Toxic chemicals and birds of prey By Stanley Cramp INTRODUCTION THE DRASTIC DECLINE in the numbers of almost all our birds of prey in the 19th and early 20th centuries, due to the increased efficiency of fire-arms, to collecting and, above all, to the spread of game pre­ servation, was followed in the cases of most of them by a general improvement in status which lasted until about the middle of the 1950's. In this, protection, the slow spread of a more enlightened attitude and the reduced activity of gamekeepers, especially during the two World Wars, all played a part. In the last few years, however, there have again been reports of declining numbers of some species, at a time when the growing use of toxic chemicals in agriculture and horticulture has provided a new threat to many forms of wild life. It is continued... 124 TOXIC CHEMICALS AND BIRDS OF PREY tempting to assume that the one has led to the other, but can this, in fact, be established? This paper examines the available evidence, much of it recent. RECENT CHANGES IN NUMBERS OF BIRDS OF PREY There are, unfortunately, all too few accurate census figures for any species of birds, either before or since the new synthetic pesticides began to be used on a large scale, and this applies to most of our birds of prey, both diurnal and nocturnal. The most recent assess­ ment was made in 1957 (Brit. Birds, 50: 129-155, 173-197) and it dealt only with the rarer diurnal birds of prey, without attempting to cover the positions of the owls or the three commoner raptors—the Sparrow- hawk, Merlin and Kestrel.* It was in the cases of two of these, the Kestrel and the Sparrowhawk, that signs of a widespread decline in numbers in recent years were first noted. After the i960 breeding season, R. S. R. Fitter, on behalf of the Council for Nature, gathered information from observers in a number of counties in England and Wales. The results showed evidence of a fall in the numbers of Kestrels in Northumberland, Yorkshire, Lincolnshire, Nottingham­ shire, Norfolk (except for marsh areas and the Broads), Suffolk, Cambridgeshire (except for the Fen Washes), Huntingdonshire, Essex and Oxfordshire, with little apparent change in Hampshire, Wiltshire, Dorset, Montgomeryshire, Merionethshire, Flint, Lancashire, Cumber­ land, Derbyshire and Leicestershire. The Sparrowhawk appeared to have been still worse hit, with decreases in Northumberland, York­ shire, Lincolnshire, Derbyshire, Leicestershire, Norfolk, Suffolk, Huntingdonshire, Essex, Oxfordshire, Hampshire, Dorset and Montgomeryshire, only Pembrokeshire and Flintshire reporting no evidence of any recent changes in numbers. Thus R. S, R. Fitter's information showed that both species had declined, often markedly, in the south and east of England and that in the case of the Sparrow- hawk this decline extended also to parts of western England and Wales. An examination of the county bird reports for i960, and, where available, for 1961, confirms this general picture, except that in York­ shire the Kestrel seems to have decreased in only parts of the county. In addition, the 1960-61 reports show that the Kestrel has now declined in Kent and probably Leicestershire, but not apparently in Herefordshire; and that the Sparrowhawk has now decreased in Herefordshire, Nottinghamshire and the London area, but not in the Bristol area. They also suggest that the Sparrowhawk may now be virtually extinct in Lincolnshire, Huntingdonshire and Norfolk. Some light on the timing and extent of this decline is provided by *The scientific names of all birds mentioned in the text can be found in the appendix on page 139; other-wise they are given only in the tables. 125 BRITISH BIRDS an analysis of the Nest Record Cards for these two species, which has kindly been made by Henry Mayer-Gross, of the British Trust for Ornithology (fig. i). The numbers of cards relating each year to nests of Kestrel and Sparrowhawk are very small and therefore liable to be influenced by chance fluctuations, but the general picture is clear and agrees closely with that drawn from the reports of observers. Whilst the total of Nest Record Cards for all species rose from about 5,000 in 1950 to between 8,000 and 11,000 in the years 1951 to i960, with a sharp rise to over 14,000 in 1961, the number of Sparrowhawk nests reported under the scheme showed signs of a general decline from about 1955, most marked in eastern and southern England, but affect­ ing the rest of Great Britain from 1957 onwards. The decline of the Kestrel is less marked on the Nest Record Cards, and so far appears to be largely confined to southern and eastern England, where it seems to have begun about 1956. In the case of a much rarer species, the Peregrine, we have far more exact data to show a recent and almost catastrophic fall in numbers. At the request of the Nature Conservancy, a census of Peregrines was organised by the British Trust for Ornithology in 1961 and 1962. This followed representations by the pigeon racing enthusiasts, who feared that these falcons were a serious threat to their birds, but it now appears that the problem is not one of regulation of numbers of the Peregrine but rather the conservation of the remnants of a seriously threatened species. In 1961, 431 of the 718 Peregrine territories known in Great Britain were visited. In southern England and Wales the majority of nesting places appeared to be quite deserted, and only eleven pairs in the 156 sites visited were known to have reared young. In northern England and southern Scotland, Peregrines were entirely absent from 45 of the 97 territories visited and only eleven pairs were known to have reared young. In the highlands and islands of Scotland the picture varied from district to district; some areas had a fairly normal nesting season, with most territories occupied and most pairs succeeding in hatching eggs, but in others (including some of the remotest) a much lower proportion of breeding pairs produced young, although the nesting populations were normal. Taking the country as a whole, of the 431 territories visited, Peregrines were absent from 173, one or both birds were present but not known to nest in 118, and of the remaining 140, where eggs were laid, only 82 pairs were known to have reared young. The general picture, with this trend of deterioration northwards, was similar in 1962, but there was evidence of a further decline, particularly in regions south of the Highlands, where breeding success was only half that of 1961. The 1962 results indicated that about half the pre- 1939 population of Peregrines in Great Britain, about 650 breeding 126 TOXIC CHEMICALS AND BIRDS OF PREY FIG. I. Numbers of B.T.O. Nest Record Cards for the Sparrowhawk (Accipiter nisus) and Kestrel (Falco tinmmculus), 1950-61, to show differences in trends between south and east England and the rest of Britain. The grand totals for all species are also shown to demonstrate that, whereas numbers of Sparrowhawk and Kestrel cards are falling, at least in south-east England, an increasing amount of attention is actually being paid to this method of recording nest data 127 BRITISH BIRDS pairs, had gone, and that only just over a quarter of the remainder bred successfully (D. A. Ratcliffe in litt.). There is no published information on recent changes in total numbers of other diurnal birds of prey in the British Isles, but observa­ tions made by Ash (i960) on a game preserve in Hampshire from 1952 to 1959 suggest that several other species may be affected. Ash's records of sightings of eight different species were made throughout the year and do not therefore refer only to the breeding populations. They show: Kestrel, a decrease from 1956; Buzzard, a decline after the advent of myxomatosis in 1954, then a revival, and then another decline; Sparrowhawk, a steady decline from 1953; Hen Harrier, a •decrease by two-thirds from 1956; Merlin, a decrease by two-thirds between 1953 and 1957, then numbers remaining steady; Peregrine, almost a two-thirds decrease, the main decline starting in 1956; Hobby, fluctuating numbers, but generally a decrease of about two- thirds; and Montagu's Harrier, the only increase (in 1958 and 1959). Moreover, the recent decline in numbers of birds of prey is not confined to the British Isles. Ratcliffe and Moore (1962) stated that they had received reports of serious decreases in Peregrines from the eastern United States, Germany, Finland and Sweden. At least in certain parts of its range in North America, the Bald Eagle has suffered severely. According to Carson (1962), in the years 1939-49 Charles Broley used annually to ring about 150 young Bald Eagles in about 125 nests on a stretch of the western coast of Florida. In 1947 the numbers of young began to decline; some nests contained no eggs and others had eggs which failed to hatch. Between 1952 and 1957 about 80% of the nests failed to produce young. In 1957 Broley found only 43 occupied nests, of which seven produced young, and in 1958 only ten nests and a single chick. At Hawk Mountain, in south-eastern Pennsylvania, the proportion of first-year eagles seen on autumn migration fell from 40% in 1935-39 to 20% in 1955-59. In the case of the nocturnal birds of prey it is more difficult, and perhaps impossible, to make any accurate assessment of total popula­ tions. There is no firm evidence of recent changes in numbers, although, as described below, owls appear to be dying from toxic chemicals on a considerable scale.