The Ecology of Closely Related Species with Special Reference to Cormorant (Phalacrocorax carbo) and Shag (P. aristotelis)

This paper is a sequel to one on the ecological aspects of species formation in passerine birds (Lack, I944). Gause (I934) contends that two species with similar ecology cannot live in the same area, and in the earlier paper this principle was shown to hold among closely related species of passerine birds, both in Britain and on oceanic islands. Such species either frequent different regions, or different habitats, or, when they live in the same place, they eat mainly different foods, the latter commonly being associated with a difference in body size. I originally intended to follow this paper with a second one analysing the extent to which the same might apply in British non-passerine birds. Among these birds, examples of closely related species which breed in a similar habitat but in mainly different regions are provided by the mute and whooper swans (Cygnus olor and C. cygnus), the curlew and whimbrel (Numenius arquata and N. phaeopus), and the common and arctic terns (Sterna hirundo and S. macrura). In each of these pairs, the first-mentioned species breeds mainly south of the second. Examples of related species which breed in the same regions but in mainly different habitats include: the long-eared owl Asio otus in woods and the short-eared owl A. flammeus in open country; the marsh harrier Circus aeruginosus in very damp marshland and the Montagu harrier C. pygargus in dryer marshland and on heaths; the goosander Mergus merganser primarily by swiftflowing streams and the red-breasted merganser M. serrator primarily by Iochs. Two species which Qverlap widely in both region and habitat but differ in food are the hobby Falco subbuteo and kestrel F. tinnunculus. Both frequent open country with trees in southern England, but the hobby feeds primarily on insects and on small birds caught on the wing, and the kestrel primarily on rodents and to a much smaller extent on small birds and insects caught on the ground. A case of a difference in both habitat and food is provided by the four species of gallinaceous birds in northern Scotland, of which the ptarmigan (Lagopus mutus) lives in the alpine zone feeding primarily on Empetrum and Vaccinium, the red grouse (Lagopus scoticus) on moorland feeding primarily on Calluna, the blackcock (Lyrurus tetrix) on the wood edge feeding primarily on Betula, and the capercaillie (Tetrao urogallus) in forest feeding primarily on Pinus needles. (Though the two latter species are placed in separate genera from the two former, they are probably closely related. In making generic distinctions, undue weight would seem to have been given to male characters. The females of the four species are rather similar in appearance, while interbreeding has been recorded not infrequently.) In the earlier paper, instances were also given in which two related passerine species frequent the same habitat, but differ markedly in body-size and in food. British non-passerine birds probably provide several further examples, but in most cases insufficient is known of the food of the birds concerned to say whether or not the size difference is correlated with a difference in diet. Examples are the great and lesser spotted woodpeckers (Dryobates major and D. minor), mallatd and teal (Anas platyrhynchos and A. crecca), great-crested and little grebe (Podiceps cristatus and P. ruficollis), little, common and sandwich terns (Sterna albifrons, S. hirundo and S. sandvicensis), and several species of gulls (Larus spp.). The idea of a complete survey of Brltish nonpasserine birds was eventually -abandoned owing to the many cases in which two closely related species appear to overlap widely in ecology, but insufficient is known to state definitely whether or not this is the case. From the information available in general works on British birds, notably Witherby (1938-41), the following species appear to come in this category: tawny and long-eared owls (Strix aluco and Asio otus), both living in woods and eating similar prey; teal and garganey (Anas crecca and A. querquedula) on southern marshes and inland waters; gadwall, wigeon and pintail (Anas strepera, A. penelope and A. acuta) on northern waters; and among sea birds, cormorant and shag (Phalacrocorax carbo and P. aristotelis), storm and fork-tailed petrels (Hydrobates pelagicus and Oceanodroma leucorrhoa), roseate and common terns (Sterna dougallii and S. hirundo) and herring and lesser black-backed gulls (Larus argentatus and L. fuscus).