Robust and hardy, the Thick-billed Murre is one of the most numerous marine birds in the Northern Hemisphere. In summer, it is found in continental-shelf waters of the Arctic Ocean and adjacent seas, shifting southward in winter. In terms of biomass, it is the dominant seabird over large parts of the Arctic Ocean in summer. It nests on precipitous sea cliffs in very dense colonies than can number over 1 million breeding birds. In the Pacific sector of the Arctic, breeding colonies are frequently shared with the congeneric Common Murre (Uria aalge), but in the Atlantic there is less overlap between the 2 species. The Thick-billed Murre is one of the deepest underwater divers of all birds, regularly descending to depths of more than 100 m, and occasionally below 200 m, and can remain submerged for more than 3 minutes. A generalist feeder, it takes most kinds of marine invertebrates and fish, from pteropod mollusks weighing less than 0.5 g up to fish weighing more than 50 g. Because of high wing-loading, its load-carrying capacity is poor and consequently chicks are provisioned only small amounts at each visit: usually a single fish. This limitation, combined with an open nest-ing site that requires chick-guarding by 1 parent throughout the rearing period and long commuting distances to feeding areas at many colonies, probably has been the main factor selecting for early departure of nestlings. Chicks leave the colony when only partially grown, at about one-quarter adult mass, and before flight feathers have begun to develop. At departure, they use the covert feathers, extended as miniature wings, to flutter and glide from the breeding site to the water. Growth is completed at sea, while being fed by the male parent alone. This unusual rearing strategy is otherwise seen only in the Common Murre and Razorbill (Alca torda).
Considering the Thick-billed Murre's remote breeding sites, its breeding biology in North America is fairly well known, with major studies at col-onies in eastern Canada (Birkhead and Nettleship Birkhead and Nettleship 1981 , Birkhead and Nettleship 1987c , Birkhead and Nettleship 1987b , and Birkhead and Nettleship 1987a ; Gaston and Nettleship 1981 ; Gaston et al. 1985 ; De Forest and Gaston 1996 ) and in the Bering Sea (Hunt et al. Hunt et al. 1981c , Hunt et al. 1986a ) and Chukchi Sea ( Swartz 1966 , Murphy et al. 1987a , Fadely et al. 1989 ). Many have been banded in Canada, Greenland, and Europe, resulting in a much better knowledge of movements than for most arctic seabirds ( Tuck 1961 , Kampp 1988 , Donaldson et al. 1997b ), although very little is known about Alaskan populations in this respect. Diet and feeding ecology have been studied intensively in summer more or less throughout the species' range (Bradstreet Bradstreet 1979 , Bradstreet 1980 ; Hunt et al. Hunt et al. 1981c , Hunt et al. 1988a , Hunt et al. 1996 ; Gaston and Noble 1985 ; Piatt et al. 1991b ; Coyle et al. 1992 ; Gaston and Perin 1993 ). Winter diet, however, remains poorly known outside Newfoundland ( Elliot et al. 1990 ). Demographics also are little known, with the only long-term study conducted in northern Hudson Bay ( Gaston et al. 1994 ).