The black-billed gull is a social and gregarious species, forming large flocks to forage and to breed. They are forced to travel long distances to feed each day as the flock can clear an area of food within a day. They scavenge less than many other gull species, but will follow ploughs to pick up earthworms and insects from the freshly-turned farmland. The birds feed from lakes and rivers as well, selecting small fish and aquatic invertebrates, as well as picking flying insects out of the air above the water (2). Food is spotted during soaring flight, and the gulls will descend on areas such as tussock grassland to eat moths, calling noisily as they do so (3).
During the breeding season the black-billed gull returns to the same site it visits every year and pairs begin to build deep depressions of twigs and grass during October. The female lays between one and three eggs on lake edges or in braided rivers, and these are incubated by both the male and female for about 22 days continuously. After hatching, the chicks are fed by their parents until they fledge around 26 days later. The family stays at the breeding site until fledging unless disturbed when they may abandon the nest as soon as the majority of their eggs have hatched, moving on to coastal habitats for the winter. The young are able to breed after two years, but more normally will not pair up until three or four years of age (3).