The great black-backed gull (Larus marinus), mistakenly called greater black-backed gull by some, is the largest member of the gull family. It breeds on the European and North American coasts and islands of the North Atlantic and is fairly sedentary, though some move farther south or inland to large lakes or reservoirs. The adult great black-backed gull has a white head, neck and underparts, dark grey wings and back, pink legs and yellow bill.
his is the largest gull in the world, considerably larger than a herring gull (Larus argentatus). Only a few other gulls, including Pallas's gull (Ichthyaetus ichthyaetus) and glaucous gull(Larus hyperboreus), come close to matching this species' size. It is 64–79 cm (25–31 in) long with a 1.5–1.7 m (4 ft 11 in–5 ft 7 in) wingspan and a body weight of 0.75–2.3 kg (1.7–5.1 lb). In a sample of 2009 adults from the North Atlantic, males were found to average 1,830 g (4.03 lb) and females were found to average 1,488 g (3.280 lb). Some adult gulls with access to fisheries in the North Sea can weigh up to roughly 2.5 kg (5.5 lb) and averaged 1.96 kg (4.3 lb). An exceptionally large glaucous gull was found to outweigh any known great black-backed gull, although usually that species is slightly smaller. The great black-backed gull is bulky and imposing in appearance with a large, powerful bill. The standard measurements are: the bill is 5.4 to 7.25 cm (2.13 to 2.85 in), the wing chord is 44.5 to 53 cm (17.5 to 20.9 in) and the tarsus is 6.6 to 8.8 cm (2.6 to 3.5 in).
The adult great black-backed gull is fairly distinctive, as no other very large gull with blackish coloration on its upper-wings generally occurs in the North Atlantic. In other white-headed North Atlantic gulls, the mantle is generally a lighter gray color and, in some species, it is a light powdery color or even pinkish. It is grayish-black on the wings and back, with conspicuous, contrasting white "mirrors" at the wing tips. The legs are pinkish, and the bill is yellow or yellow-pink with some orange or red near tip of lower bill. The adult lesser black-backed gull (L. fuscus) is distinctly smaller, typically weighing about half as much as a great black-back. The lesser black-back has yellowish legs and a mantle that can range from slate-gray to brownish-colored but it is never as dark as the larger species. A few superficially similar dark-backed, fairly large gulls occur in the Pacific Ocean or in the tropics, all generally far outside this species' range, such as the slaty-backed (L. schistisagus), the western (L. occidentalis) and the kelp gull (L. dominicanus).
Juvenile birds of under a year old have scaly, checkered black-brown upper parts, the head and underparts streaked with gray brown, and a neat wing pattern. The face and nape are paler and the wing flight feathers are blackish-brown. The juvenile's tail is white with zigzag bars and spots at base and a broken blackish band near the tip. The bill of the juvenile is brownish-black with white tip and the legs dark bluish-gray with some pink tones. As the young gull ages, the gray-brown coloration gradually fades to more contrasting plumage and the bill darkens to black before growing paler. By the third year, the young gulls resemble a streakier, dirtier-looking version of the adult. They take at least four years to reach maturity, development in this species being somewhat slower than that of other large gulls. The call is a deep "laughing" cry, kaa-ga-ga, with the first note sometimes drawn out in an almost bovid-like sound. The voice is distinctly deeper than most other gull species.