An opportunistic predator and scavenger with an omnivorous diet, there is little that the great black-backed gull will not eat. It feeds on fish, birds, small mammals, insects, eggs, berries, carrion and occasionally even large prey such as sickly lambs (1) (2) (5) (6). Small prey items are swallowed whole, but larger items are broken up before being consumed. Harder food items, such as molluscs and eggs, are dropped on hard surfaces to break them open (2) (7).
The great black-backed gull forages alone or in groups, from the shoreline to far out at sea (5). Groups of this gull and other seabird species often gather in areas where prey is in abundance, or follow humpback whales (Megaptera novaeangliae) to eat fish forced up to the surface. Prey is also pirated from other birds and scavenged from refuse dumps (5) (7).
The great black-backed gull forms breeding pairs in March or April, with pairs nesting in colonies with the herring gull (Larus argentatus), and occasionally with other species (2) (5). Both the male and female scrape several nests into the ground, but the nest site with the best shelter from the prevailing wind is chosen, usually behind a large object such as a log or rock. The nest is then filled with grass, moss and seaweed (2) (5). One to three eggs are laid over a six day period, between late April and late June. The eggs are incubated for up to 28 days by both adult birds, which continue to care for the chicks after they have hatched, alternating between foraging for food and protecting the young. The chicks fledge after seven or eight weeks, but are cared for by the adults for up to six months. Sexual maturity is reached at four or five years (5).