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.
EVIDENCE OF DEATHS OF BIRDS OF PREY FROM TOXIC CHEMICALS Since i960 the Joint Committee of the British Trust for Ornithology and the Royal Society for the Protection of Birds on Toxic Chemicals has, in co-operation with the Game Research Association, been collect ing evidence of birds killed by toxic chemicals. Their first three reports (Cramp and Conder 1961; Cramp, Conder and Ash 1962 and 128 TOXIC CHEMICALS AND BIRDS OF PREY 1963) show that 72 deaths of diurnal and nocturnal birds of prey analysed during 1960-62 were apparently due to toxic chemicals. These are summarised in table 1.
Table 1. Reported deaths of birds of prey apparently due to toxic chemicals, British Isles, 1960-62
i960 1961 1962
Buzzard (Buteo buteo) 2 2 2 Sparrowhawk (Accipittr nisus) 2 1 Kestrel (Falco tinnunculus) 1 7 8 Barn Owl (Tyto alba) 1 1 18 Little Owl (Athene noctud) 2 Tawny Owl (Strix aluco) 2 8 4 Long-eared Owl (Asio otus) 3 Unidentified owls 5
6 28 35
The figures in table 1 cannot necessarily be taken as reflecting the actual scale of deaths in each year, as it is possible that increased publicity led to more observers being on the alert for casualties in 1961 and 1962. On the other hand, it is also likely that for some if not all of the species the total populations, and therefore the numbers at risk, were diminishing over the period. In 1961 it was possible to obtain more definite evidence on the causes of death when a number of bodies of birds of prey were sent to a London firm of public analysts and consulting chemists for chemical examination. The results are given in table z; it should be noted that at this time the methods of analysis allowed the amounts of organic chlorine to be determined, but not the identification of the specific chlorinated hydrocarbon involved.
Table 2. Quantities of toxic chemicals (parts per million) found in predators in analyses sponsored by the R.S.P.B. in i96r
No. of Mercury Organic chlorine bodies Max. Min. Average Max. Min. Average
Sparrowhawk (Accipifer nisus) 1* 1.5 Kestrel (Fa/co tinnunculus) 2* 2-5 2.0 7.5 Tawny Owl (Strix aluco) 3* 1.8 0.5 0.8 7.3 0.2 Long-eared Owl (Asio otus) 1 4-7 3-9 *One specimen in each case not examined for organic chlorine I29 BRITISH BIRDS In 1962, however, more refined and more expensive techniques made it possible for the specific chlorinated hydrocarbon concerned to be identified in most instances, and more bodies were analysed. I am grateful to P. J. Conder for generously supplying the details summarised in table 3. Table 3. Quantities of toxic chemicals (parts per million) found in predators in analyses sponsored by the R.S.P.B. in 1962 No. of Organic Heptachlor County bodies Mercury chlorine Aldrin Dieldrin BHC epoxide
Buzzard {Buteo buteo) Leics 1 0.3 Sparrowhawk {Aaipiter nisus) Dorset 1 !-5 Kestrel (Falco timumculus)Lane s 1 0.04 0.3 0.05 1 I.I 4-5 present Lines 1 0.6 1 1.0 1.8 Wilts 1 0.1 0.6 0.02 Berwicks 1 0.4 0.1 Ireland 1 0.01 Little Owl {Athene noctua) Kent 1 0.9 0.5 6.5 Barn Owl (Ty/o alba) Lines 1 0.2 Notts 1 1.2 2.0 Northants 1 0.6 0.4 0.2 Beds 1 I.I 0.8 present Berks 2 i-3 0.6 1.0 Bucks 1 1.6 Cornwall 1 1.2 Essex 1 0.7 3.0 Kent 1 0.6 11.8 0.1 Tawny Owl (S/rix alucd) Scotland 1 0.8 1.2 I.I Norfolk 1 i-5 3.2 0.2 3.0 London 1 0.3 3.0 0.01 0.1 Kent 1 3-3 0.04
Details have also been published (Ratcliffe and Moore 1962) of the results of analyses carried out by the Pest Infestation Division of the Ministry of Agriculture, Fisheries and Food. They concern some predators found dead in the spring of 1962 and details are given in table 4. In the United States studies have been made to test whether there is any connection between the striking decline in the numbers of the Bald Eagle, described earlier, and the use of chlorinated hydrocarbons, such as DDT to control the salt marsh mosquito on the marshes and 130 TOXIC CHEMICALS AND BIRDS OF PREY coastal areas of Florida where the eagles forage for food. In an interim report DeWitt and Buckley (1962) stated that of 27 field specimens of eagles analysed (two Golden and 25 Bald) all but one, a Bald Eagle from Alaska, contained measurable amounts of DDT or its meta bolites. The residues contained in tissues from these birds were: liver, 0.0 to 53.1 parts per million (average 7.4); kidneys, 0.0 to 58.6 (average 13.6); heart, less than 0.5 to 44.0 (average 9.3); brain 0.0 to 71.5 (average 14.4); breast muscle, 0.2 to 23.4 (average 6.0).
Table 4. Quantities of insecticides (parts per million) found in predators by the Ministry of Agriculture, Fisheries and Food in spring 1962
Heptachlor epoxide Dieldrin Gamma BHC Flesh Liver Flesh Liver Flesh Liver
Kestrels (Fako linnunculus) (a) 17-7 44.0 13.7 42.0 14.7 28.8 (b) 18.2 32.0 10.0 5.0 0.0 0.0
Unidentified owl 11.3 54-3 12.0 24.0 0.0 0.0
In Sweden, analyses have shown the following birds of prey to contain pesticide residues: a Merlin (aldrin or mercurial fungicides), one each of Golden Eagle and White-tailed Eagle (minor amounts of mercurial fungicides), and two Eagle Owls (aldrin or mercurial fungicides; thallium sulphate rodenticide) (K. Curry-Lindahl, in Hit.). Finally, amongst the casualties in Holland in i960 following the use of parathion (an organo-phosphorus pesticide) were Buzzard, Sparrow- hawk, Goshawk, Marsh Harrier, Montagu's Harrier, Peregrine, Merlin, Kestrel, Barn Owl, Little Owl, Tawny Owl and Long-eared Owl (Morzer Bruijns 1962). There is abundant evidence, therefore, that many species of birds of prey in several different countries have been found dead in circum stances suggesting that toxic chemicals were the cause, and analyses here, in Sweden and the United States have shown varying amounts of poisons, mainly chlorinated hydrocarbons and mercury, in their tissues. They must have derived these poisons from their prey, as few birds of prey are likely to feed directly on dressed grain or sprayed crops. This underlines the fact that the chlorinated hydrocarbons, unlike the organo-phosphorus compounds, are highly persistent and pass readily from one organism in the food chain to another, often concentrating in the process. Perhaps the best known example is that of the Western Grebes of Clear Lake, California, which were killed by DDD applied to destroy a small, non-bloodsucking gnat. The DDD was used with extreme care, so as to cause the minimum harm to fish, and the greatest concentration applied at any one time
131 BRITISH BIRDS (there were three applications in all) was one-fiftieth part per million. Yet the poison built up, reaching about 5 parts per million in the plankton organisms, from 40 to 300 parts per million in plant-eating fishes, and 1,600 parts per million in the fatty tissues of the grebes. In a similar way, hawks and owls obtain poison from their prey. This has been shown experimentally by Bernard (1962) in the United States. He fed House Sparrows with DDT, less than 200 gm. in all, and then fed parts of the sparrows to an American Sparrowhawk (the New World equivalent of the European Kestrel). The hawk died five days later in tremors, and residues of DDT were found in its organs—85 p.p.m. in the brain, 76 p.p.m. in the breast muscle, 97 p.p.m. in the heart and 212 p.p.m. in the liver. So far as I am aware, the lethal dose of the various toxic chemicals for the different species of birds of prey in this country has not been determined. This dose varies considerably, not only among different species of animals, but among different individuals of the same species. As little as 3.3 p.p.m. of aldrin or 5.8 p.p.m. of dieldrin can be lethal to adult Bob-white Quail (DeWitt 1956), while, according to evidence quoted by Murton and Vizoso (in press), the LD 50 dose for feral pigeons is 67 p.p.m. for dieldrin, 55 p.p.m. for aldrin and 167 p.p.m. for heptachlor. DeWitt and Buckley (1962) fed captive Bald Eagles on diets containing 10 p.p.m. and upwards of DDT. One fed at 10 p.p.m. escaped but another died, whilst all those fed at higher levels of DDT (except for one sacrificed after being fed for 112 days on a 160 p.p.m. diet) died after developing severe tremors. Some of the birds of prey analysed in Britain (tables 2-4) may have been carrying less than lethal doses of poisons and may actually have died from other causes; others almost certainly died from the pesticides they had accumulated indirectly from their prey. Also, as tables 3 and 4 make clear, a number of specimens were found to contain more than one poison and there is some evidence that in certain cases one poison may increase the toxicity of another.
THE EFFECT OF SUB-LETHAL DOSES ON FERTILITY Sub-lethal doses of poisons can also have disastrous effects. DeWitt (1955, 1956) and Rudd and Genelly (1956) showed, in the case of Bob-white Quail and Pheasants that chlorinated hydrocarbons in doses too small to kill the parent birds could seriously affect their fertility by reducing egg production, the percentage of fertile eggs and chick survival. There seems to be little doubt that some birds of prey are being similarly affected. Thus, one of the striking features of the Peregrine survey was the large proportion of pairs occupying sites and not breeding, or else nesting unsuccessfully. This was also found by
152 TOXIC CHEMICALS AND BIRDS OF PREY Treleaven (1961) in his study of Cornish eyries. The numbers of Peregrines in Cornwall recovered well after the campaign waged against them by the Air Ministry during the war, and by 1955 there were 17 pairs compared with about 20 in the 1930's. But by 1958 only seven eyries were occupied and only two produced young, and by i960 only three sites were occupied by ist June and only one pro duced young. In the United States, the decline in the fertility of the Bald Eagle has already been described; even more striking is the fate of a colony of 62 Osprey nests near the mouth of the Connecticut River, where only a single chick was reared in 1962, all the other pairs having had addled eggs (R. T. Peterson per G, Waterston). Little is known of reduced fertility in other species of raptor. The Nest Record Cards for Kestrel and Sparrowhawk have been analysed by Henry Mayer-Gross to see whether they reveal any evidence of a recent decline (table 5). The numbers are too small for the Sparrow- hawk for any significant conclusions to be drawn. In the case of the Kestrel, where a larger sample is available for recent years, the figures cannot be pressed too far, but they do not lend any support to a marked decline in fertility. More evidence is badly needed on this point. Table 5. Hatching and fledging success (excluding losses from predation) of Spairowhawk (Accipiter nisus) and Kestrel (Fako tinntmculus) South and east England Rest of Great Britain h % % % Egg s Youn g Youn g Reare d Reare d Egg s Hatche d Hatche d
SPARROWHAWK 1940-54 5° 42 84% 21 21 100% 41 34 83% 13 12 92% 1955-61 4 3 (75%) 3 3 (100%) 35 26 74% 8 8 (100%)
KESTREL 1951-54 8} 67 81% 57 5° 89% 53 49 9*% 31 28 9°% 1955-61 37 32 87% 17 17 100% »5 84 99% 43 39 91%
For the Peregrine, Bald Eagle and Osprey, however, there is clear evidence of a high level of infertility, in circumstances suggesting that toxic chemicals might be responsible. The next step is to establish if chemicals are present in the addled eggs of these birds, and this has recently been done for all three species. Moore and RatclifFe (1962) have described how an addled egg from a Perthshire eyrie was found to contain DDE (a metabolite of DDT), dieldrin, heptachlor epoxide (a metabolite of heptachlor) and smaller quantities of heptachlor and gamma-BHC, amounting to a total of between 4 and 5 p.p.m. of chlorinated hydrocarbon residue. DeWitt and Buckley (1962) analysed eggs of Bald Eagles from unsuccessful nests in New Jersey,
133 BRITISH BIRDS and found that they contained 24.3 p.p.m., 11.4 p.p.m. and 36.9 p.p.m. of DDT respectively, and three Osprey's eggs from the ill-fated Connecticut River colony were found to contain 316 to 411 micro grams of DDE and 79 to 134 micrograms of DDD (Peter L. Ames per Roland C. Clement). Finally, an addled egg from a Montagu's Harrier nest in Wales in 1962 has been found to contain both DDE and dieldrin (N. W. Moore in lift.).
DISCUSSION In Great Britain, therefore, there is definite evidence that the Peregrine population has declined in recent years and there are marked, though less precise, indications that the Kestrel and Sparrowhawk have also suffered. It is not known whether the total populations of other diurnal birds of prey or of owls have been affected, though there is a clear suggestion from field observations in Hampshire that other raptors began to diminish there in the mid-i 9 5 o's. Three of our diurnal birds of prey and four of our owls have been found dead in circumstances suggesting toxic chemicals as a cause, and three species of each have been shown on analysis to contain pesticide residues. Eggs of two diurnal species have been found to contain residues, and the fertility of one of these is known to have declined. Similar evidence has also been obtained for other species abroad, especially in the United States. The evidence for a link between the recent declines in some of our predators and the growing use of toxic chemicals is strong. However, Moore (1962) has stressed that there are a number of other factors which may have been acting on bird populations in recent years. They include increased motor traffic, hedgerow destruction, overhead wires, myxomatosis (in 1953), the post-war rise in game preservation, and the reduction in numbers (and to some extent variety) of weed-plants and their accompanying insect fauna. The precise effects of these, and possibly other factors, on the populations of different species is not known (see, however, Moore 1957 for the effect of myxomatosis on the numbers and ecology of the Buzzard), although it seems obvious that, for example, the widespread habitat changes in eastern England and the reduced supplies of weed-seed and insect food could affect certain birds considerably. Even deaths from poisoning on a sizeable scale do not necessarily reduce the population of a species permanently. Thus Murton and Vizoso (in press) show that after a kill of at least 8% of the Woodpigeons in one area of Cambridgeshire in 1961, the population had recovered by the 1962 breeding season, when deaths of Woodpigeons from seed-dressings had been virtually abolished there after the voluntary ban on the more toxic seed-dressings had come into operation from January 1962.
134 TOXIC CHEMICALS AND BIRDS OF PREY Two considerations suggest, however, that the influence of toxic chemicals on our birds of prey has been considerable. The first is the geographical distribution of the decline. Fig. 2 shows the number of incidents of mortality due to toxic agricultural chemicals (seed- dressings and sprays) in each county in 1961. This is influenced to some extent by the distribution of observers, but the general picture is in striking agreement with that provided by the analysis of crops and the use of pesticides in Great Britain in 1961 made by Moore (1962). This may be summarised as follows (the letters S, M and E indicating that crops are grown extensively in the areas concerned):
% treated annually Principal areas where crops grown Crop with pesticides North West South Midlands East
Hants Cereals 80% E. Scotland M E — Berks Fruit 9°% . — — S Worcs E Vegetables, sugar 20-60% Lanes S M E beet, stock foods —
Moore also stated that other crops (grass, rough grazing and timber) were subject to only limited treatment by pesticides in 1961 (under 5%) and that, among these, the chlorinated hydrocarbons were used only on timber. From both these sources (Moore and fig. 2), it appears that the heaviest use of pesticides was in southern and eastern England and the Midlands, with more limited use in northern and western England and east Scotland, and very little in the rest of Scotland or most of Wales. This agrees very closely with the pattern of decline described earlier for the Kestrel and the Sparrowhawk—most marked in eastern and southern England and to a lesser extent the Midlands, and spread ing in the case of the Sparrowhawk to the west and north also. The Peregrine is absent as a breeding species from eastern England and the Midlands, but its decline is most marked in southern England, includ ing the south-west, followed by those areas (Wales, north England and south Scotland) nearest to the areas of greatest pesticide use. The second correlation is with the timing of the decline. The use of the new synthetic pesticides increased steadily throughout the 1950's, and the highly toxic mercury and chlorinated hydrocarbon seed-dressings were brought into use in the spring of 1956, not widely at first, but reaching a peak in 1959-61, after which the voluntary ban operated for some of them. The general decline of the Sparrowhawk appears to have begun about 1955 and that of the Kestrel about 1956. 135 FIG. 2. The number of incidents reported of mortality due to toxic agricultural chemicals in each county in 1961 (reproduced from Cramp, Conder and Ash 1962, by permission of the R.S.P.B.) 136 TOXIC CHEMICALS AND BIRDS OF PREY The start of the Peregrine decline is less certain, but Ash noted it in Hampshire from 1956 and Treleaven recorded it in his Cornish breed ing population after 1955 but before 1958. Ash's observations in Hampshire also showed a drop in the numbers of Merlins from 1954, of Hen Harriers from 1956, and of Hobbies from 1956 or earlier. The increasing infertility noted in the Peregrine in Great Britain and in the Bald Eagle and the Osprey in the U.S.A., since the growing use of chlorinated hydrocarbon pesticides, coupled with the fact that addled eggs of all three species have been found to contain pesticide residues, and the experimental proof that pesticides can reduce fertility in species such as Pheasants and Bob-white Quail, seems to establish a clear connection, and I am aware of no other possible explanation. Such a reduction in fertility may well be more effective in causing a population decline than the killing of adults, as witness the success of the campaign to eliminate the Screw-worm Fly {Cochliomyia americana) in Curacao and Florida (Carson 1962).
CONCLUSION After reviewing the evidence available, it is difficult to avoid the conclusion that the increased use of toxic chemicals, especially the persistent chlorinated hydrocarbons, has been the major factor in the recent decline of some of our diurnal birds of prey. If present trends continue, some species could be faced with extinction. The ban on the spring use of the three most toxic seed-dressings from January 1962 should result in some improvement, and deaths of birds of all species were much reduced in the spring of 1962. It should be noted, however, that less toxic chlorinated hydrocarbons, such as BHC, may still be used in seed-dressings, and the ban on aldrin, dieldrin and heptachlor does not extend to seed sown in the autumn in areas where attacks from the Wheat-bulb Fly are a danger, nor to their use in other pesticides, such as sprays. More important, perhaps, some of our birds of prey may have escaped the full brunt so far because their main breeding strongholds are in the north and west, where chlorinated hydrocarbons have so far been used on a much smaller scale. The danger exists, however, for BHC, DDT and dieldrin are used as sheep dips (Moore and Ratcliife 1962) and Golden Eagles feeding on sheep carrion could be affected. And if the spraying of forests with chlorina ted hydrocarbons, to combat either insect pests, as in the U.S.A., or voles, as in Germany, is begun on a large scale, this last refuge would be menaced.
ACKNOWLEDGEMENTS My grateful thanks are due to Henry Mayer-Gross of the British Trust for Ornithology, R. S. R. Fitter of the Council for Nature, Roland C. 137 BRITISH BIRDS Clement of the National Audubon Society, Drs. N. W. Moore and D. A. Ratcliffe of the Nature Conservancy, and P. J. Conder of the Royal Society for the Protection of Birds, for allowing me to use un published material or for other help.
SUMMARY (i) There is definite evidence of a sharp decline in the Peregrine in Great Britain in recent years, and strong indications of decreases also in the Kestrel and Sparrow- hawk. Little information is available about the total populations of other diurnal species or of owls, but observations in southern England suggest that some of the former have also been affected. The Bald Eagle has decreased markedly in the U.S.A., and the Peregrine both there and elsewhere in Europe. (2) Pesticide residues have been found on analysis in the bodies of the Kestrel, Sparrowhawk, Buzzard, Barn Owl, Little Owl, Long-eared Owl and Tawny Owl in Great Britain, in the Golden and Bald Eagle in the U.S.A., in the Merlin, White- tailed Eagle and Eagle Owl in Sweden, and in the Goshawk, Marsh Harrier and Montagu's Harrier in Holland. (3) A decline in fertility has been shown in the Peregrine in Great Britain, and in the Bald Eagle and Osprcy in the U.S.A., and eggs of all three species have been found to contain pesticide residues. (4) These facts, together with the timing and geographical distribution of the decreases, suggest that the increased use of toxic chemicals, especially the persistent chlorinated hydrocarbons, which both kill directly and have sub-lethal effects on fertility, is the major factor in the recent decline of some of our birds of prey. The position of these and the remaining species could be made worse if the use of such chemicals is extended in forests and other habitats in the north and west of Britain.
REFERENCES ASH, J. S. (i960): 'Bird of prey numbers on a Hampshire game-preserve during 1952-1959'. Brit.Birds, 53: 285-300. BERNARD, R. F. (1962): 'Secondary DDT poisoning in a Sparrowhawk'. Auk, 79: 276-277. CARSON, RACHEL (1962): Silent Spring. Boston. CRAMP, S., and CONDER, P. J. (1961): The Deaths of Birds and Mammals connected with Toxic Chemicals in the first half of i960. Report No. 1 of the B.T.O.-R.S.P.B. Committee on Toxic Chemicals. and ASH, J. S. (1962): Deaths of Birds and Mammals from Toxic Chemicals, January-June 1961. The second report of the Joint Committee of the B.T.O. and R.S.P.B. on Toxic Chemicals, in collaboration with the Game Research Association. • (1963): Deaths of Birds and Mammals from Toxic Chemicals, September 1961-August 1962. The third report of the Joint Committee of the B.T.O. and R.S.P.B. on Toxic Chemicals, in collaboration with the Game Research Association.
PLATE 22. Two specialists: the Short-toed Eagle {Circaetus gallicus), declining where cultivation is reducing snakes (page 146); and Griffon Vultures {Gypsfulvus) (also Egyptian Vulture, Black Kite and Raven) which have to face modern hygiene and the poisoning of carcases (pages 143-144) {photos: Eric Hoiking) 138 TOXIC CHEMICALS AND BIRDS OF PREY DEWITT, J. B. (1955): 'Effects of chlorinated hydrocarbon insecticides on quail and! pheasants'. /. Agric. and Food Cbem., 3: 672-676. • (1956): 'Chronic toxicity to quail and pheasants of some chlorinated insecticides'. /. Agric. and Food Chem., 4: 863-866. and BUCKLEY, J. L. (1962): Studies on Pesticide-Eagle Relationships. Interim- report to the National Audubon Society Convention, Corpus Christi, Texas. MORZER BRUIJNS, M. F. (1962): 'De massasterfte van vogels in Nederland door vergiftiging met bestrijdingsmiddelen in het voorjaar van 1960'. l^andbouwkundig Tijdscbrift, 74-14: 578-588. MOORE, N. W. (1957): 'The past and present status of the Buzzard in the British, Isles'. Brit. Birds, 50: 173-197. (1962): 'Toxic chemicals and birds: the ecological background to con servation problems'. Brit. Birds, 55: 429-435. MORTON, R. K., and Vizoso, M. (in press): 'Dressed cereal seed as a hazard to- Woodpigeons'. Ann. Appl. Biol. MOORE, N. W., and RATCLIFFE, D. A. (1962): 'Chlorinated hydrocarbon residues in the egg of a Peregrine Falcon from Perthshire'. Bird Study, 9: 242-244. RUDD, R. L., and GENELLY, R. E. (1956): 'Pesticides: their use and toxicity in relation, to -wildlife*. Calif. Dept. of Fish and Game, Game Bulletin, 7: 1-209. TRELEAVEN, R. B. (1961): 'Notes on the Peregrine in Cornwall'. Brit. Birds, 54:. 136-142.
Appendix—Scientific names of species mentioned in the text
BIRDS OF PREY Golden Eagle (Aquila chrysaetos) Hobby (Falco subbuteo) Buzzard (Buteo buted) Peregrine (Falco peregrinus) Sparrowhawk {Accipiter nisus) Merlin {Falco columbarius) Goshawk (Accipiter gentilis) Kestrel (Falco tinnunculus) White-tailed Eagle (Haliaetus albicilld) American Sparrowhawk (Falco sparveriusy Bald Eagle {Haliaetus leucocephalus) Barn Owl (Ty/o alba) Marsh Harriet (Circus aeruginosus) Eagle Owl (Bubo bubo) Hen Harrier (Circus cyaneus) Little Owl (Athene noctua) Montagu's Harrier (Circus pygargus) Tawny Owl (Strix aluco) Osprey (Pandion haliaetus) Long-eared Owl (Asia otus)
OTHER BIRDS Western Grebe (Aechmophorus occidentalis) Feral pigeon (Columba livid) Bob-white Quail (Colinus virginianus) Woodpigeon (Columba palumbus) Pheasant (Phasianus colchicus) House Sparrow (Passer domcsticus)
PLATE 2 5. Two harriers: the adaptable Hen Harrier (Circus cyaneus), much persecuted but holding its own by nesting in many habitats from moorland to reed-beds (page 145); and the much more specialised Marsh Harrier (Circus aeruginosus), decreasing wherever marshes are drained (page 144) (photps: Eric Hosking)
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