In general, any of 22 species of diving birds (family Alcidae), especially the little auk and the razorbill, or razor-billed auk.
Auks are 6–16 in. (15–40 cm) long, with short wings and legs and webbed feet. They occur only in Arctic, subarctic, and temperate regions (with a few species south to Baja California). Auks nest colonially on cliff ledges or in rock crevices or burrows near the sea; many spend the winter far from land. They feed on fish, crustaceans, mollusks, and plankton. True auks are black and white and stand erect on land. See also great auk .