Gulls are members of the cosmopolitan family Laridae, of the order Charadriiformes, along with the terns and jaegers. Saskatchewan has records of twenty-three members of the approximately 104 species worldwide. Members of the family are short-legged, short-necked, webbed-footed birds found around water, particularly on the coasts. They are ground nesters, often in colonies; the sexes have similar plumage. The gulls are typically grey-backed, white-bellied birds with strong bills, feeding on a variety of plant and animal material. The terns are also usually grey and white, with crests in breeding plumage, and long pointed bills used for catching small fish by diving into the water. In both groups the plumage of different species can be very similar, and they can be challenging to identify. The gulls have taken advantage of human habitats and are often very abundant around harbours, garbage dumps, and city lakes. They follow farm machinery, catching insects and small rodents startled up by the movement of the machinery. Most species are migratory in Saskatchewan, although some individuals may remain on open water over winter.
The common gulls are of two types: the black-headed and the white-headed. The black-headed gulls are Franklin's gull (Larus pipixcan), a common prairie species, and Bonaparte's gull (L. philadelphia), which nests in trees in the boreal regions and is a transient in the south. The most common white-headed gull is the ring-billed gull (L. delawarensis), named for its identifying characteristic. Other common gulls are the California gull (L. californicus) and the large herring gull (L. argentatus). The mew gull (L. canus) breeds in the north. Other regular transients are: the parasitic jaeger (Stercorarius parasiticus), a predatory gull-like bird; the very pale Iceland gull (L. glaucoides); and the glaucous gull (L. hyperboreus).
The terns are seen hovering and diving over large marshes, lakes and rivers. The common tern (Sterna hirundo) nests throughout the province, commonly on the islands and sandbars of the larger lakes, reservoirs, or rivers. Forster's tern (S. forsteri) is a very similar, but less common, bird seen around deeper-water marshes of the south; it is best told by having a deeper-voiced call than the common tern. The black tern (Chlidonias niger) is a marsh nester in the southern two-thirds of the province; its numbers appear to be decreasing with periodic drying and draining of the wetlands. The Caspian tern (S. caspia), a large, red-billed cosmopolitan species, is rare and sporadically distributed; the Arctic tern (S. paradisaea) nests in the far north and is seen only rarely farther south.
A number of marine stragglers have also been reported: pomarine jaeger (Stercorarius pomarinus), long-tailed jaeger (S. longicaudus), little gull (L. minutus), lesser black-backed gull (L. fuscus), slaty-backed gull (L. schistisagus), great black-backed gull (L. marinus), Sabine's gull (Xema sabini), black-legged kittiwake (Rissa tridactyla), and least tern (Sterna antillarum).