Description
This is the largest gull in the world, noticeably outsizing a Herring Gull. Only a few other gulls, including Pallas's Gull and Glaucous Gull, 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 10 in–5 ft 7 in) wingspan and a body weight of 0.75–2.3 kg (1.7–5.1 lb). In 209 adults from the North Atlantic, males were found to average 1,830 g (4.0 lb) and females were found to average 1,488 g (3.28 lb). An exceptionally large Glaucous Gull was found outweigh any known Great Black-backed Gull, although usually that species is slightly smaller. It is bulky and imposing in appearance with a large, powerful bill. The standard measurements are as follows: the bill is 5.4 to 7.25 cm (2.1 to 2.85 in), the wing chord is 44.5 to 53 cm (17.5 to 21 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. 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 has a mantle that can range from slate-gray to brownish-colored but it is never as dark as the larger species. The Lesser Black-backed is also distinctly smaller, with different bare part coloration. A few superficially similar dark-backed, fairly large gulls occur in the Pacific ocean or in the tropics, such as the Slaty-backed, the Western and the Kelp Gull.
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.
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