Читать книгу Man and Birds - R. K. Murton - Страница 8
ОглавлениеTHE 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.
