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Known in Eurasia as Common Guillemot, and in North America as Common (or Thin-billed) Murre (Uria aalge), this species frequents cooler, continental shelf waters of the sub-Arctic/low-Arctic, and is one of the most numerous marine birds in the Northern Hemisphere. It numbers more than 8.3 million breeding pairs, with 5.0 million in the Pacific region, and 3.3 million in the Atlantic region. Its range overlaps extensively with the congeneric, but generally more northern Brünnich's Guillemot or Thick-billed (or Brünnich's) Murre (Uria lomvia) in the Pacific region, though less so in the Atlantic. Pacific and Atlantic forms of Common Murre are morphologically and genetically distinct; in the Pacific, 2 poorly differentiated subspecies are recognized, and in the Atlantic, 2–5 subspecies are recognized, all of which occur in Europe, while just 1 occurs in eastern North America.
Among the largest of the living Alcidae and a consummate diver, Common Murre reaches depths of more than 100 m in search of small fish but also invertebrates (such as euphausiids and cephalopods). It often forages in flocks, including multispecies assemblages. Its relatively small wings, a compromise for underwater wing-propelled diving and aerial flight, require rapid beats in order for the murre to remain aloft in the air. This high wing-loading, plus diving in cool waters with 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; it has no nest but fiercely defends its egg-laying site or territory that is little more than the space a pair occupies. It has a unique breeding strategy, the major elements of which are high breeding density; high degree of laying and, especially, phenological synchrony; and departure of chicks at just 3–4 weeks of age, with most development taking place at sea in the company of the male parent. Initially the chick is flightless, but in time its flight feathers grow sufficiently for flight before it becomes independent of its parent at about three months in age. These attributes require that adults find abundant, energy-rich prey within 60–70 km (usually) of breeding ledges. For 1–2 months after departure, male parent and chick swim to where prey is abundant and predictable, and where intraspecific competition is less intense than near to breeding sites.
While this is one of the most intensively studied avian species in the world, and we know much about most aspects of its natural history, much more needs to be learned about demography, including movement of individuals among neighboring breeding sites and breeding populations of differing size. Such a wealth of information exists owing to a long interaction with humans, dating to prehistoric times. For example, its dependence on abundant prey brings this species into interaction with commercial fisheries, in some cases to advantage and in others disadvantage. 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. Mortality events occur owing to anomalous ocean conditions, more recently including climate change-related food shortages. The species' susceptibility to oil spills has driven a good deal of research and management, often funded by environmental impacts of oil development, as well as fines from pollution sources.