species of their own. We need not be surprised at this indication that the Atlantic’s bird fauna is derived from that of other oceans if we accept Wegener’s theory of the origin of the Atlantic; but whether the Wegener theory is true or not it is quite clear that the North Atlantic has not been the home in which any important group of sea-birds has evolved. This is not to say that there has been no sea-bird evolution in the North Atlantic; but it has not usually gone beyond the differentiation of species. Of this it has, indeed, much to show. Some of the classic examples which E. Mayr (1942) has discussed are North Atlantic species. Mayr’s thesis is that one species can only become two after it has been differentiated geographically. He opposes the notion which has found favour in some quarters that speciation may occur by ecological differentiation or by the differentiation of behaviour.
So far the available evidence appears to uphold Mayr’s view—at all events, for birds. During the present century much systematic work in the description and measurement of birds has been conducted in American and European museums, and much practical and theoretical work on evolution has also been done. But it needed the persuasions of Mayr and Julian Huxley (1942), amongst a few others, to collate the work of the systematists and the evolutionary zoologists. Sea-birds lend themselves to evolutionary study because they are so largely confined to coasts for breeding purposes. This makes their distribution often linear rather than of the ordinarily spatial two-dimensional type; and this linear distribution makes it easy to apply Huxley’s concept that the characteristics of animals tend to grade from one part of their range to another in an orderly way. Some of these gradations had been recognised long before Huxley thought of the word “cline” because they are adaptations to the environment. For instance Bergmann’s Rule states that from the warmer parts of an animal’s distribution-area to the colder parts there tends to be an increase in its size. Thus the puffins, black guillemots and eider-ducks of the Arctic are considerably bigger than those of Britain. The main adaptive reason for this is that larger animals have less surface in proportion to their weight, and consequently heat is not lost from them (if warm-blooded) so rapidly as it is from small animals. Another rule, Allen’s Rule, states that warm-blooded animals of cold climates tend to have their heat-radiating surfaces decreased by a reduction in size of their extremities and limbs such as ears, tails, necks, legs and noses. There is also a general tendency (Gloger’s Rule) for animals to become darker as humidity increases.
If we examine those sea-birds which are widely distributed, we find clines in various characteristics, notably in size, i.e. total size, and also size of limbs and extremities, beak-length, wing-length, etc., and in colour. There are also clines in shape; for instance the fulmars of the north-east Atlantic have very thick bills, those of Baffin Island rather more slender bills, those of the North Pacific more slender bills still, and those of the Antarctic very slender bills indeed. No sea-bird is arranged quite evenly in its geographical distribution. Just as the distribution in space is never even, so are the gradations in character never even. From one part of the geographical distribution of a species to the other, change often occurs more as a series of steps rather than as continuous ramp.
Most working ornithologists today will agree that there are more subspecific names about than a true understanding of bird evolution requires. It is the species which has reality and significance. In this book we have tried to be sparing in the use of subspecies, and have rejected some that appear in many current text-books. Nevertheless, a study of the geographical races of the species of the North Atlantic sea-birds will lead us to examine here some of the more fascinating examples of geographic differentiation. The classic example among the sea birds is the chain of the Larus argentatus and fuscus group, the herring-gulls and lesser blackbacks, which may include some birds which are regarded as separate species, e.g. the California gull L. californicus, and the so-called ‘Iceland’ gull, or better the Greenland herring-gull, L. glaucoides or leucopterus. The relationships of this superspecies (Fig. 4a) were first worked out by B. Stegmann (1934): we have included the results of subsequent systematic work in this map (Fig. 4b) and in the discussion which follows.
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
