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Auks are small to medium-sized diving birds having bills that are variably pointed and compressed but never acuminate and are sometimes covered with colorful horny sheaths in breeding adults. Feathering densely covers the lores and often extends to the nostrils, which vary from linear to oval. There are 10 functional primaries and 15–19 secondaries; the greater secondary and primary coverts are usually lengthened. Molting of the primaries is usually simultaneous but is gradual in some species; the wings are used for underwater propulsion, and the feet are then used mainly for steering. The 12–18 rectrices are short and normal in shape. The feathers are dense, water-resistant, and shiny; aftershafts and adult down feathers are present. The legs are set fairly far back on the body; walking is done in an erect posture with the weight on the toes (digitigrade). Takeoff from level ground is difficult and infrequent; the birds usually take flight from cliffs or into the wind from water. The tarsus is compressed laterally, is reticulated or scutellated, and is usually shorter than the middle toe. The hallux is absent or vestigial, and the front toes are fully webbed. The wings are relatively short (one recently extinct species and several fossil species were flightless), bowed, and pointed, and the body is robust, with a short neck and large head. Adults have white to dark grayish underparts and usually are blackish dorsally. Adults of most species exhibit crests, facial tufts, or other distinctive plumage or horny bill adornments in both sexes during the breeding season. All species are monogamous and monomorphic, often having distinct breeding and wintering plumages. The eggs are laid on rock ledges, in crevices or burrows, or rarely among the branches of trees. The eggs (1 or 2) are often spotted and are pyriform to ovate. The young are down-covered, unpatterned to bicolored, and are nidifugous to seminidicolous. The family is Holarctic in distribution, and the species are entirely marine in winter but usually breed colonially (sometimes solitarily) along coastlines. Their foods mainly consist of fish, plankton, and other marine fauna. There are twenty-two extant species, twenty of which breed in North America, particularly along the northern Pacific coast.