Gulls are mainly scavengers and prey on anything they can find, sometimes far inland. They forage along ocean and inland shores, when they pick up dead animal matter or catch fish in shallow waters. In some areas they are known to carry hard-shelled molusks aloft and drop from on pavements or rocks to break the shell. Some species rob smaller birds of their catches, and gulls often gang up on wounded larger birds. Some species, such as the ring billed, Bonaparte's and
Gulls usually nest in colonies on islands. Two or three eggs are laid in a shallow nest composed of stones, shells seaweed, or stalks of vegetation. Incubation periods vary from 20 to 30 days, and the young, sparsely clothed with natal down, remain at or near the nest for several days before wandering off. Mortality is high in the nesting colonies, but survivors may live up to 30 years or more. The birds have been favorite subjects for behavior studies, and their movements and migrations are well known.
The great black-backed gull (left) is often seen with smaller and more common herring gull (center), which often breaks the shells of clam and other mollusks by dropping them on rocks or paved areas. Fanklin's gull (right), found inland in freshwater habitats, feeds on insects and other prey in plowed fields.