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Largest and heaviest gull in North America, and one of the largest in the world: length 71–79 cm, wingspan 152–167 cm, mass 1,300–2,000 g. Adult is white with black back and wings; has yellowish bill with subterminal red spot; legs chalky or whitish pink; iris gray or pale yellow that may be flecked with brown. Overall appears heavy and robust; heavy bill with prominent gonydeal angle. In adults, no significant seasonal or sexual differences in plumage.
Adult plumage similar to that of several other North American gull species. Adult Western Gull (Larus occidentalis) is very similar in plumage but restricted to coast of w. North America and extremely unlikely to be encountered in same place as Great Black-backed Gull. Vagrants of either species encountered outside of their normal ranges (e.g., in interior of North America) could pose an identification challenge. Western Gull is slightly smaller with paler back and wings, pinker legs, and (in northern birds) a darker eye. Western Gull also has white subterminal spot on outermost primary tip, while entire tip of this feather is white in Great Black-backed Gull. Entirely white tip of outermost primary also helps distinguish Great Black-backed Gull from Yellow-footed Gull (L. livens) and Lesser Black-backed Gull (L. fuscus). Adult Yellow-footed Gull, while restricted to Gulf of California region, is very similar to Western Gull but has yellow legs and feet. Adult Lesser Black-backed Gull, which occurs in very small numbers throughout North America, but most frequently along East Coast and Great Lakes, is also similar to Great Black-backed Gull. Lesser Black-backed Gull is significantly smaller and less robust in proportions, and its legs and feet are (usually) yellow, and (in Basic plumage) it has distinctive black streaking on face. Race of Lesser Black-backed Gull typically observed in North America (L. fuscus graellsii) has slate gray, rather than black, back and wings. Other races of Lesser Black-backed Gull that have black back and wings (L. f. fuscus or L. f. intermedius) are very rarely observed in North America, and differ from Great Black-backed Gull in all respects that L. fuscus graellsii does. Slaty-backed Gull (L. schistisagus), rare in Alaska and Pacific Northwest, is slightly smaller and has distinctly paler back and wings, pinker legs and feet, and (in Basic plumage) streaked head. Kelp Gull (L. dominicanus) is also similar, but is an extremely rare vagrant in North America.
Great Black-backed Gull attains Definitive (adult) plumage by fourth year (see Appearance, below). Juvenal and Basic I characters include very large size, whitish head and underparts, barred or checkered back and wings, and large, black bill. Can be confused with juvenile Herring and Lesser Black-backed gulls in e. North America; to lesser extent with juvenile Glaucous Gull (L. hyperboreus) in Greenland. Differentiated from Herring and Lesser Black-backed gulls by larger size, whiter head and breast relative to rest of underparts, grayish and blackish brown markings on back and wings, diffuse and broken blackish subterminal band on tail, and large bill with white area at tip (Grant 1986). Herring Gull (North American race) has browner back and wings and dark tail. Subsequent preadult plumage stages also difficult to identify; best characters are size (larger than any other North American gull), combination of pattern and coloring of head, size of bill, pattern and coloring of remiges and wing-coverts, and tail (Nikander 1996). Alternate I–III plumages same as preceding Basic plumages, but with whiter heads and paler, more faded colors throughout. Rare hybrids reported with Herring (Jehl 1960, Andrle 1972, Foxall 1979, Veit and Petersen 1993) and Glaucous (Harrison 1983) gulls. For more information on identification of adult and subadult stages, see Grant 1986 and Harris et al. 1989 .