Sea-Birds. James Fisher

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      FIG. 5

      Breeding distribution of Larus canus, the common gull, and the closely related L. delawarensis, the ring-billed gull

      Among the North Atlantic sea-birds are others whose species have differentiated geographically and whose range-end populations have become different enough to occupy the same geographical area—but separate ecological niches, and thus preserve their identity. For instance, it is probable that the ring-billed gull Larus delawarensis, of North America, and the common gull of the Old World, L. canus, have not long since shared a common ancestor, though a subspecies of the common gull, which has probably spread across the Bering Straits from the Old World, now occupies Alaska and parts of the Canadian North-West, where it overlaps with the western element of the ring-billed gull (Fig. 5). Here the two act as different species. The glaucous gull and the great blackback, which overlap in eastern North America, Iceland and parts of the European Arctic (Fig. 6) may be not long ago descended from a common ancestor. They very rarely hybridise. How the three species of terns—the arctic, common and Forster’s—which are very closely related, arrived at their present distribution (Fig. 7) is difficult to imagine at this stage of their evolution, but they all may be descended from a common tern of north-east Asia or an arctic tern of the North Pacific—from which part of the world the species has probably spread, differentiated and overlapped.

      Various suggestions could be made as to the origins of the two guillemots, the common and Brünnich’s guillemot (Fig. 8). Possibly the original guillemot was a common guillemot (Uria aalge) type which got divided into two subspecies in the Atlantic and Pacific by the Ice Age, but not before it had had time to give rise to an arctic race adapted to the harder life. After the Ice Age, with the ameliorating conditions, perhaps both the Atlantic and the Pacific guillemots began pushing north again, this time to meet and overlap with their arctic descendant, which, meantime, had differentiated sufficiently to offer no direct competition. It is interesting to note that the most arctic of the common guillemot races, Uria aalge hyperborea of Iceland, Novaya Zemlya, and Lapland, has a very thick bill and a considerable resemblance to Brünnich’s guillemot, with which it, however, does not interbreed, nor apparently compete. Perhaps it is recapitulating some of the early stages in the origin of Brünnich’s guillemot. To some extent Brünnich’s guillemot, with its razorbill-like beak, appears to replace the razorbill in the arctic, where it may occupy the same ecological (feeding and breeding) niche in relation to the common guillemot as the razorbill does in relation to that bird in the south part of the common guillemot’s range.

      The student of variation will find much material for his researches among the North Atlantic sea-birds. Several species of North Atlantic birds, notably the common guillemot, the three smaller skuas and the fulmar, are polymorphic or dimorphic. They exist in several so-called phases. Some common guillemots have a white ring embracing their eye from which a white line runs back towards the back of their heads. These are called ‘bridled’ guillemots, and were for long actually thought to be of a different species. The phases of the skuas range from very light phases with yellow over their ears and the back of their necks, white throats and bellies, to those which are almost uniformly brown. The breeding fulmar population of Britain, the Faeroes, Iceland, Jan Mayen and West Greenland are all light-coloured with white bellies, necks and breasts, but in Baffin Island, Spitsbergen and Franz Josef Land the fulmars are nearly all very dark coloured. Between the light forms of Britain, etc., and the dark forms of Spitsbergen, there are a number of puzzling intermediates, most in evidence on Bear Island (and often to be seen at sea in the Rockall area), and the situation among the fulmars is therefore one not of dimorphism but of polymorphism, as it is among the skuas.

      FIG. 6

      Breeding distribution of a group of closely-related gulls: Larus occidentalis, the western gull: L. glaucescens, the glaucous-winged gull; L. schistisagus, the slaty-backed gull; L. hyperboreus, the glaucous gull; and L. marinus, the great black-back. Areas of overlap shaded. Black areas in Canadian Arctic represent outpost breeding-places of L. marinus

      Southern, who has carefully studied the problem of the differential distribution of the bridled guillemot, thinks that its ‘bridle’ is probably controlled by a single Mendelian factor, which appears to control also a slight difference in the skull structure and the shape of the tail-feathers. He organised counts of the percentage of bridled guillemots throughout Britain in the years round 1939 and again in those round 1949; and he has also collected as much evidence as he could from the rest of the guillemot’s range. Two main conclusions are apparent: first, the percentage of bridled birds increases from SSE to NNW (with a reversal in Iceland); and secondly the percentage is not always constant at any one place—there are signs of trends towards increase or decrease, and of shifts, or drifts, of the balance. Possibly the possession of a bridle gives a guillemot an advantage over other guillemots in some environments, and a disadvantage in others, though we do not know why: the alternative is that possession of the bridle is the result of an advantageous mutation that is spreading through the population; which is unlikely to be the case on the evidence, though Southern has been careful to show that the possibility still exists. There is no indication that bridled guillemots prefer to mate with each other rather than with unbridled guillemots; mating in a mixed colony appears to be completely, or almost completely, at random.

      FIG. 7

      Breeding distribution of three closely-related terns: Sterna hirundo, the common tern; S. paradisaea, the arctic tern; S. forsteri, Forster’s tern. Areas of overlap shaded. S. h. turkestanica is a doubtful subspecies

      Southern shows that the percentage of bridled birds marches fairly closely with humidity and cloudiness; but, as he points out, many other factors may be involved. The changes between c.1939 and c.1949 may be linked with the climatic amelioration, but “might very well be due to random fluctuation.” The actual percentages as recorded in the paper of Southern and Reeve (1941) and Southern (1951), and in a few notes published by other observers, are shown on the maps (Figs. 9a, 9b). The results of Southern’s enquiry of 1949 have shown that out of the very many colonies studied in Britain at only five has a significant^* change been recorded in ten years, four of which show decreases of the percentage of bridled СКАЧАТЬ