It is sometimes considered as a separate species (Larus heuglini) but is now usually treated as a subspecies of the lesser black-backed gull.[1][2][3] Birds in the eastern part of Heuglin's gull's range are often paler grey above and sometimes considered to be a separate subspecies Larus fuscus taimyrensis (Taimyr gull). It is possible that they are a result of hybridization between Heuglin's gulls and Vega gulls.
They are large gulls with a rounded head, strong bill and long legs and wings. Length is from 53 to 70 cm (21 to 28 in), wingspan is from 138 to 158 cm (54 to 62 in) and body mass is from 745 to 1,360 g (1.642 to 2.998 lb).[4] Among standard measurements, the wing chord is 40.5 to 46.9 cm (15.9 to 18.5 in), the bill is 4.5 to 6.5 cm (1.8 to 2.6 in) and the tarsus is 5.9 to 7.8 cm (2.3 to 3.1 in).[4] The back and wings are dark grey, variable in shade but often similar to the graelsii race of the slightly smaller lesser black-backed gull. In winter the head is only lightly streaked with brown but there is heavier streaking on the hindneck. The legs are usually yellow but can be pink.
Moulting takes place later than in most of their relatives so birds still have unstreaked heads and worn primaries in September and October. The primary feathers may not be fully grown until February or March when the head is still streaked.
^Burger, J.; Gochfeld, M.; Kirwan, G.M.; Christie, D.A.; de Juana, E.; Garcia, E.F.J. (2020). "Lesser Black-backed Gull (Larus fuscus)". In del Hoyo, J.; Elliott, A.; Sargatal, J.; Christie, D.A.; de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions. doi:10.2173/bow.lbbgul.01. S2CID218849076. Retrieved 21 April 2017.
^ abGulls: Of North America, Europe, and Asia by Klaus Malling Olsen & Hans Larsson. Princeton University Press (2004). ISBN978-0691119977.
