EVOLUTION 39 



calif amicus, 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 dind 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, winter- 

 ing 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 



