Man and Birds. R. K. Murton
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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