Subspecies
Larus glaucoides (Sibley and Monroe 1990, 1993) has been split into Iceland Gull (Larus glaucoides) and Thayer's Gull (Larus thayeri) following AOU (1998).
Formerly listed as Larus leucopterus. Closely related to European Herring Gull (Larus argentatus), and sometimes treated as conspecific.
Form kumlieni sometimes considered a separate species, as birds with grey wingtips occur alongside those with white wingtips in Nearctic. Eastern-western cline apparent, from pale glaucoides to slightly darker kumlieni to darker Thayer's Gull (Larus thayeri).
There is increasing tendency to lump Thayer's Gull (Larus thayeri) with Iceland Gull (Larus glaucoides) refuted, as such treatment would make the species undiagnosable; yet kumlieni hybridizes with Thayer's Gull (Larus thayeri), and may be closer to latter than to subspecies glaucoides of Iceland Gull (Larus glaucoides).
The following 3 subspecies are recognised:
kumlieni Brewster, 1883 - North-eastern Canada, on Baffin I and north-western Ungava. Winters from Labrador south to eastern Great Lakes and Virginia. Considered by some authors to be a distinct species, Kumliens' Gull (Larus kumlieni).
glaucoides Meyer, 1822 - Southern and western Greenland. Winters south to northern Europe.
thayeri Brooks, WS, 1915 - Hudson Bay to western Greenland. Winters British Columbia to Baja. Considered by some authors to be a distinct species, Thayer's Gull (Larus thayeri).
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Birds in Colour Campbell, B., 1960, Penguin Books Ltd
The Pocket Guide to Nest and Eggs Fitter, R.S.R., 1954, Collins
RSPB Handbook of British Birds Holden, P., Cleeves, T., 2002, A & C Black ISBN 0 7136 5713 8
Birds of Britain and Europe Sterry, P., et al., 2001, AA Publishing ISBN 0 7495 3068 5
The Popular Handbook of British Birds Hollom, P.A.D., 1973, H.F. & G. Witherby Ltd ISBN 0 85493 002 7
BirdLife International Species Factsheet