It will be seen that the subspecies is composed of three chains of subspecies which unite in Central Siberia, where the resident breeding subspecies is Birula’s herring-gull Larus argentatus birulai. The two northerly chains link round the Polar Basin, the two end links of one overlapping with the two end links of the other. Where they overlap, the two races of one chain-end are ‘herring-gulls,’ of the other ‘lesser blackbacks.’ These behave as different species. It can be found convenient to make the ‘species’ separation in the chain, between the two races birulai and heuglini, thus calling the latter Larus fuscus heuglini (it is the first really dark-mantled gull in the chain). This is more practical than splitting the chain into argentatus and fuscus in the Bering Strait area, though this is probably the place of origin of the ancestral gull that gave rise to the whole chain; for if all the palearctic group were fuscus some confusion would surround the light-mantled Mediterranean forms.
Special comments can be made on various members of the chain. In the zone of overlap in Western Europe the herring-gulls are distinguished from the lesser blackbacks not only by form but by many habits. The lesser blackbacks breed often inland on moors, and when coastal tend to colonise flattish ground set back from the cliff-tops beloved of the herring-gulls. While the herring-gulls are dispersive in winter, the lesser blackbacks are almost entirely migratory, wintering south of all but their most southerly breeding-places, though some of the dark L. f. fuscus of Scandinavia winter in Britain, and recently a minority of the British race L. f. graellsii has ‘revived’ an old habit of wintering in England, especially in Cheshire and Lancashire. Both species are also extending their breeding-range north; L. a. argentatus has colonised east and north-east Iceland since 1909, and a herring-gull of this or the Scandinavian race omissus was breeding on Bear Island in 1932, though not 1948. The graellsii lesser blackback has established itself in south Iceland since about 1925, and a group intermediate between graellsii and fuscus in Denmark since 1922.
The North American situation is of great interest. As the herring-gulls range north-east they become generally paler in colour. The much-discussed Kumlien’s herring-gull L. a. kumlieni was for a long time held to be a hybrid between the ‘Iceland’ gull of Greenland and L. a. thayeri, Thayer’s gull of the Canadian Arctic and Thule corner of north-west Greenland. But there seems no doubt that it is a valid race (Taverner, 1933) with its own discrete breeding-distribution in southern Baffin Island, though on the western marches of its distribution there are apparently some forms intermediate between it and thayeri (Hørring, 1937) and colonies off south-west Baffin Island have been described as mixed (Soper, 1928).
FIG. 4
Breeding distribution and relationships of all subspecies of Larus fuscus, L. argentatus and related forms. a Diagram of forms, with leg and mantle-colour
The palest of all the herring-gulls is the ‘Iceland’ gull. Unquestionably this extremely pale bird, with pale flesh legs, is a herring-gull, and conspecific with the other herring-gulls of North America. Reports of its breeding in the Canadian arctic archipelago are due to confusion with thayeri; there is no evidence whatever of its overlapping with this or any other subspecies of L. argentatus anywhere; and its similarity in size, structure and plumage is obvious. It is just a very pale kind of herring-gull; and at the same time happens, through convergence, to be extraordinarily similar to, though smaller than, the glaucous gull.* It is entirely confined to Greenland, breeding north to Melville Bay on the west (this inhospitable coast separates it from thayeri) and to Kangerdlugssuaq (at the south end of the Blosseville coast) on the east. Evidence of its breeding farther north in east Greenland, and elsewhere (e.g. Franz Josef Land, Novaya Zemlya) is quite unsatisfactory, and probably due to confusion with the glaucous gull; on Jan Mayen it was stated by F. Fischer to be as abundant as the glaucous gull in 1882–83, and to be nesting on low ledges, but it has not been proved to breed there since.
FIG. 4bBroken line: in Eurasia, L. fuscus; in North America southern limit of possible area of overlap between L. californicus and L. argentatus smithsonianus. Fig. 4a gives the key to the numbered forms.
In a complex situation, such as this, a confusion of scientific names is to be expected. In other cases it is often found that the vernacular name is less equivocal, and certainly more stable, than the scientific name! Such is not the present case, however; for the name ‘Iceland gull’ makes confusion worse confounded. It has never bred in Iceland. Hørring and Salomonsen (1941) have already used the English name Greenland Gull to describe it, and regard it as a race of Larus argentatus. We commend to our readers, and to the compilers of the Lists of the American and British Ornithologists’ Union : the Greenland Herring-Gull, Larus argentatus glaucoides (= L. a. leucopterus).
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