marked fluctuations occur on English farmland, not only because of the changing seasons, but also because ploughing, harvesting and other farm operations impose drastic changes. As a result, the farmland birds occupying the various niches available for ground-feeders show a wide character displacement; we find a plover, three passerines (rook, starling, lark), a partridge and a pigeon, other species being only transient visitors, or primarily dependent on other habitats. No bird can afford to be too conservative in its niche requirements in a fluctuating environment, while the need for each species to show more tolerance reduces the number of ecologically isolated forms. Therefore, we should expect modern farm mechanisation, which enables whole farms to be ploughed within a fortnight, to be detrimental to bird life compared with the old methods which ensured some degree of stability by leaving land fallow and by transforming stubbles into bare ground more gradually. Klopfer and MacArthur (1960) have similarly emphasised that the major factor accounting for a decrease in the number of species away from the tropics, while the number of individuals of each species increases, does not result from a decrease in habitat complexity, but to a decrease in the similarity of coexisting species. The principle can obviously be extended to any situation where man simplifies the environment.

An important feature of complex ecological communities is that interactions between members damp out oscillations in the numbers of any one species (see here) and so help to introduce a high degree of stability and energy utilisation. For one thing, available food is more fully exploited, which is not the case in arctic environments for instance, where considerable seasonal changes occur. Hence, the amount of energy needed to maintain a stable community is less than that required for an unstable one. Man’s activities have tended to reduce complexity and introduce monotony, through monocultures of crops, uniform stands of trees, or rows of similar houses. In consequence, the animals inhabiting these environments usually fluctuate much more than those of more complex ecosystems, often to the extent of becoming pests (see here). One feature of stabilisation is that natural selection can favour anticipatory functions – for example, the breeding season of northern birds has become approximately geared to seasonal daylight changes – in unstable environments opportunism must set more of a premium. Because more energy goes into maintaining fluctuations in simple ecosystems, often short-term fluctuations, these systems offer more scope for rational exploitation, giving more production per unit of biomass. In general, pestiferous birds and game species can be cropped very intensively, but the corollary also applies in that the energy needed to counteract fluctuations, the efforts of pest control, must often be so considerable as to be impracticable. On the other hand, mature and complex ecosystems can be disturbed relatively easily. In the Eltonian food chain the predators at each level become rarer and larger, because the energy passing from link to link is only in the region of 10%-20%. This not only sets limits on the number of links in a food chain (five seems to be the maximum) and rules out the possibility of a super-predator but makes the top predators particularly vulnerable to small but cumulative changes in the food chain. The loss and increased rarity of so many of the birds of prey depends not so much on persecution, but on the reduced complexity of the environment through human ‘progress’. Clearly our future policies should not concentrate too much on bird protection per se, but rather on the creation and maintenance of as much diversified habitat as possible.

The long-term or ultimate value to a species in settling in an appropriate habitat will depend on the bird’s ability to find suitable food and produce surviving progeny, and this ability will be conditioned by the structural and behavioural adaptations of the species. The immediate or proximate factors which determine how a bird chooses an appropriate habitat are unlikely to involve these same factors. Instead, natural selection has enabled each species to respond to immediate signals, which can be reliably taken as indicators that other more basic needs will be satisfied. In this way a bird which lives in oak woodland might respond to the configuration of an oak tree, because natural selection will favour this appropriate response provided it leads the bird to find in the oak woods all the various foods which are appropriate to its needs and feeding adaptations. Natural selection can favour the emergence of appropriate proximate responses which are anticipatory.

In practice, it is generally agreed that birds respond to a range of releasing stimuli which combine to provide the best cues. A meadow pipit may respond innately to open country, thereafter to specific elements of the habitat, such as the height of grass, the presence of song posts and nest sites. These considerations are important when man radically alters the habitat, without necessarily altering its food value. As various features of the environment combine to produce a response, they need not always be present in the same proportions, and some may even be absent, for a response still to occur. Species vary in the capacity to respond when some stimuli are absent. Within any area intraspecific competition ensures that the most favourable sites are filled at the start, after which less complete habitats can be occupied. At low population densities, when, for example, a species is at the edge of its range, only the best habitats are occupied (stenotopy) whereas when population explosions occur, marginal areas are also utilised (eurytopy). This is well illustrated by the wide range of nesting sites accepted by the various gulls which have undergone a spectacular population explosion in Britain – nesting colonies occur on rocky and sandy sea shores, estuarine and freshwater marshes, inland lakes, and on moors and fells.

It becomes clear how an originally montane bird like the house martin should come to accept the sides of houses for its nest site instead of cliffs. Similarly the absence of a species from what appears to be a suitable habitat may be attributable to the absence of some apparently trivial factor which must be satisfied. Already in south Europe it is known that the presence of electric and telephone pylons and cables in otherwise open country facilitates colonisation by species like the collared dove and various shrikes.

The psychological response of the reed bunting to a limited range of habitat cues seems to have been the only reason for its past restriction to wetland habitats and its ecological isolation from the yellowhammer. But, as will emerge later, this segregation is no longer maintained and the invasion of yellowhammer habitats by the reed bunting is possibly the result of a genotypic change which has removed this psychological restriction. Another explanation is also tenable. Although most habitat recognition is innate, birds are able to reinforce or even modify, to a variable extent, these innate responses by learning processes. By this means, adults often return to traditional areas, even though these change drastically, whereas young birds breeding for the first time avoid entering habitats which do not release appropriate responses – either innate or acquired by imprinting in early life. For instance, Peitzmeier (1952) found that in a study area in Germany the curlew typically nested only in boggy areas and avoided surrounding cultivated land. When the marshes were drained and cultivated, the adults not only remained faithful to the area, but after learning its new characteristics, also spread to other tilled land which before they had avoided. It could be that this situation has applied in the case of the reed bunting. Peitzmeier attributes the further spread of curlews in arable environments to the imprinting of young which have been reared in these new habitats. Such processes explain why local populations of birds come to acquire unusual habitat associations – for example, the stone curlews which used to nest on the shingle of Dungeness and Norfolk beaches, or yellow wagtails which breed in fields of growing potatoes. Peitzmeier accounts for a remarkable and fairly sudden change in the nesting habitat of the mistle thrush in the same way – originally confined to continuous woodland dominated by conifers it began, in 1925, to nest in parkland-type habitat, and in small groups of deciduous trees in cultivated country. That very recent changes may have occurred in the habitat of an animal must always be remembered when trying to understand their adaptations – they may have evolved in conditions quite unlike those in which the birds are seen today.

CHAPTER 3

SOME PREDATORS AND THEIR PREY

THE last chapter was primarily concerned with the factors governing bird numbers and distribution, showing some of the ways in which man does or does not influence the natural balance. The importance of the food supply was stressed. In many cases the food supply can be considered as an independent variable. For example, the quantity of beech-mast varies in different years in response to climatic and other factors and is not initially determined by the birds which use it as food. Nevertheless, the availability of beech-mast profoundly affects the numbers of those species which have to depend on it for food. More bramblings winter in Britain in good beech-mast years. On the other hand, when short-eared owls feed on voles they may themselves become an important factor determining vole numbers. Watson (1893) quotes an interesting example chronicled for 1580. In that year a vole plague developed in the marshes near Southminster, Essex, which so depleted the grasses that cattle died and men were powerless to take any preventive action. The situation was supposedly saved by the arrival of ‘such a number of owls as all the shire was not able to yield; whereby the marsh-holders were shortly delivered from the vexations of the said mice’. When a reciprocal interaction exists between an animal and its food supply, it is termed a predator–prey relationship. The principles involved in such predator–prey interactions are fundamental to almost all aspects of economic ornithology, from the daily routine of the gamekeeper to the effects some forest birds may have on various insect pests. In discussing certain economically important examples it seems desirable to outline some of these principles.

A complex range of variables determines the nature of predation. For one thing the availability of other foods in the environment influences the extent to which a predator concentrates on any particular prey. This also depends on how specialised the predator has become in feeding on restricted categories of prey; kestrels are better equipped to catch ground-living rodents in open country than are sparrowhawks, and these in turn are more efficient at catching small birds in flight in wooded areas. Prey-species have evolved an enormous range of anti-predator devices, the more so if they are subject to intense attack. These protective devices vary from breeding in colonies to the various forms of camouflage and cryptic behaviour and the possession of special defence organs. Some invertebrates and the eggs of some birds are distasteful to predators, and the list could be longer. Whatever anti-predator adaptation has evolved, it is likely that this is also subject to limitations. For example, camouflage can only be effective if sufficient concealing backgrounds are available and when these are saturated surplus animals may derive no benefit from being cryptically coloured. Accepting the existence of all these modifying influences, there are two basic aspects to the response shown by a predator to changes of its prey (Solomon 1949). First, there is the response of the individual predator to changing numbers, or, better, to the changing density of its prey (food), this being the functional response of the predator. Second, predators may respond to increases of prey density by increasing their own numbers through immigration or by breeding, and vice versa, and the change in population size of the predator is the numerical response.

The simplest functional response shown by a predator to changes in food density is depicted in Fig. 8 based on the number of cereal grains eaten per unit time by wood-pigeons according to grain density on stubbles or sowings. The response is simple because the birds have little or no other food choice when they search the stubbles; unlike the related stock dove they do not normally respond to the presence of weed seeds. It can be seen that once grain density reaches a particular threshold the birds’ intake rate cannot be increased; this limitation is imposed because a constant amount of time is needed to pick up, manipulate and swallow each grain. The stock dove’s ability to find weed seeds on stubbles and sowings probably depends on it having shorter legs so that it is nearer the ground. In such ways birds have evolved different feeding mechanisms which are efficient in a limited range of feeding situations.

In most circumstances predators have a choice of prey and the particular item they select depends not only on prey density but also on learned individual preferences. This learning ability introduces a sigmoid stage to the functional response curve as shown in Fig. 9. This type of response curve is much more common among vertebrates and has, for instance, been found to apply to the predation by titmice on forest insects (Tinbergen 1960, Mook 1963), and has been produced experimentally with mammals in the laboratory by Holling (1965). The same curve is also found when bait, in the form of beans or peas, is spread on a grain sowing where wood-pigeons are feeding. This characteristic curve has been explained, as follows, by Leopold in 1933 and more recently by L. Tinbergen. At very low densities of the specific prey (density 1 for curve A in Fig. 9), few or none are found, and the food of the predator consists entirely of other items (100% other prey). As the specific prey density increases, a point is reached when some individuals are found by chance (density 2 in Fig. 9) and for a while the curve rises with density as chance encounters increase. But at some stage, which varies with the attractiveness of the prey, the predator learns that this particular food is available and makes a special effort to find it. The food is now found more often than by chance alone, and this causes the sigmoid stage to appear in the response curve (between densities 2 and 3 in Fig. 9). In the terminology of Tinbergen the bird now adopts a specific searching image for the prey in question. At even higher prey densities the predator again introduces variety into its diet and from now on the prey is taken at a constant rate (from density 3 onwards for curve A in Fig. 9). The level at which the intake rate of prey or the number of prey caught becomes constant depends on its palatability, that is, to what extent it is the preferred food of the predator. A high level (curve B in Fig. 9) would be found for a highly preferred prey (or in the absence of a very good alternative), as when wood-pigeons feed on tic beans spread on a clover pasture. The beans (curve B) are much preferred to clover. If the tic beans are scattered on a grain sowing, the response to beans more closely approximates to curve A because cereals are a preferred food. Nevertheless, the shape and characteristics of the response curve remain unchanged. Buzzards, as will be discussed, feed to a large extent on rabbits, if these are available. In Fig. 9 the rabbit could be represented by curve A and at pre-myxomatosis densities by the vertical line 3, that is, up to 80% of the buzzard’s diet is comprised of rabbits, other prey making up the remainder. Following myxomatosis, which virtually eliminated rabbits for a few years, the buzzards’ diet had to change in favour of other prey, and their feeding response with regard to rabbits could now be represented by the vertical line 1. As with the simple response already considered, it is important to note that above a certain point increase in prey density still does not result in a higher proportion being eaten. There is no reason why the activities of a pest-control operator or a gamekeeper would not obey curves of this kind. When an operator can kill only relatively small proportions of a pest animal, it is necessary to ensure that he does not switch from one pest to another depending on the ease of catching. For example, a rabbit catcher might undesirably be ignoring rabbits at low densities, to concentrate on catching and killing moles, pigeons and other species.

FIG. 8. Number of cereal grains eaten per minute by wood-pigeons depending on grain density. Note that the scale on the abscissa and the actual graph are broken to save space. (From Murton 1968).

Any numerical responses shown by bird predators to changes in the density of their prey can most rapidly be achieved by emigration or immigration; more permanent changes dependent on reproduction must necessarily be slow and delayed, because birds have restricted breeding seasons. Figure 10, based on Mook (1963), shows how the numbers of bay-breasted warblers which settled to breed in certain Canadian conifer forests, after they had returned from migration, varied according to the density of the third instar larvae of the spruce budworm Choristoneura fumiferana. The response did not depend on the reproductive rate of the warblers: it resembles the behaviour of certain arctic birds of prey, like the snowy owl, which settle to breed in the Canadian arctic and in Scandinavia in those years when lemmings are abundant, and is similar to the behaviour of the short-eared owls already mentioned. There are, however, many cases in which a bird predator shows no numerical response to a specific prey component. An example is discussed below (see here) where it was found that numbers of wintering oystercatchers showed little variation over several seasons, although their preferred prey, second year cockles, fluctuated widely in numbers. This was because in seasons when second year cockles were scarce the birds fed on cockle spat and the older age groups, so that while oystercatcher numbers may have been related to the total cockle population they were not related to this one specific age group.

FIG. 9. Functional response of a predator to changes in prey density. The horizontal line C represents the total food of the predator equal to 100%. The proportion of a specific prey, either A or B, eaten by the predator is also shown, depending on changing density of A or B. Consider a predator eating B. At density 1 none is found and the predator’s diet is composed of 100% of C (this could be many different items considered in toto). At density 3, 80% of the diet is composed of B and 20%, i.e. C—B, of other things again making a total of 100%C. (Based on Holling 1965).

The synthesis of the numerical and functional responses shown by a predator determines the nature of predation and its significance for the prey concerned, the main possibilities being represented in Fig. 11. Because bird predators must in most cases take time to respond to changes in prey density, any interaction must usually be delayed. The classical predator–prey relationship is shown at A in Fig. 11. Here the predator increases when prey is abundant, causing the prey to decrease. This results in a decrease of the predator, followed again by an increase of the prey. This ideal balance was first demonstrated in laboratory cultures of the protozoan Paramecium by Gause (1934), and was derived theoretically by Lotka (1925) and Volterra (1926). It is rare to find such perfect examples in wild populations, usually because predators have other prey which they turn to when their major source becomes scarce. An apparent case is shown in Fig. 12 which refers to the number of barn owls and kestrels ringed each year, and which may be taken as an index of their actual abundance. Snow (1968) has shown that most of the peaks of kestrels depend on high numbers being ringed in the north of England and south Scotland and they reflect fluctuations in the number of families ringed, not differences in the size of broods. There is no evidence for similar periodic fluctuations in southern England. Moreover, there is good evidence that the vole Microtus agrestis has been particularly abundant in the north of England in the same years that large numbers of kestrels have been found for ringing. But the curves seem to represent a numerical response of the predator, more kestrels settling to breed when vole numbers are high. There is no evidence that the survival rate of nestling or adult kestrels has varied in the different years in a way that would be expected in a classical predator–prey situation.

FIG. 10. Numerical response of predator to changes in prey density as shown by the number of nesting pairs of bay-breasted warblers per 100 acres of forest in relation to the number of third-instar larvae of the spruce budworm per 10 sq. ft. of foliage. (From Mook 1963).

FIG. 11. Three possible interactions between a predator and its prey. On the left the numbers of predator are plotted against numbers of prey and successive points on the time scale chosen (months, years, etc.) joined in chronological order. In the right hand graphs the numbers of prey (solid line) or predator (dotted line) are plotted against time.

A. An increase in prey numbers if followed by an increase of predators which eat the prey, causing a decline in the prey which is followed by a decline in the predator so that with time a steady balance is maintained.

B. Predator density increases relative to prey density so that the regular oscillations shown at A become damped.

C. Prey density increases relative to predator numbers and the system become unstable with violent oscillations.

FIG. 12. Number of nestling kestrels (top) barn owls (middle) or sparrowhawks (bottom) ringed each year as a percentage of all nestlings ringed under the British Trust for Ornithology scheme. The ease with which observers find nestlings to ring is assumed to be an index of the populatuon at risk. The kestrel and barn owl feed on the same small rodent species and it is noticeable that their numbers fluctuate in parallel in a manner reminiscent of the classical predator–prey curve depicted in Fig.11a. In contrast, the sparrowhawk feeds on small birds (see Table 3) and its numbers do not fluctuate in the same way. The suggestion of a decline in sparrowhawk numbers is almost certainly a true indication of the changed status of the species due to contamination of its food supply with persistent organochlorine insecticides. The risks of such contamination are very much less for species feeding on small rodents.

Simple systems of this kind are theoretically liable to change to the kind shown at C in Fig. 11, where the oscillations between predator and prey become self-destructive and lead to the extinction of one or the other. Feeding patterns like those shown above density 3 in Fig. 9, where increased prey density is not compensated by an increased predation (in practice more animals would usually move in to take advantage of such good feeding conditions), tend to produce violent fluctuations. One reason that such oscillations rarely occur depends on the complexity of natural ecosystems as was discussed in the previous chapter (see here). Prey is effectively isolated in groups so that if one group is accidentally exterminated it is re-populated in a density-dependent way according to its own food supply; predators tend to be less efficient at very high prey densities; and some prey have refuges enabling them to escape predation. These factors plus the existence of more than one kind of predator all help to dampen the kind of expanding oscillation seen in Fig. 11c.

When the percentage of predation at first increases with rising prey numbers, there is a high probability that oscillations will be damped as in Fig. 11b. If the numbers of an insect increase, a proportional increase in the amount of predation by birds could bring the system back to its old level. There is good evidence from L. Tinbergen’s researches that this is what certain insectivorous birds may achieve in preying on forest insects in the manner shown in the sigmoid part of Fig. 9; fluctuations in prey density can be reduced and predator–prey oscillations damped, so reducing the risk of an infestation developing. But it is clear that if insect density rises beyond the level where the predation curve is S-shaped in Fig. 9, that is, if the prey achieves densities where a smaller proportion is taken with rising density, then the predator could not be held to have a regulating effect. As will be discussed in Chapter 4, birds cannot control an insect plague once it has developed, but they may help prevent it developing in the first instance. From the point of view of pest control or conservation one general lesson following from the above is that predator–prey interactions will be most stable in environments with a diverse structure supporting a wide variety of predators and prey, as for example, natural undisturbed oak woodland. Monocultures of introduced conifers would be expected to provide unstable conditions. Voute’s (1946) observation that outbreaks of insect pests are commoner in pure than in mixed stands of trees is, therefore, of considerable interest and adds weight to the suggestion that forestry policy should aim at intermixing deciduous trees in conifer woods (see here).

It can often happen that a predator takes only a fixed number of the prey with which it is in contact, satisfying its food requirements and allowing the surplus prey to escape. Again, this is an unstable situation which cannot last, either because predator numbers would eventually increase leading to new relationships, or because if the same predators persist in their attacks they will eventually exterminate the prey. Examples are the temporary concentration of birds seeking the invertebrates disturbed by a farmer ploughing a field, the gathering of swallows and martins to feed on the insects blown from a wood in strong wind, and the birds which gather round a locust swarm. Here, the number of prey eaten depends on the number of predators that chances to arrive on the scene, and is not a function of prey density. The scale of losses inflicted by birds on locust swarms seems usually slight. Around a small locust swarm in Eritrea, which covered about ten acres, Smith and Popov noted two or three hundred white storks, many great and lesser spotted eagles, and several hundred Steppe eagles, as well as smaller numbers of black kites, lanner falcons, marabou storks and other species. Shot and dissected storks each proved to have eaten up to 1,000 locusts. But most observers agree that this scale of predation has a negligible effect. More important is the suggestion (e.g. Vesey-Fitzgerald 1955) that birds may be useful in preventing a rapid build-up of locust numbers. Recent evidence from the Rukwa Valley indicates that this is not the case. First, because the preferred feeding habitat of the birds mostly concerned – white stork, cattle egret and little egret – is the short grass area of the lakeshore, whereas the locusts prefer and breed in the long grass associations covering much of the plains. Second, locusts are most abundant from March–June, when bird numbers are low, and decrease for other reasons in July when large numbers of immigrant birds arrive.

When man preys upon wood-pigeons by shooting them on their return to their roosting woods, the number he kills increases slightly when the total population increases, but the percentage shot declines. Each man shoots at the passing flock but can potentially kill only two birds because he uses a double barrel 12-bore gun. More flocks pass when pigeon numbers are high, enabling a higher total of birds to be shot, but the flocks are also much larger and a smaller proportion of each can be killed. Hunting methods impose a limit on a man’s kill, and the only way to achieve a higher rate of predation would be for more men to shoot or for each man to use a faster firing, or otherwise more efficient gun. The first possibility is limited by social considerations; the number of men interested in shooting, either for sport or for monetary gain, is restricted because today there are so many more outlets for pastoral relaxation and the financial reward for shooting pigeons is low. The battue shoots did not usually begin until the end of the pheasant shooting season, in late January and early February, because only then were gameowners prepared to let pigeon shooters wander over the estates. They ended in early March when shooting at dusk on the longer days interfered with the other attractions of evening, the village dance or local hostel. The alternative of using a more lethal weapon would be opposed to the arbitrary code of sportsmanship current in Britain today, a code which imposes operose conditions on the ways animals can be ‘taken’ – a euphemism for ‘killed’. In days when cumbrous muzzle-loading guns prevailed, shooting birds sitting on the ground was acceptable, but fashion changed to ‘shooting flying’ during the eighteenth century as guns improved, and today shooting a sitting bird is unthinkable. Today a similar pretentious scorn is poured upon the American repeater, just as it was upon the double barrelled 12-bore when it first appeared. As Markland (1727) says in Pteryptegia or The Art of Shooting Flying:

he who dares by different means destroy

Than nature meant, offends ’gainst Nature’s law.

A viewpoint all right in sport but with no place in serious pest control.

Sometimes predators take all, or at least a large proportion of, a prey species only when the prey exceed a certain minimum number. This minimum may result from a fixed number of safe refuges, physically or behaviourally determined, in the environment; or may occur because the predator finds it unrewarding to search for prey below a fixed density and moves off elsewhere. Errington showed that a given area of range in Iowa could support a relatively fixed number of bob-white quail in winter irrespective of the autumn population, while the surplus birds were taken by predators. A similar story applies to the red grouse in Glen Esk which have been studied in great detail by D. Jenkins, A. Watson and G. R. Miller. It has already been noted that a territorial system relates grouse numbers to the carrying capacity of the habitat, forcing excess birds to move into less favourable marginal areas. Most of this dispersal takes place during two distinct seasons, from November to December and from February to April, the displaced birds suffer much more from predators than those resident in territories. Knowledge of a territory presumably enables the individual to find hiding places when danger threatens. Of 383 birds individually tabbed which had territories in November the remains of 2% were later found killed by predators, whereas of 261 tagged birds known to be displaced from territories in November as many as 14% were found killed. On high ground (700 m. and above) eagles and foxes accounted for about half the grouse preyed upon, while on low ground (500 m. and below) foxes and hen harriers were about equally responsible for the losses. Table 2 also shows how grouse suffered much more from predators in years when the number known to be dispersing was high. The number of raptors actually hunting in the study area also increased in such years, although roughly a similar total was present in the general area each year. Jenkins et al. presume that the grouse were but one of a number of suitable foods and were only taken when they were particularly vulnerable. In this case the number of predators was not determined by the availability of the specific prey studied; their numbers may have been related to the abundance of all prey animals combined, but this is at present unknown. There are times when it is only the birds dispersed to marginal habitats in ways like those outlined above, that cause economic problems – one case is given below (see here).

At Glen Esk gamekeepers persecuted the predatory birds and mammals at every opportunity. In spite of this, Jenkins et al. showed that slaughter was not controlling these predators because a similar number appeared every year. They point out that the number of predators expected to be killed on the moors of Perthshire and Kincardineshire today are similar to figures quoted in 1906 by Harvie-Brown. Keepers may well reduce breeding numbers slightly, or prevent the breeding of some individuals in early summer, but the relatively large number of young produced on the estates in question and near by, is always sufficient to make good these losses. In other words, gamekeepers merely crop an expendable surplus of predatory birds and mammals, in much the same way as these predators crop their prey – as a man does when he shoots grouse. In the circumstances, it is probably a waste of time for the keepers to set their traps and patrol their estates in search of so-called vermin. As far as grouse are concerned, the most useful employment for the keeper would be to lend a hand with heather burning and help to improve moor management, for this is really what affects grouse numbers, not predator control.

It would be a sophism to infer from this account that the activities of gamekeepers have never been detrimental to the birds of prey, but it is likely that in many cases the senseless slaughter has at most accelerated processes brought about by more fundamental changes, particularly in land use or loss of habitat. Once a species declines and becomes restricted in range through lack of habitat it is far more vulnerable to persecution by man. The osprey was probably always rarer as a breeding bird than some authors have implied, as it was restricted by a need for large lochs with good fish populations, but man’s greed and his continuous persecution eventually caused its extinction in Britain at the turn of the century. The marsh harrier, too, has long been persecuted. In one Suffolk locality two or three pairs regularly nested up to 1951, in spite of egg-collectors (the mentality of one of the men concerned is shown by the fact that in one day he took nine clutches of shoveller from the level where the harriers bred to demonstrate clutch and egg-size variation) and shooting by the keeper of a nearby estate. But it was drainage of the marsh to improve cattle grazing that spelt the final doom for the harriers at this site in the mid 1950s; not just persecution, senseless though it had been.

Unlike the marsh harrier, the hen harrier has proved remarkably adaptable in its habitat requirements: indeed in the Americas it is the only harrier and occupies the combined niche of all the Old World species. In the north of Britain it has increased, despite fairly heavy persecution, and is doing particularly well in the new conifer plantations of Scotland, which are rich in ground rodents. On the whole, the hen harrier seems to be making good the ground it lost in the nineteenth century, when an even more intensive slaughter banished it from the Scottish mainland as a breeding species. In eastern Europe, the pallid harrier has expanded and increased its range from the Russian Steppes, in close association with the spread of agriculture. Thus human disturbance alone does not necessarily disturb birds of prey. This is shown by the distribution and breeding of the osprey on the eastern end of Long Island Sound in coastal Connecticut and New York with little regard for human activity; it nests on artificial man-made platforms as does the stork in Europe.

The difficulties of measuring the contribution of habitat change and the persecution of gamekeepers, skin and egg-collectors to the decline of the raptors is well illustrated in the case of the buzzard. The changing status of this bird has been very carefully documented by Dr N. W. Moore following a survey sponsored through the British Trust for Ornithology. Until the early nineteenth century the buzzard was to be found over virtually the whole of the British Isles. Then a serious decline occurred in East Anglia, the Midlands and much of Ireland in the mid-nineteenth century, followed by some recovery in the twentieth century. Today densities of 1–2 pairs per square mile can be expected in suitable habitats. The decline cannot be attributed directly to the spread of agriculture during the nineteenth century because the species underwent increases and decreases both during times of agricultural advance and recession. Also during this period the rabbit, one of the main foods of the buzzard, became more common. Similarly, more urbanisation took place between 1915 and 1954 when the buzzard was increasing, than during the years 1800–1915 when it was decreasing. Furthermore, in the 1954 survey, which indicated a British population of 20–30,000 birds, the highest buzzard density was recorded on mixed agricultural moorland, rather than in pure forest or on extensive moorland, where nesting sites seem to be in short supply. In fact, Moore attributes the early decline of the buzzard to the game-preservation which boomed from 1800–1914. Convincing evidence is provided by his maps, which show an inverse correlation between areas of intensive game-preservation, judged by the number of gamekeepers per square mile, and the distribution of buzzards. His view is also supported by the fact that the biggest recovery took place during the two world wars, when there was much less game-preservation, and many keepers were fighting a different adversary. However, the early decline of the buzzard in the nineteenth century is also temporally related to a marked decline of sheep farming, particularly in East Anglia, and, as discussed below in the case of the raven and carrion crow, the associated loss of carrion may have provided the initial cause, being only accelerated by keepers. Nor does persecution account for the disappearance of the buzzard from Ireland.

Myxomatosis was confirmed at Edenbridge in October 1953, and from two original outbreaks it rapidly spread until by early 1955 rabbits throughout the mainland of Britain were infected with a 99% lethal strain of the virus (Armour and Thompson 1955). The 1954 buzzard survey was carried out before there had been widespread reductions in rabbit numbers, but already many poultry farmers and shooting men were afraid that the bird would now turn to other forms of prey, particularly chickens and game-birds. The same concern was accorded the fox, but fortunately an investigation of this animal’s feeding habits had been made before myxomatosis by Southern and Watson (1941) and this was repeated by Lever (1959) on behalf of the Ministry of Agriculture in 1955. The results showed that in the absence of rabbits, foxes concentrated on other small rodents which would normally have been their second most important prey; the incidence of poultry or game-birds in stomach remains did not increase. This turned out to be roughly what happened in the case of the buzzards. Deprived of rabbits, they turned to other small rodents, but took no more game-birds or poultry than before. In the normal course of events the buzzard is not a very specialised feeder and takes a wide range of prey, including rabbits, small rodents, birds and invertebrates, so that their response to a density change in one prey species was as discussed above (see here). Although the buzzard could turn to other prey, it proved much more difficult for the birds to obtain enough food. The immediate consequence of myxomatosis was that many pairs failed to breed, while those that did attempt to nest laid fewer eggs and were much less successful than usual at rearing the young.

The deforestation in north-west Scotland which caused the loss of the roe deer, great-spotted woodpecker and other species (see here), also opened up the Western Highlands for sheep grazing (around 1800); at first good on the rich woodland soil, but subsequently poor as a result of soil degeneration and moor burning. Muirburn resulted in the loss of woody, nourishing and palatable plants leaving only those species resistant to fire. Associated with this spoliation of the habitat, the numbers of all local animals decreased, including grouse, mountain hares, woodcock, snipe and red deer. These last die in large numbers in winter because the impoverished habitat provides much too poor a food supply at the critical time – ideally, good management should ensure a better balance between summer and winter resources. For roughly the same reasons, many sheep die each winter and few lambs survive. A good deal of carrion therefore exists in the form of deer, lambs and ewes already doomed to die and this provides food for golden eagles in the area. Dr J. D. Lockie, who has examined the problem in detail in Wester Ross, has taken great care to discover to what extent eagles prey upon live sheep. Lambs taken as carrion have often lost their eyes as a result of crow attack, or have had limbs or ears bitten off by foxes. In catching live lambs the eagle’s talons cause considerable haemorrhage and bruising of the back, which can be recognised at a post-mortem. It is, therefore, fairly easy to distinguish the two sorts of prey by examining lamb carcases in eyries, and for 22 remains found at one eyrie between 1956 and 1961, 10 could be so categorised. Three of these lambs had been killed by eagles and seven taken as carrion. What could not be determined was how many of the live captures were weakling animals or twins – an important consideration in the case of attacks by ravens and carrion crows (see below). Nevertheless, the anti-eagle policy adopted by so many shepherds is understandable. Lockie was able to show that the percentage of lamb in the eagle’s diet averaged about 46% in years when lamb survival was average or poor, that is, when conditions for lamb rearing were poor; but it fell to 23% in years of high lamb survival. Hence, when lamb was not abundant the eagles compensated by turning to other prey. Clearly, sensible sheep management is the answer to any eagle problems, and it is not fair to attribute poor lamb seasons directly to eagle predation.

On the Isle of Lewis, complaints that eagles had been attacking sheep in 1954 were investigated by Lockie and Stephen on behalf of the Nature Conservancy. Here the main prey comprises rabbits, lamb and sheep carrion, supplemented by a few hares, grouse, rats, golden plover and hooded crows. Occasionally the eagles do attack live lambs and a pair which were seen to attack 5 lambs sparked off the complaints. Actually, out of thirteen local farmers and crofters interviewed, only two had seen eagles in the act of killing lambs, though two others believed that eagles did attack lambs. The eagle has increased on Lewis since about 1946, coincident with a decline in mountain hares, grouse and rabbits, but an increase in sheep. As the eagle density is now, if anything, higher on Lewis than in other areas of Scotland, where a much richer wild fauna exists, it seems that the high density is maintained by the sheep carrion, of which there is an excessive amount because sheep mortality is high. Overgrazing occurs and deficiency diseases are frequent. In one two-mile walk on 20 April, 28 carcases were counted. Again the basic problem is one of land management, the inefficient farmer being the one who suffers most.

The Western Highlands are mostly deer-forest where, with the exception of some shepherds, the hand of man is not specially directed against birds of prey. The attitude is that if these eat grouse they do good because an accidentally flushed grouse frightens deer and hinders the stalker. The attitude varies again in north-eastern Scotland. In parts of the southern Cairngorms, where Watson found about 12 eagle pairs in 220 square miles of suitable country, sheep are rare on the hills in winter and their density in summer is also low compared with Wester Ross and Lewis. Here eagles are rarely disturbed. Their food in summer comprises about 60% red grouse and ptarmigan and around 30% mountain hares and rabbits. On lower ground, which is grouse moorland, and also on the grouse moors of the southern Grampians, any bird with a hooked bill is considered a potential competitor with man for the grouse stocks – a totally unjustified view as we have seen. The effects of persecution are well-illustrated by a study made by Sandeman of breeding success among eagles in the south Grampians. Successful birds reared on average 1.4 young per year, but making allowance for non-breeders, or birds whose eggs or young were destroyed, gives a figure of 0.4 young per pair per year for the whole area. In the northern part of the area studied by Sandeman the land is primarily deer-forest and sheep ground, where eagles are little disturbed. Here the average success was 0.6 young per pair, which compares with a production of only 0.3 young per pair on nearby areas predominantly given over to grouse-management and sheep-grazing, and where persecution is considerable. The consequences of killing adult eagles are also reflected in the number of immature birds mated to old birds. In 24 territories on deer ground which were occupied over the years 1950–56, no member of any pair was ever immature and no bird was without a partner. In contrast, on the grouse and sheep moors where 51 occupied territories were watched over the same period, immature birds were paired to adults in four territories, while in eight territories only one member of the pair was present. Males or females mated to immature birds either did not breed, or if eggs were laid these were often infertile; killing could thus result in a suppression of breeding success in following years among the survivors. Immature birds were replacing lost adults and, although this replacement may have been insufficient as to saturate the pre-breeding population, it is possible that post-breeding numbers were little below par due to immigration. It is perhaps surprising that intensive killing had so little effect on this slow breeding species, but the area in question probably relied on immigration from areas with a higher breeding success, and were it not for the existence of such reservoirs killing would certainly have depressed total numbers. In the southern Cairngorms, Watson found that the average number of young leaving a successful nest was similar to the above at 1.3 young per pair. However, more pairs were successful and five which were closely studied by Watson reared 0.8 young per year. It is presumably from areas such as these that excess birds are produced which can replace the losses inflicted by man on the grouse estates.

The population of eagles in the deer-forest country of the remote North-West Highlands has probably long been near the maximum carrying capacity of the habitat, in spite of constant harrying by man in supposed defence of his sheep. It required the more subtle action of toxic insecticides to upset this balance, it being suggested that these derived from sheep dips containing organo-chlorine insecticides, particularly dieldrin. These chemicals contaminated carcases and were then accumulated by feeding eagles, with the result that their breeding efficiency was seriously impaired. Lockie and Ratcliffe (1964) found that the proportion of non-breeding eagles in western Scotland increased from 3% in 1937–60 to 41% in 1961–3, and the proportion of pairs rearing young fell from 72% to 29% in the same periods.

There is much stronger evidence that the peregrine has suffered drastically since toxic chemicals were introduced. In 1961 and 1962 Ratcliffe undertook a survey of the species for the B.T.O., primarily because pigeon fanciers had claimed that the species was increasing and threatening their interests. As it happened quite the opposite was found. The average British breeding population from 1930–9 had been about 650 pairs with territories, but in 1962 only about half these territories proved to be occupied, and successful nesting occurred in only 13% of 488 examined. There had been some depletion in the south of England during the war years of 1939–45, because the bird was outlawed as a potential predator of carrier pigeons with war dispatches, and was rigorously shot by the Air Ministry; it was almost exterminated on the south coast. Subsequently there was a rapid build-up in numbers in southern England, which were nearly back to the pre-war level by the mid-1950s. Then the second much more drastic and this time national decline took place, associated with a fall in nesting success and the frequent breaking and disappearance of eggs which the birds appeared to be eating themselves (see here). While the evidence that toxic chemicals were responsible was necessarily circumstantial, it was such that no reasonable person could wait for cut and dried scientific proof while there was a grave risk of losing much of our wild life in the meantime, and a voluntary ban on the use of these chemicals was agreed. All the same, the recovery of dead peregrines and their infertile eggs containing high residues of organo-chlorine insecticides, together with the coinciding of the decline with the increased usage of the more toxic insecticides, seems to indicate that pollution from these chemicals does account for the loss of these birds. In fact, fifteen infertile eggs from thirteen different eyries in 1963 and 1964 all contained either D.D.T., B.H.C., dieldrin, heptachlor or their metabolites. The distribution and residue level of these insecticides in adults and eggs shows that birds at the top of the food chain are highly susceptible to contamination. A sample of 137 of those territories examined in 1962 was again checked in 1963 and 1964. In 1962, 83 of these were occupied and in 42% of these young were produced (this is the best measure of nesting success), in 1963 only 62 of these territories were occupied but 44% produced young while 66 were occupied in 1964 and 53% produced young. There thus seems some hope that the alarming decline in numbers has been halted and that breeding success is returning to a more normal level. To complicate the picture, though certainly unconnected with the effect of toxic chemicals, there is some evidence that there has been a gradual fall in the peregrine population of the Western Highlands and Hebrides since the start of the century. Whether or not this decline followed the depletion of vertebrate prey in the region already referred to, is not at all clear.

Peregrines capture live prey, usually in flight, and, as Table 3 shows, domestic pigeons form a large proportion of the food in the breeding season. The peregrine is called duck hawk in the United States, and it can sometimes be seen on the estuary in winter instilling panic into wigeon and teal flocks, although duck form a relatively unimportant prey in the summer. It is surprising that the wood-pigeon is not taken more frequently, but it is likely that the adults, which average 500 gms, are too big; domestic and racing forms of the rock dove weigh 350–440 gms. In fact, the only wood-pigeons I have seen killed by the peregrine, and this was in S. E. Kent, were juveniles about 2–3 months out of the nest. In this area of Kent, peregrines seemed to do much better in autumn by concentrating on the flocks of migrants, particularly starlings, which pour into the country over the cliffs at Dover. It is not known to what extent peregrines take domestic or racing pigeons which have become lost and have joined wild populations and as a result are of no value to their owners. Ignoring this factor, but making various allowances for breeding and non-breeding birds, Ratcliffe estimated that the pre-war peregrine population (650 pairs) would consume about 68,000 pigeons per annum, while the depleted population in 1962 would eat about 16,500. This latter figure represents about 0.3% per annum of the total racing pigeon population of Britain, numbering about five million birds. To put this in proportion, there are about 5–10 million wood-pigeons in Britain, depending on the season, which are widely regarded as a pest of mankind – yet mankind happily finds food for 5,000,000 domesticated pigeons. In Belgium, the home of racing pigeons (one-third of the world’s pigeon fanciers are Belgian and one-fifth are British), the Federation of Pigeon Fanciers was offering a reward of 40 francs for evidence of the killing of red kite, sparrowhawk, peregrine or goshawk, in spite of the fact that Belgium has ratified the International Convention for the Protection of Birds under which such subsidies are forbidden. While education is again the answer to this kind of attitude it is slow to take effect. A big problem arises because pigeon racing, like greyhound racing, provides a relaxation which can be coupled with betting. As some pigeons are fairly valuable, and the loss of a race through a bird failing to home results in lost prizes or betting money, it is all too easy to lay the blame on a bird of prey.

There is much evidence that predators select ailing prey, and when this additional allowance is made it seems ludicrous to claim that peregrines can really do significant harm to racing pigeon interests. Rudebeck observed 260 hunts by peregrines. Of these only 19 were successful and in three of the cases the victim was suffering from an obvious abnormality. For 52 successful hunts by four species of predatory bird (sparrowhawk, goshawk, peregrine and sea eagle) he recorded that obviously abnormal individuals were selected in 19% of the cases – a much higher ratio of abnormal birds than would normally be expected in the wild. Thus when Hickey (1943) examined 10,000 starlings collected at random he reckoned that only 5% showed recognisable defects. M. H. Woodward, one time secretary of the British Falconers’ Club, quotes the case of 100 crows killed in Germany by trained falcons belonging to Herr Eutermoser. Sixty of these crows were judged to be fit, but the remainder were suffering from some sort of handicap, such as shot wounds, feather damage or poor body condition. But of 100 crows shot in the same district over the same period, only 23 were judged abnormal on the same criteria.

FIG. 13. Seasonal changes in the number of wood-pigeons (top figure) or domestic pigeons (lower figure) in the diet of the goshawk in Germany. The dotted line is based on Murton, Westwood & Isaacson 1964 and represents seasonal changes in the population size of the wood-pigeon. Goshawks take more pigeons when the population size of their prey is swollen by a post-breeding surplus of juveniles, domestic pigeons having their peak breeding season earlier than wood-pigeons. (Based on data in Brüll 1964).

Table 3 summarises the diet of two other birds of prey, the sparrowhawk and goshawk. Apart from demonstrating how two closely related species differ in their food requirements, enabling them to co-exist in the same deciduous woodland habitat without competition, the table shows the importance of the wood-pigeon in the diet of the goshawk. The fact that the goshawk is slightly larger than the peregrine and is also a woodland species accounts for its ability to take those larger pigeons which the peregrine rarely utilises. Many people have suggested that the goshawk should be encouraged to settle in Britain to help control the wood-pigeon population, but there is no evidence that it would take a sufficient toll to be effective, for the same reasons that eagles and harriers do not control grouse numbers. Fig. 13 supports this view by showing the proportion of wood-pigeons in the prey of goshawks at different seasons, against seasonal changes in wood-pigeon numbers. Clearly wood-pigeons are mostly eaten at the end of the breeding season when many juveniles are available, and in mid-winter when population size is still high. In spring, when the goshawk could potentially depress population size below normal – and hence really control numbers – it turns to other more easily captured prey. In contrast, feral and domestic pigeons breed earlier in the year and have a population peak in June; this is when they are most often caught by goshawks.

Neolithic husbandmen were doubtless familiar with the presence of ravens and crows near their domestic animals, long before biblical shepherds were tending their flocks aware that these birds were a potential menace to a young or weakly animal – the eye that mocketh at his father … the ravens of the valley shall pick it out (Proverbs 30: 17). Predacious habits and black plumage, burnt by the fires of hell, long ago made the crows prophets of disaster. A suspicion of such augury still persists among those who today think it appropriate to hang corvids and birds of prey on some barbed wire fence or makeshift gibbet; while these crucifixions may well release human frustrations, they do nothing whatever to deter the survivors (see Chapter 12).

Ravens are no longer widely distributed throughout Britain as they were in medieval and even more recent times, but there are still frequent complaints from hill farmers and shepherds in parts of Wales, northern England and Scotland that ravens, and hooded or carrion crows, sometimes kill or maim lambs and even weakly ewes. According to Bolam (1913) sheep, mostly in the form of carrion, comprise the major part of the diet of ravens in Merionethshire, sheep remains being found at least three times more frequently in castings than remains of any other food item (these including rabbits, rats, voles and mice, moles, birds, seashore and other invertebrates, snails and large beetles and some vegetable remains of cereals and tree fruits). Similarly, E. Blezard (quoted by D. Ratcliffe 1962) found sheep remains in over half the castings he examined from birds in northern England and southern Scotland, the next most important item being rabbit, which occurred in only a quarter of the castings. The examination of castings probably underestimates the importance of rapidly digested invertebrates or vegetable foods, but it is clear that sheep (probably as carrion) are an important food source, although the raven, like the crow, is very much an omnivore and carrion feeder. There is no reason to doubt that the raven had similar food habits in the past, when it occurred throughout lowland Britain; in fact, we know that shepherds in Suffolk around 1850 were bitterly hostile to the bird – ‘five were among Mr Roper’s sheep at Thetford in August 1836’. Like the buzzard, the raven was a reasonably common breeder in Norfolk and Suffolk until about 1830, but it declined markedly thereafter, coincident with the rise of intensive keepering, and it had vanished by the end of the nineteenth century. While continued persecution was doubtless responsible for the final elimination of the bird, and was also probably responsible for making the carrion crow very rare in the second half of the nineteenth century, other factors doubtless contributed to the initial decline. Loss of carrion is usually given as the cause, and it seems likely that it was specifically the loss of sheep carrion that was responsible. In Norfolk and Suffolk this coincided with the period of active enclosure, particularly that of waste land and sheep walks from 1800 to the mid-nineteenth century. According to Arthur Young, half Norfolk yielded nothing but sheep feed until the close of the eighteenth century, when with enormous speed – enclosure was mostly achieved in twenty years – the land was covered with fine barley, rye and wheat. The rapidity with which enclosure was completed is manifested in west Norfolk by straight roads and compact villages, the result of planning on a large scale, whereas in the east of the county the winding lanes, isolated churches, farms and homesteads derive from centuries of slow economic evolution.

Although improvements in hygiene, a lack of carrion and the extensive use of firearms may have eliminated the raven from most of lowland Britain, in relatively undisturbed areas, like the Welsh and Scottish Highlands, its density has probably been altered less. But even in such areas man has much reduced the upland forest habitat of the species and caused it to depend on cliffs for breeding. For more recent times, Ratcliffe (1962) has been able to show that breeding populations in four areas he studied have not dropped by more than 14% since 1945, and average only 6% below the maxima ever recorded. Some increases may even have occurred in areas where the bird previously suffered intensive persecution; in the Scottish borders tree-nesting, but not rock-nesting, has increased since 1945, indicating an increase in local populations which are again able to exploit traditional nesting sites. In Ratcliffe’s four inland study areas the average size of a raven’s territory ranged from 6.6 to 17.6 square miles (in these same areas the breeding density of the raven was about 2½ times that of the peregrine) but higher densities may occur in favourable coastal areas, for example, four pairs in two miles of cliffs in Anglesey. In Pembrokeshire, R. M. Lockley estimated the raven population at 80 pairs in 1949. In 1953 M. G. Ridpath (Report to Ministry of Agriculture, Fisheries and Food, 1953) searched 25 miles of cliff, in the 140 miles of apparently suitable coast-line, and found an average of one breeding pair every two miles.

Ridpath spent three weeks (220 hours) between 9 and 30 March, 1953, watching a lambing flock of about 1,500 sheep in the Prescelly Mountains, Pembrokeshire. During this period he saw two lambs killed by ravens, and in addition nine other attacks on lambs and eight on adult ewes. Attacks on lambs were concentrated on the eyes, lips, umbilical cord and anus. In one case two ravens persistently attacked a four-day-old lamb in spite of the mother’s efforts to defend it. At first the ewe managed to ward off the birds, but eventually one of them managed to peck at the lamb, at which point the mother walked away leaving the birds to finish the kill. In many other cases when attacks were first witnessed, the ewes were active in defence of their young and successfully repulsed the birds.

One of the local farmers, an experienced observer, showed Ridpath a young lamb which he had seen killed by a raven as it was being born. By weight and appearance it did not seem to have been a weakling. Both ravens and crows are certainly attracted by the afterbirths at lambing time, and sometimes newly born lambs are not readily distinguished. Lambs are most vulnerable during the moment of delivery (especially if parturition is at all difficult, or twin births occur) when the mother cannot guard them, and for two or three days afterwards. Weaklings are attacked most often, as they are easier to kill than healthy lambs, which make vigorous attempts to escape. Ravens and crows (and for that matter golden eagles in Scotland) rely mostly on carrion, which is fairly common owing to the larger number of sheep which perish in the rigorous hill environment (see below). Attacks on sheep are most frequent at lambing time and during the winter months when parties of ravens or crows are attracted to the supplementary feed put out in the vicinity of the sheep flocks. There is reason to believe that much of the trouble is caused by non-breeding or immature individuals of either species. Thus during Ridpath’s study up to nineteen ravens were seen associating with the sheep, but though paired they seemed to be non-breeding birds, and were probably immatures not yet holding territories. Ratcliffe considers that established pairs keep very much to their own territories and do not associate in flocks in this way. It is very likely that these bold attacks on lambing sheep are largely made because birds excluded from the large territories, which must normally contain ample stocks of carrion, are short of food.

Good shepherding in the hills of Britain is a tradition that goes back for centuries. It has always included burying carcases which attract predators and cause disease, the regular and frequent surveillance of lambing flocks and help for ewes in difficulties and for weakling lambs – bad cases are even brought down from the hills. Ridpath concluded that any trouble could be greatly reduced by returning to these practices. If and when control is really needed (the raven is rare and is protected by law) it should be aimed only at the birds causing the damage. It should not involve indiscriminate killing of all the corvids in the area, most of which probably cause very little harm.

Unlike the raven, the carrion crow has increased considerably throughout Britain after suffering a marked suppression from the 1860s until the early twentieth century. A decline in the intensity of game-preservation after two world wars has certainly been a big factor, but it is clear that the crow’s feeding habits have enabled it to become re-established in areas now unsuitable for the raven. It seems likely that it has benefited from changes in agriculture and is the best adapted avian scavenger of the new farm environment. That its numbers are still increasing over most of Britain is shown by a B.T.O. inquiry recently conducted by Prestt (1965) for the period 1953–63. It is probably significant that the only region where no increase has occurred over the last ten to fifteen years is East Anglia, where game-preservation remains most intensive.

Burgess (unpubl.) recently organised a survey of carrion crows over an area of 6,000 acres, near the confluence of the North and South Tyne rivers in Northumberland. This is predominantly a pasture area, lying 2–300 feet above sea level, and consists of large fields surrounded by untrimmed hedgerows with many mature trees. The survey involved the destruction of all occupied nests that could be found in mid-May and a repeat of this operation in late June and August, partly to check for repeat nests or those previously overlooked. The first search for nests was begun in April. For the whole area, including those overlooked in the first operation, there were about 103 occupied nests in May 1961, 134 in May 1963, 128 in 1964 and 137 in 1965 (old nests which were never used were noted and totalled about as many nests again in each year). The results indicate a breeding population averaging one pair to about 50 acres, excluding an unknown number of non-breeding individuals. They also suggest a remarkable constancy in the size of the breeding population in different years, a feature also noted for the raven by Ratcliffe. Population fluctuations in birds of prey, including some corvids, seem to depend largely on the number of non-breeding individuals, partly because the size of a breeding territory seems less flexible than in many bird species, and sets a relatively constant limit on the size of the breeding population, which thus remains stable over long periods. This is not to deny that if long enough periods are considered, or different habitats, the size of the territory is ultimately adjusted to the food supply available. As virtually all successful breeding was prevented in 1961 by the nest destruction, it is clear that this had no depressing effect on the subsequent breeding population, a result in keeping with other similar studies and to be expected.

Because of their smaller size, crows seem less of a danger to ewes at parturition and to young lambs, but because of their large numbers and wider range they provide a greater potential threat to the sheep flocks. In Wales, Ridpath saw two carrion crows kill a ewe and her lamb during delivery, and during his three-week watch he also recorded 30 abortive attacks on sleeping lambs, where the crows crept up to the animal and then pounced at the head or tail base.

It is most distressing for a shepherd to contemplate such savage attacks; to see his defenceless lambs with their eyes pecked out obviously rouses deep emotions which make it hard to keep the problem in perspective. It is difficult to obtain objective and unexaggerated estimates of damage. Burgess (1963) did try to overcome this problem and organised an inquiry covering 155 selected hill-farms in Cumberland and 59 in Westmorland in 1962, after a good deal of publicity to ensure that all incidents would be reported. These farms between them supported some 82,000 ewes and in all 16 attacks on ewes were reported (0.02%). In nearly all cases the ewes attacked were in some difficulty, trapped in snow drifts or hedges, lying on their backs or giving birth. About two-thirds of the attacked ewes did not survive, but a little over half of these were already sick, many suffering from staggers. On the same farms there were 69 attacks reported on lambs, approximately 77,000 being at risk. About half the attacks were made on live lambs (0.04% of lambs at risk) while half again were fit lambs that should have survived. Allowing for unreported cases, the loss of lambs due to crow attack must be well under 0.5% of those at risk.

A pilot survey was conducted in Argyllshire between May and July 1964 (Gailey in litt.) by officers of the Department of Agriculture and Fisheries for Scotland. On 50 farms holding 48,390 ewes and 36,292 lambs*, losses attributable to hoodie crows were 192 ewes (0.4%) and 366 lambs (1%). Losses due to all predatory birds (including eagles, ravens and great black-back gull) amounted to 1% of the total sheep stock, hoodies being responsible for 0.65% of this total. Again, this is a very low percentage of damage particularly as this area of Scotland is generally reckoned to suffer the highest level of crow damage. In Argyll, the average mortality of ewes is 7.4% from November to July and 1.6% from July to November, according to McCreath and Murray (1954). These authors give lamb losses as 13% between birth in April and marking in June, and 5% between marking and sales in September. The sheep stock for the county of Argyll is approximately 450,000 breeding ewes and 338,000 lambs. The application of these results to the whole county would mean a loss of 2,926 ewes and 2,195 lambs to hoodies, which at £5 and £3 per head respectively at first suggests £21,000 worth of damage. But this is the kind of calculation made by the farmer and is quite unjustified. It can be calculated from McCreath and Murray’s mortality data that around 16,000 ewes (3.6%) and 60,800 lambs (18%) would die in the county between April and the September sales, a level of normal wastage far above that of the damage attributable to crows.

The level of damage to sheep seems to be markedly similar in widely separated areas. A survey in Radnorshire, Breconshire and Montgomery in 1969 showed that under 0.01% of 114,751 ewes at risk were attacked by crows, while 0.6% of 119,680 lambs at risk were attacked, the figure becoming 1.4% if only farms where attacks actually occurred are included (K. Walton, in litt.). In Australia, Smith reckoned that avian predators were responsible for the death of less than 2% of the lamb crop, and other Australian studies indicate that the live lambs attacked are already ailing; many have no milk in their stomachs and seem not to be receiving proper maternal care. The work of Alexander et al. (1959) in Australia has shown that sheep in their lambing flocks react relatively little towards foxes, more towards crows and most of all towards dogs. Unlike foxes, crows make very determined efforts to attack lambs.

As already discussed (see here), losses are not additive in these circumstances and it is likely that deaths caused by predatory birds simply improve the survival chances for the remaining animals, so that the final yield is unaffected. There would have to be a very much lower death-rate of sheep and lambs from natural causes before it could be accepted without qualification that predatory birds were depressing the output. The survival of sheep must depend largely on the carrying capacity of the hill, and an effective reduction in sheep mortality would best be obtained by improvements in land management. In large areas of Britain overgrazing and bad land management have been responsible for much sheep carrion, and this in turn supports the predator population. It is this sort of problem that needs evaluation and the immediate answer is not an out-and-out war on the birds. In some circumstances these birds may indeed be troublesome, even allowing for natural losses – but biologists cannot accept the extrapolation of damage costs, as in the example above.

In Britain it is common to see starlings, jackdaws, and less often magpies, associating with livestock and even perching on the backs of the animals. They catch the insects flushed from the ground by the animals’ movements or those attracted to the beasts, such as various flies. In addition, they sometimes search the fur for ticks and other parasites, like the tick-birds of Africa. The habit does not cause trouble in this country but in the U.S.A. magpies (a subspecies of the European form, which has a ring distribution extending all round the world) sometimes become more adventurous. Schorger (1921) and Berry (1922) have described how the birds learned to peck open a small hole in the sheep’s back, which they gradually enlarged until they located the kidneys which provided a favoured delicacy. Unshorn sheep on open range were sufficiently protected by the thick fleece, and it was only after shearing, when the animals were confined to untended paddocks, that the trouble began; possibly the birds were originally attracted by small wounds left by the shearers. Even small sores provide sites for secondary attack by blowflies. This kind of damage is reminiscent of the attacks of the kea parrot of New Zealand.

The progression from a commensal to a parasitic association between bird and mammal host is well seen in the red-billed oxpecker in Kenya. These birds feed on the ticks and other insects gleaned from the larger game animals, and help the host by warning it of impending danger. Occasionally, they also make the most of blood clots and fragments of skin from any abrasion or wound and will purposely open up a sore with hammer-like blows to eat the serum and blood discharged. Van Someren (1951) comments that the wounds inflicted on the livestock are smooth saucer-like depressions, 1–3 inches in diameter, which do not suppurate, perhaps because the birds keep them clean. Oxpeckers feed on open sores by nibbling with a scissor-like motion as if squeezing out the blood and serum. The attacked animals seem untroubled and their wounds rapidly heal if protected from bird attack. The dependence of the oxpecker on ticks is emphasised in districts where insecticide dips are extensively used. In these places the bird has declined drastically rather than become more prone to flesh feeding as some people feared. The European starling has also been recorded as inflicting extensive wounds on cattle in Texas, by pecking at warbles (McCoy 1941). Apparently the birds were first stimulated to attempt this mode of feeding when more normal food supplies were inaccessible through frozen ground.

CHAPTER 4

BIRDS AND FORESTS

TO see oak woodland today in anything like its natural state, with a field layer of bluebells, bracken and brambles, and a shrub zone of holly and other small deciduous trees, one must seek out places like the New Forest or the Forest of Dean, or search for other isolated remnants, mostly derived from secondary plantings, which are scattered in small patches throughout the land. Oak in something like this condition had covered much of Britain for around 4,000 years, to be almost totally destroyed since the enclosures of the last 300 years. At sites in Breckland, pollen analysis reveals a decrease of tree pollen in about 3,000 BC followed by the appearance of grass ling and heather and there seems no doubt that Neolithic man was able to clear some quite extensive areas, to produce the heathland that exists today; that this has been maintained does of course owe much to sheep and rabbit grazing. These early efforts at forest clearance, though effective, were certainly made on a very local basis. The open nature of oak woodland allows the development of a rich associated flora of shrubs and herbs supporting a consequently diversified fauna; this diversity has also been facilitated by a long period of establishment enabling a wide variety of plants and animals to reach Britain from Europe. In contrast, beech woodland only spread to Britain at the end of the Atlantic climatic period, roughly at the time when the English Channel was formed and the land bridge with Europe was severed. With the beech came the sweet chestnut, hornbeam and various poplars, and a few other species. None of these have become so widely established as the oak, partly because they are more demanding in their physiological and ecological requirements. In times of much more severe preboreal climate birch was the dominant vegetation, and large areas reminiscent of these ancient times still survive in many parts, particularly in the north of Scotland. Native Scots pine, which preceded the southern deciduous forests, is now confined to the ‘black woods’ of the Highlands. This, like birch, provides a more uniform habitat than oak and supports a less rich, but none the less interesting, avifauna.

After systematically spoiling almost all the native woodland, man has replanted the landscape with comparatively few small stands of hardwoods, a fairly considerable acreage of trees in orchard and hedgerow, and an increasing area of exotic conifers, from the Japanese larch to the north American Sitka spruce and Douglas fir. These new woods have been colonised in varying degrees by our native sylvan birds. Deafforestation, reafforestation (often with new and exotic species) and afforestation (the planting of trees where none previously grew, at least in recent times) have certainly altered the bird fauna, as we shall see in the next chapter. Here our task is to discover whether the birds have any clear effect on man’s interests. The more general question of the role played by birds in the ecosystem is much more difficult to answer, because the interactions involved are complex and not readily measured. There have been many general statements made about the whole plant and animal community, which can be neither refuted nor proved. For example, it is often claimed that some birds, by eating forest seeds, may hinder natural regeneration and, conversely, that birds actually help to distribute seeds and fruit. While it is undoubtedly true that many plants have evolved dispersal mechanisms which rely on animals (the mistletoe is a good example), no quantitative data on such inter-relationships are available. Turček considers that jays, which are specialist feeders on acorns, are practically the only agents able to move acorns uphill. Mellanby (1968) has also considered the role of animals in this respect and disagrees with the many ecologists who consider that most animals prevent natural regeneration of oak woodland. Those animals which destroy the most acorns by feeding on them also appear to be of most importance in causing oak regeneration. Turček also has data to show the importance of birds, particularly jays and blackbirds, in disseminating the sweet cherry in some spruce forests in the central Slovakian mountains. During the summer a mean of 18 casted seeds and 7 seedlings were found per square metre, most within 50 metres of the fruiting tree. Similarly, nutcrackers transport the seeds of Pinus cembra

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