Hybridization and changes in the distribution of Iceland gulls (Larus glaucoides kumlieni/thayeri)

Three Iceland gull taxa were defined mainly from adult wingtip melanism. Up until about 1860, nominate glaucoides (no melanism) was known to breed fro m Greenland to western High Arctic Canada, but by about 1900 it was essenti ally confined to Greenland. Until 1860, thayeri (most melanism) was known o nly from western High Arctic Canada, but from 1900 to 1980 it was found thr oughout High Arctic Canada and in a small part of north-west Greenland. At high latitudes in Canada it replaced glaucoides, with which it was formerly sympatric in the west and probably interbred. The first known kumlieni (in termediate, variable melanism) were from west Greenland in the 1840s, and b y 1900 the western and northern limits of most of its breeding range in the eastern Canadian Low/High Arctic were known. The range of kumlieni lies be tween those of thayeri and glaucoides and overlaps both; kumlieni bred in G reenland by 1964. It freely interbreeds with thayeri and probably with glau coides. Winter ranges of glaucoides and thayeri have changed little since t hey were first determined for glaucoides by 1860 and for thayeri by the 192 0s. However, winter adult kumlieni was unknown from Greenland to the Britis h Isles until 1900; there were a few records prior to 1915 and progressivel y more after 1950. The study adds to the evidence that kumlieni represents introgressive hybridization by western thayeri into eastern glaucoides.