Auk is a common name for any member of the auk family (Alcidae). Auks are compact, duck-shaped birds with very dense plumage. Coloring is austere, either two-colored with black upperparts and white underparts, or dark. The most colorful and bright are the bare parts such as the bill, mouth, and legs. The male and female species are alike. Recent species vary in size between 100 g and 1.3 kg.

Auks are true seabirds, inhabiting oceanic and coastal waters of the Arctic, boreal, and temperate zones. During the nonbreeding season, they may move further south, but some remain to winter over in the High Arctic among ice.

All auks are superior swimmers; while on land they walk with a waddle on their tarsal bones. The legs are typically positioned toward the rear of the body, which accounts for the bird's upright posture and often a comical appearance of human expression. Ecologically, they are counterparts of the penguins in the Northern Hemisphere. Both groups have much reduced wing size, resembling oars and specialized to swim underwater. Auks are thought to have originated from gulls during the process of adaptation to explore the ocean's depths in pursuit of prey. They not only show an example of parallel evolution with penguins but also demonstrate the way in which penguins have evolved. Finally, penguins have lost their ability to fly in air and can only swim underwater, whereas auks retain flying and swimming capabilities in both elements. Auks have wing-loading close to the maximum permitted to fly, and hence their flight is headlong, slightly maneuverable with rapid wing-beats. The wings are the only underwater advancer, while the webbed feet are used as a rudder.

Auks come ashore only to breed. They settle typically in colonies, often in great numbers, along the sea-coasts. No nest is constructed, and eggs are laid in the open on rock ledges; in species seeking shelter in crevices or burrows, a very simple nest is built. Most auks breed at a considerable distance from their feeding grounds, and typically are able only to provide enough food for a single chick. Incubation lasts for three to five weeks, and both parents incubate in turn. The time that chicks stay at the nesting sites may vary considerably between species, ranging from 2 to 50 days.

Auks feed at sea only. The diet consists of a variety of small fishes and invertebrates (mainly crustaceans).

Twenty-two recent species with greatest variety are found in Beringia, which is thought to be the center of auk origin. Nearly a third of the auks are common in the Arctic, including little auk, guillemots, puffins, and razorbill. The overall number of alcids is assumed to be 100,000,000 birds; the little auk followed by guillemots are the most abundant.

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