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 birds and one an increase. One of the decreases is at St. Kilda, where the expedition of 1939 found 16.5 per cent. of the guillemots bridled and that of 1948 only 10.3 per cent. (one of us took part in both counts). Other decreases in Britain have been significant, as at the Isle of May, 5.3 to 3.2 in ten years; and at Unst in Shetland—23.8 to 16.9 per cent. in the same period. There has also been a significant decrease—of about one-third—in Iceland; thus at Grimsey in the north, from 8.7 in 1939 to 6.9 per cent. in 1949; at Hafnaberg, in south-west Iceland, from 29 per cent. in 1939 to 18.1 per cent. in 1949; in the Westmann Islands a parallel decline from 75 per cent. in 1935 (Lockley, 1936) to 50 per cent. in 1949.
FIG. 8
Breeding distribution of two closely-related guillemots or murres: Uria aalge, the common guillemot; and U. lomvia, the arctic guillemot (Brünnich’s guillemot* in the Atlantic, Pallas’s murre in the Pacific). Areas of overlap shaded.
Increases noted in the 1939–1949 enquiries were several, but only one, at Foula in Shetland, was significant and by checked observers (from 24 per cent. in 1938 to 29.4 per cent. in 1948–49). Increases on the margin of significance were recorded from St. Bee’s Head in Cumberland, Marwick Head in Orkney, and the Fair Isle. Apart from these small increases in the last decade, there was a significant increase of the percentage on Noss in Shetland from 15.5 in 1890 to 26.5 in 1938, which seems great enough to embrace a possible slight observer-error.
Unfortunately, too few of the early bridled guillemot counts are reliable, though some from Berneray and Mingulay (‘Barra Head’) in the Outer Hebrides may be so. This had 20.2 per cent. in 1871; 12 in 1939; 9.8 in 1949; 12.6 in 1950. The decrease between 1871 and 1939 is significant, though the other apparent changes are not so. Elsewhere we have followed Southern in discarding such vague records as ‘about one in every nine or ten.’
Nothing is yet known about the percentage of bridled guillemots along the coast of Norway, except that it has remained slightly over 50 per cent., at Bear Island from 1932 to 1948. At the Karlov Islands off the Murmansk coast the percentage was 42 in 1938. It seems likely, from the rather scanty figures from Novaya Zemlya, which Southern slightly misdates and misplaces, that the percentage may be about the same on islets in Pukhovy and Bezymiannaya Bays off that island (36.4 and 50).
These changes are curious and it is clear that much remains to be solved about this interesting problem in distribution and evolution. Nor is much known about the distribution of the bridled form in the New World, save the following: H. F. Lewis found 128 bridled out of a sample of 724 (17.7 per cent.) in the colonies along Quebec Labrador in 1929. One of us found 51 bridled out of a sample of 295 (17.3 per cent.) at Cape St. Mary, on the south-west corner of the Avalon Peninsula of Newfoundland, in 1953. In June, 1940 at Funk Island and other parts of the east coast of Newfoundland within forty miles of it W. Templeman (1945) collected twelve common guillemots (? at random) of which six (50 per cent.) were bridled. When Hørring and Salomonsen (1941) compiled a list of all the common guillemots that had been then collected on the west coast of Greenland they recorded six out of thirty-two (18.7 per cent.) as bridled, but knew of no breeding-colony. Soon afterwards Salomonsen (1944) became aware of the colony in the Sukkertoppen district; but no count has apparently yet been made there.
FIG 9a The principal breeding-colonies of the common guillemot in Britain. The percentages of bridled forms in the breeding-populations, as determined chiefly by H. N. Southern and his colleagues, are shown. Minus and plus signs in brackets indicate changes in the decade c. 1939–c. 1949 which are significant, or on the borderline of significance. Crossed circles mark sites of former