The loss at sea of oil-carrying vessels during the 1914–18 war resulted in a large increase in the numbers of oiled seabirds. This led the Royal Society for the Protection of Birds to publish figures in 1921 which played a large part in the introduction of the ‘Oil in Navigable Waters Act’ soon after. In the Second World War most tankers carried petroleum spirits, and the destruction of shipping presented less hazard. But this situation has changed as the needs of a modern industrialised world have led to an enormous expansion in oil traffic at sea; nowadays crude oil is carried to refinement plants near the destination in giant tankers. Constant pollution arises from ships washing-out at sea after a voyage and purposely releasing oil, while occasional accidents can have widescale repercussions. Thus, during the night of 18 September, 1966, the German tanker Seestern allowed 1,700 tons of crude diesel oil to escape into the Medway Estuary on a flood tide, polluting 8,000 acres of saltings. Probably about 5,000 birds died immediately. Certainly, 936 black-headed gulls, 927 great black-backed gulls, 184 dunlin, 165 herring gulls, 135 redshank, 98 common gulls, 90 oystercatchers, 65 curlew and various other birds (including an American pectoral sandpiper) were picked up. In this case the number of birds using the area had declined, by 20–100% depending on the species, in the following winter, but has since recovered so possibly no permanent damage has been done, yet for years an area could be denuded of suitable plant and animal food for birds and other wildlife. Only a small oil slick of about 87 tons hit the Tay estuary, the most important wintering ground for eiders, in March 1968. Fortunately, most of the birds had left, but between 7–26% of the national eider population was wiped out: the happy state of affairs pertaining to eiders and described above could soon be reversed. These modern incidents compare with R.S.P.B. estimates made for the 1940s and 50s that between 50,000–250,000 birds were being killed per year in home waters.
On 18 March, 1967, the Torrey Canyon ran on to the Seven Stones Reef and in all about 60,000 tons of crude oil were lost into the sea. At least 10,000 birds were collected, and many more must have died unbeknown, of which 9 out of 10 were guillemots. In the aftermath we now know (see special review by Bourne 1970, and Parslow 1967) that guillemots at some breeding colonies in Scilly and the north Cornish coast have been considerably reduced; in Cornwall, but not Scilly, fewer shags and herring gulls were breeding in 1966; kittiwake numbers at a colony within the worst polluted area were reduced in 1966. Only a few razorbills and apparently no puffins suffered in Britain, though these last are difficult to census. This was not the case in the Sept Iles in Britanny, the most important seabird colony in France, where careful protection had allowed a recovery in seabird numbers following the persecutions of the last century. While aerial species such as the gannet had escaped damage, such divers as the shag had suffered markedly and the auks very severely: counts before and after the incident show that the number of pairs of guillemots fell from 270 to 50, of razorbills from 450 to 50, and of puffins from 2,500 to 400. This is the first incident where really detailed knowledge has been available, making a fairly comprehensive ecological survey possible so that wildlife interests are given more than passing regard. The public has been aroused at the prospects of ruined beaches and the ‘overriding concern of the Government throughout has been to preserve the coasts from oil pollution and to adopt a course most likely to achieve this end’. Unfortunately the enormous quantities of detergents used for this purpose have done vastly more immediate and probably long-term harm to intertidal organisms than the oil itself. Whatever the pros and cons of the whole sad story, it illustrates the dilemma man finds himself in today. In some fields his technology has progressed far too quickly, while in others it has lagged, so that when accidents occur he is too often forced to resort to ill-conceived panic measures. Since this book has been in press there has been another major wreck of auks, this time in the Irish Sea in August 1969. At first attributed to gales, it seems that many birds came ashore under conditions of not particularly unfavourable weather. Analyses show the bodies to contain high levels of polychlorinated biphenyls, agents used in the paints and plastics industry. This new chemical hazard, unrealised when Mellanby prepared his book, highlights the complexity of the interaction of man and wildlife and the need for drastic measures if man is to avoid becoming the ultimate victim of this extensive environmental pollution, which wildlife is indicating.
A THOROUGH insight into the relationships between birds and humans demands some understanding of population ecology, a knowledge of how animal numbers fluctuate and change through births and deaths and of the factors which determine these processes. Populations have dynamic properties and these cannot be neglected by people concerned with wild life management, whether as game preservers, conservationists, pest controllers or farmers. This chapter attempts to set out some of the basic principles with pertinent examples, but these same principles will emerge in later sections in various guises. Our knowledge of the subject was first collated and clearly enumerated by Dr D. Lack (1954) in a stimulating book The Natural Regulation of Animal Numbers since when more field studies have been made by various workers, which Lack (1966) has summarised in his Population Studies of Birds. Both books should be consulted by the reader who is really interested in this subject. In this account I have drawn on examples which, wherever possible, have direct relevance to economic ornithology.
Farmers tend to the pessimistic belief that all problem birds become more abundant every year. Yet careful counts of most such species living in stable environments usually show there to be no clear tendency towards a steady increase, or even decrease, though numbers may fluctuate from year to year. For instance, many farmers believe that the wood-pigeon has increased drastically during this century to become more of a pest, though in conditions of stable agriculture this is unlikely to be true. In fact, there is evidence of a decline since the early 1960s associated with a reduction in the acreage of winter clover-leys and pastures. Certainly the species has moved into newly developed marginal land; places like the east Suffolk heathlands, which have been ploughed and claimed for agriculture since the Second World War, have been colonised by wood-pigeons. Fig. 1 gives some indication of how wood-pigeon numbers have varied on a Scottish estate near Dundee since 1887, and it is evident that fluctuations have occurred within narrow limits, with no evidence at all for a sustained rise or fall. In contrast, Fig. 1 also shows how the closely related stock dove has increased over the same period; this species first colonised in Scotland in 1866, reached Fife in 1878 and increased dramatically in Scotland in the next ten years. Every year each pair of wood-pigeons rears on average just over two young and it is evident that if all these survived to breed in their turn, population size would increase exponentially. That this does not normally occur indicates that some form of regulatory mechanism must be operating. Furthermore, this regulation must be density-dependent, that is, it must become proportionately more effective at high population densities and proportionately less effective at low ones. If the regulatory factor (s) operated without regard to density, it is evident that population size could fluctuate widely without reference to a particular level – to the constant mean represented by the dotted line in the figure.
FIG. 1. Annual number of wood-pigeons and stock doves shot