The Pigeon Guillemot, a seabird endemic to the North Pacific, is one of three extant members of the genus Cepphus .
Found along rocky coastlines between Alaska and California, this auk nests in burrows or in rock cavities, often on small islands that provide protection from predators; small colonies often form, although this bird does nest as isolated pairs. It occurs in similar habitat in western Siberia, from the Chukot Peninsula south to the Kurile Islands. Its breeding numbers seem to be limited by the availability of food and of suitable nest sites. Pigeon Guillemots generally feed inshore, in water 10 to 30 meters deep, diving directly to the sea bed where they probe rocky recesses and vegetation with their bills for benthic fish and invertebrates.
The estimated population of this species is about 235,000, with the largest breeding concentrations on the Farallon Is., CA, and in the Chukot Peninsula-about 2,200 birds at each locale. Although the Pigeon Guillemot is locally vulnerable to threats such as oil pollution, gill-netting, and mammalian predators, its widespread distribution along most coastlines decreases this vulnerability at the population level. There is an extensive literature on Pigeon Guillemot biology and distribution during the breeding season, but less so than for the closely related Black Guillemot of the North Atlantic (C. grylle; Cramp 1985a , Nettleship and Birkhead 1985 ). Little is known of the Spectacled Guillemot (C. carbo), which inhabits inshore waters of the Seas of Okhotsk and Japan ( Ewins et al. 1993 ).