The Great Black-backed Gull (Larus marinus), also known as the Greater Black-backed Gull or, informally, as the Black-back, 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 Black-backs move farther south or inland to large lakes or reservoirs.
The Great Black-backed Gull was one of the many species originally described by Linnaeus in his 18th-century work, Systema Naturae, and it still bears its original name of Larus marinus.
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. The Great Black-backed Gull 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. In other white-headed North Atlantic gulls, the mantle is generally light gray 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 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, both generally far outside this species' range, 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, 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.
Distribution and habitat
This species can be found breeding in coastal areas from the extreme northwest portion of Russia, through much of coastal Scandinavia, on the Baltic Sea coasts, to the coasts of northwestern France, the United Kingdom and Ireland. Across the northern portion of the Atlantic, this gull is distributed in Iceland and southern Greenland and on the Atlantic coasts of Canada and the United States. Though formerly mainly just a non-breeding visitor south of Canada in North America, the species has spread to include several colonies in the New England states and now extends as far south as a breeding bird to North Carolina. Individuals breeding in harsher environments will migrate south, wintering on northern coasts of Europe from the Baltic Sea to southern Portugal, and regularly down to coastal Florida in North America. Exceptionally, the species can range as far south as the Caribbean and off the coast of northern South America.
The Great Black-backed Gull is found in a variety of coastal habitats, including rocky and sandy coasts and estuaries, as well as inland wetland habitats, such as lakes, ponds, rivers, wet fields and moorland. They are generally found within striking distance of large bodies of water while ranging inland. Today, it is a common fixture at refuse dumps both along coasts and relatively far into land. The species also makes extensive use of dredge spoils, which, in the state of New Jersey, comprise the most prevalent nesting sites for them. It generally breeds in areas free of or largely inaccessible to terrestrial predators, such as vegetated islands, sand dunes, flat-topped stacks, building roofs and sometimes amongst bushes on salt-marsh islands. During the winter, the Great Black-backed Gull often travels far out to sea to feed.
Diet
Like most gulls, Great Black-backed Gulls are opportunistic feeders. They will investigate any small organism they encounter and will readily eat almost anything that they can swallow. They get most of their dietary energy from scavenging, with refuse, most provided directly by humans, locally comprising more than half of their diet. The proliferation of garbage or refuse dumps has become a major attractant to this and all other non-specialized gull species in its range. Like most gulls, they also capture fish with some regularity and will readily capture any fish smaller than itself found close to the surface of the water.
Unlike most other Larus gulls, they are highly predatory and frequently hunt and kill any prey smaller than themselves, behaving more like a raptor than a typical larid gull. Lacking the razor-sharp talons and curved, tearing beak of a raptor, the Great Black-backed Gull relies on aggression, physical strength and endurance when hunting. When attacking other animals, they usually attack sea bird eggs, nestlings or fledgings at the nest, including smaller gull species as well as eiders, gannets, terns and various alcids. Adult or fledged juveniles of various bird species have also been predaceously attacked. Some flying birds observed to be hunted in flight by Great Black-backed Gulls have included Anas ducks, ruddy ducks, buffleheads, pied-billed grebes, terns, puffins, coots, hen harriers, glossy ibises and even rock pigeons. When attacking other flying birds, the Great Black-backed Gulls often pursue them and attack them on the wing by jabbing with their bill and hoping to exhaust the other bird. They will also catch flying passerines, which they typically target while the small birds are exhausted from migration and swallow them immediately. Great Black-backed Gull also feed on land animals, including rats at garbage dumps and even sickly lambs.
Most foods are swallowed whole, including most fish and even other gulls. When foods are too large to be swallowed at once, they will sometimes be shaken in the bill until they fall apart into pieces. Like some other gulls, when capturing molluscs or other hard-surfaced foods such as eggs, they will fly into the air with it and drop it on rocks or hard earth to crack it open. Alternate foods, including berries and insects, are eaten when available. They will readily exploit easy food sources, including chum lines made by boats at sea. They are skilled kleptoparasites who will readily pirate fish and other prey captured by other birds and dominate over other gulls when they encounter them. At tern colonies in coastal Maine, American Herring Gulls occasionally also attack nestling and fledging terns but in a great majority of cases were immediately pirated of their catch by Great Black-backs. In one observation, an adult Great Black-back was seen to rob a female peregrine falcon of a freshly caught gadwall. Due to their method of using intimidation while encountering other water birds, the species has been referred to as a "merciless tyrant". Naturally, these gulls attracted to surface activity of large marine animals, from Atlantic bluefin tuna to humpback whales, to capture fish driven to the surface by such creatures.
Breeding
This species breeds singly or in small colonies, with pair formation occurring in March or April. They make a lined nest on the ground often on top of a rocky stack, fallen log or other obstructing object which can block the contents from the elements. Usually, several nest scrapes are made before the one deemed best by the parents is selected and is then lined with grass, seaweed or moss or objects such as rope or plastic. The female lays one to three eggs sometime between late April and late June. The eggs are greenish-brown with dark speckles and blotches. Both parents participate in the incubation stage, which lasts for approximately 28 days. The breeding pair are devoted parents who both take shifts brooding the young, defending the nest and gathering food. Young Great Black-backed Gulls leave the nest area at 50 days of age and may remain with their parents for an overall period of around six months, though most fledglings choose to congregate with other immature gulls in the search for food by fall. These gulls reach breeding maturity when they obtain adult plumage at four years, though may not successfully breed until they are six years old.
Longevity and mortality
This is a relatively long-living bird. The maximum recorded age for a wild Great Black-backed Gull is 27.1 years. This species is rarely kept in captivity, but domestically kept Herring Gulls have been known to live for over 44 years and generally larger birds can outlive smaller ones. Mortality typically occurs in the early stages of life, when harsh weather conditions (including flooding) and starvation can threaten them, as well as predators. Chicks and eggs are preyed on by crows, cats, other gulls, raccoons and rats. The Bald Eagle, Golden Eagle, and White-tailed Eagle are the only birds known to habitually predate healthy, fully grown Great Black-backed Gulls. A Great Skua was filmed in Scotland unsuccessfully attempting to kill a second or third year Great Black-backed Gull. The slightly smaller Pomarine Skua has been observed to have been predated by Great Black-backed Gulls. Killer whales and sharks also reportedly prey upon adult and juvenile birds at sea. In some biomes, where large eagles are absent the Great Black-backed Gull may be considered the apex predator.
Status
Historically, the Great Black-backed Gull was harvested for its feathers, which were used in the hat-making trade, and this species was removed from large parts of its range as a result of this exploitation. Today however, its adaptability to human presence and the use of urban environments as artificial nesting sites has resulted in the Great Black-backed Gull rapidly increasing in number and range. It is now a widespread and abundant species in its range and its numbers have increased to such high levels in some areas that it is often seen as a pest species, especially near airports where it risks collisions with airplanes, and in some coastal areas where it predates upon and competes with rarer seabirds such as Atlantic puffins. Although there are no known major threats to the Great Black-backed Gull, high levels of toxic pollutants, which are ingested with contaminated prey, are often found in individuals and eggs, reducing reproductive success. Breeding is also interrupted by human disturbance, which can lead to eggs being abandoned, leaving them vulnerable to exposure and predation. The great black-backed gull is additionally hunted for sport in Denmark.
source: http://en.wikipedia.org/wiki/Larus_marinus
