The Common (or Thin-billed) Murre, a species found in cooler, continental shelf waters of the Sub-arctic-low Arctic, is one of the most numerous marine birds in the Northern Hemisphere. Its breeding numbers total 13 to 21 million birds, perhaps more, with 4.0 to 8.4 million in western North America and 1.2 million in eastern North America. Its range overlaps extensively with the congeneric Thick-billed (or Brünnich's) Murre (Uria lomvia) in the Pacific region, less so in the Atlantic. Pacific and Atlantic forms are morphologically and genetically distinct; 2 poorly differentiated subspecies are recognized in western North America and 1 in eastern North America (2 to 4 more in Europe). Especially in poor light and with individuals in Basic plumage, this species is virtually impossible to distinguish at a distance from the Thick-billed Murre and Razorbill (Alca torda); both have much deeper bills than the Common Murre, which is known to hybridize with both.
Among the largest of the living Alcidae and a consummate diver, the Common Murre reaches depths of more than 100 meters in search of prey, mainly fish but also invertebrate s (euphausiids, cephalopods). It often forages in flocks, including multispecies assemblages. Its smallish wings, a compromise for underwater wing-propelled diving and aerial flight, require rapid beats to remain aloft. This high wing-loading, plus diving in cool waters and possessing poor insulation, contributes to an energetically costly life style.
Highly social, this species breeds on island cliff ledges, slopes, and flat surfaces, usually shoulder to shoulder, with no breeding site or territory extending beyond its individual space, which it defends fiercely. It has a unique breeding strategy, the major elements of which are high nesting density; high degree of laying and, especially, departure synchrony; and colony departure of chicks at just 3 to 4 weeks of age, with most development taking place at sea in the company of the male parent. These attributes require that adults find abundant, energy-rich prey within 60 to 70 kilometers (usually) of breeding ledges. For 1 or 2 months after departure, male parent and chick swim to where abundant prey is predictable and where intraspecific competition is less intense than near to colonies.
This is one of the most intensively studied avian species in the world and certainly the best known of the Alcidae; all aspects of its natural history have been well investigated, although information on demography in North America is rudimentary. Such a wealth of information exists owing to a long interaction with humans, dating to prehistoric times. Dependence on abundant prey brings this species into competition with commercial fisheries. All age classes in many local populations incur regular, high mortality due to oil spills and gill-netting, and in certain regions (for example, Newfoundland and Labrador), to human predation/hunting. The species' susceptibility to oil spills drives much of its research and management, often funded by fines from pollution sources.