Collins New Naturalist Library. R. Murton K.

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СКАЧАТЬ 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 СКАЧАТЬ