Southern populations are generally sedentary, while those breeding in the far north (northern Norway, northwest Russia) move farther south in winter.

The adult has a white head, neck and underparts, dark blackish-grey wings and back, pink legs and the bill yellow with a red spot.

The lesser black-back has yellowish legs and a mantle that can range from slate-grey to brownish but it is never as dark as the larger species.

[7][11] Juvenile birds of under a year old have scaly, checkered black-brown upper parts, the head and underparts streaked with grey brown, and a neat wing pattern.

As the young gull ages, the grey-brown colour gradually fades to more contrasting plumage and the bill darkens to black before growing paler.

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.

North of the Åland islands, the sea often freezes all the way from Sweden to Finland, and then the bird migrates to open waters.

They get much of their dietary energy from scavenging, with refuse, most provided directly by humans, locally constituting more than half of their diet.

The proliferation of garbage or refuse dumps has become a major attractant to this and all other non-specialised gull species in its range.

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.

Some fully-fledged or adult birds observed to be hunted in flight or on the ground by great black-backed gulls have included Anas ducks, ruddy ducks (Oxyura jamaicensis), buffleheads (Bucephala albeola), Manx shearwaters (Puffinus puffinus), pied-billed grebes (Podilymbus podiceps), common moorhens (Gallinula chloropus), terns, Atlantic puffins (Fratercula arctica), Little auks (Alle alle), coots (Fulica ssp.

), glossy ibises (Plegadis falcinellus), rock doves (Columba livia) and even predatory birds such as hen harriers (Circus cyaneus).

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.

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.

[29] In one observation, an adult great black-back was seen to rob a female peregrine falcon (Falco peregrinus) of a freshly caught gadwall (Anas strepera).

They make a lined nest on the ground often on top of a rocky stack, fallen log or other obstructing object which can protect the eggs from the elements.

Usually, several nest scrapes are made before the one deemed best by the parents is selected and then lined with grass, seaweed or moss or objects such as rope or plastic.

The breeding pair are devoted parents who both take shifts brooding the young, defending the nest and gathering food.

[39] This species is rarely kept in captivity, but domestically kept European herring gulls have been known to live for over 44 years and generally larger birds can outlive smaller ones.

[40] Mortality typically occurs in the early stages of life, when harsh weather conditions (including flooding) and starvation can threaten them, as well as predators.

[48][16] 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 sometimes outcompetes or hunts rarer seabirds, such as Atlantic puffins, possibly resulting in conservationist intervention.

[13] 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.