Common Murre

The common murre (Uria aalge), also called the common guillemot, is a large auk of northern seas. Adults are roughly 38–43 cm long, have a wingspan of about 64–71 cm and weigh around 900–1,300 g. In breeding plumage the head, back and wings are dark brown-black and the underparts white. In winter the face and throat become white while the crown remains dark.

The closely related thick-billed murre (Uria lomvia) has a shorter, deeper bill with a pale line along the cutting edge and generally cleaner white flanks. Common murres have a longer and more pointed bill and usually show dark streaking along the sides of the body. In some Atlantic colonies a white eye-ring and line behind the eye occur in a colour form known as the “bridled” morph.

Common murres spend almost the entire year at sea, using cold and temperate shelf waters, coastal seas and productive ocean fronts where schools of small fish are concentrated. They only need land for breeding and then gather on steep cliffs, rocky islands, sea stacks and broad ledges that are difficult for land predators to reach.

Colony location is closely tied to the surrounding marine food supply. Adults must repeatedly commute between the cliff and feeding areas while incubating and raising the chick, so changes in fish distribution can quickly affect breeding success even when the nesting cliffs themselves remain unchanged.

Small schooling fish dominate the diet, including capelin, sand lance, herring and young cod, with squid and crustaceans also taken. The exact prey changes between regions and seasons. On Bear Island in the Barents Sea, capelin is especially important.

Common murres dive from the sea surface and use their wings to “fly” underwater in pursuit of prey. Most hunting dives are much shallower than their maximum diving capacity, but birds caught in deep-set fishing nets off Newfoundland showed that common murres can reach depths of at least 180 m, making them among the deepest-diving members of the auk family.

Foraging effort is flexible. Studies in Alaska found that adults can devote more time to searching and diving when prey becomes less available, helping them compensate to some extent for large year-to-year changes in food supply.

At colonies, common murres spend much of their time resting, preening and remaining alert between trips to sea.

Common murres are intensely social at breeding colonies and can stand in direct physical contact with several neighbours. Adults interact frequently with both partners and nearby birds through mutual preening, body contact and occasional aggression, and familiar neighbours can form stable relationships within the same crowded part of a colony.

The density of a colony is not simply a matter of limited space. Group size and the immediate social surroundings can influence how birds respond to predators and how safely eggs are guarded when disturbance causes birds to move away from their sites.

After breeding, common murres leave the colonies and disperse across nearby shelf seas and more distant wintering areas. The movement is better described as broad marine dispersal than a single fixed migration route, and different breeding populations use very different parts of the North Atlantic and North Pacific.

Common murres breed in extremely dense colonies and do not build a conventional nest. A female lays a single large egg directly on bare rock or a very small patch of debris on a cliff ledge. Both parents incubate for about 32 days, standing almost shoulder-to-shoulder with neighbouring birds.

The egg is strongly pointed at one end. It has long been repeated that this shape makes the egg roll in a tight circle and therefore prevents it from falling from the cliff, but experimental work has found little support for that simple explanation. Other ideas include greater resistance to impacts and keeping the broader, more porous end of the egg cleaner in crowded colonies. The exact evolutionary reason for the shape is still debated.

The chick stays on the ledge for about 20–23 days and then leaves while still unable to fly. It jumps or scrambles down from the cliff towards the sea, usually after or alongside a parent. The male then accompanies the chick at sea and continues feeding it for several weeks, while the female normally remains associated with the colony for longer. This early departure is therefore part of an extended period of parental care rather than simple independence from the nest.

The common murre is classified as least concern on the IUCN Red List because of its very large population and broad northern distribution. Regional trends are much less uniform. Colonies can be strongly affected by changes in fish availability, oil pollution, entanglement and drowning in fishing gear, disturbance and unusually warm marine conditions.

Because the species produces only one egg in a breeding attempt and adults are long-lived, population recovery after major losses can be slow. Productive marine feeding areas are therefore as important to long-term population health as the breeding cliffs themselves.