The Black Guillemot, also known as the Sea Pigeon, Tystie, and Greenland Dove, is a medium-sized alcid with a circumpolar distribution, breeding from arctic eastern Canada south to the coast of Maine, then eastward at the fringes of the Arctic across Eurasia, just reaching North America again in isolated colonies in northern Alaska and the Yukon Territory. In summer, its black plumage with white wing-patches and scarlet legs and feet are distinctive within its range. As is typical of all 3 species of guillemots (genus Cepphus), and unlike most alcids, the Black Guillemot lays 2 eggs instead of only 1 and prefers inshore waters to open ocean, foraging on fish and invertebrates on or near the bottom in shallow water. Breeding colonies are rather widely scattered and small in the southern part of its range, perhaps due to evenly dispersed prey; larger colonies of 2,000-10,000 pairs have been reported in the high Arctic. The ability of this guillemot to feed in small patches of open water may explain its widespread distribution at higher latitudes, where adults often concentrate at ice edges feeding on epontic and mid-depth prey. Black Guillemots generally remain close to their breeding colonies even in winter, except in areas where open water is limited by ice. One recent estimate of the world population was 250,000-500,000 pairs, but small colony size and cavity-nesting make accurate censusing of this species difficult.
Details of Black Guillemot breeding biology, food preferences, habitat characteristics, and behavior are available from studies of populations in eastern North America ( Preston 1968 ; Cairns Cairns 1980 , Cairns 1981 , Cairns 1987c , Cairns 1987a ), Iceland ( Petersen 1981a ), Denmark (Asbirk Asbirk 1978 , Asbirk 1979a , Asbirk 1979c , Asbirk 1979b ), and the United Kingdom (Ewins Ewins 1988a , Ewins 1988d , Ewins 1988c , Ewins 1989 , Ewins 1992 ). Geographic variation of morphology and plumage patterns have been detailed by Salomonsen ( Salomonsen 1944 ) and Storer ( Storer 1952a ), while molecular genetic variation among populations has been studied only recently ( Friesen et al. 1996a ; Kidd and Friesen Kidd and Friesen 1998a , Kidd and Friesen 1998b ). Despite these studies, the status of proposed subspecies designations remains unresolved. The only published long-term study of Black Guillemot population biology was conducted in Iceland ( Frederiksen 1998 ; Frederiksen and Petersen Frederiksen and Petersen 1999b , Frederiksen and Petersen 1999a , Frederiksen and Petersen 2000 ).