Auk, a diving seabird related to the puffins, murres, and guillemots. Auks are found in the arctic regions; a few species spend the winter in temperate areas. An auk resembles a penguin, having legs set far back on its body so that it walks almost upright. It is dark brown or black on its upper parts, white or light gray underneath. The males of some species have brightly colored plumes on each side of the head during the breeding season. Auks have knifelike bills, pointed wings, and short tails. They use their wings as well as their webbed feet in swimming. Auks range in height from about 6 to 16 inches (15 to 40 cm).

Auks feed on fish, shrimp, and other small sea animals. In spring, auks gather in large numbers in their arctic breeding grounds. The female lays one or two eggs in a crevice among rocks, or on a bare ledge. The parent birds tend the young for several months, then return to the sea to spend the winter.

Auk colonies have long been raided by man. Eggs and flesh are used for food, and feathers and skin are put to many uses by the Eskimos. The great auk, a flightless bird that stood about two feet (60 cm) tall, was killed in such numbers that it became extinct in 1844.

Several species of auks visit the shores of the United States. The razor-billed auk winters along the Atlantic coast as far south as New York. The paraquet auklet, the rhinoceros auklet, and Cassin's auklet winter along the Pacific coast as far south as southern California.

Auks belong to the family Alcidae. The great auk is Pinguinus impennis. The razor-billed auk is Alca torda; the paraquet auklet, Cyclorrynchus psittacula; the rhinoceros auklet, Cerorhinca monocerata; Cassin's auklet, Ptychoramphus aleuticus.