Diet
Great Black-backed Gulls are opportunistic and will eat almost any small organism that they access. Like most gulls, they will eat almost anything that they can swallow. They get most of their food from scavenging, with refuse locally comprising more than half of their diet. 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, including Anas ducks, terns, 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. 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, in order to capture fish driven to the surface by such creatures.
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