The Great Northern Diver, known in North America as the Common Loon (Gavia immer), is a large member of the loon, or diver, family. Adults are typically 73-88cm (28-36in) in length with a 122-148cm wingspan, slightly smaller than the similar White-billed Diver. Common Loons weigh between 2.7 and 6.3 kg with a mean value around 4.1 kg.

It breeds in Canada, parts of the northern United States, Greenland, and Alaska. There is a smaller population (ca. 3000 pairs) in Iceland. The female lays 2 eggs on a mound of vegetation very close to water. Both parents build the nest and feed the young.

It winters on sea coasts or on large lakes over a much wider range in Northern Europe and the British Isles as well as in North America.

Distinguishing identifiers: The only diver with head all black and a white necklace; black bill distinguishes it from the White-billed Diver.

Breeding adults have a black head, white underparts, and a chequered black-and-white mantle. Non-breeding plumage is drabber, with the chin and foreneck white. Its bill is grey or whitish and held horizontally.

It flies with its neck outstretched.

This species, like all divers, is a specialist fish-eater, catching its prey underwater, diving as deep as 200 feet. Freshwater diets consist of pike, perch, sunfish, trout and bass; salt water diets consist of rock cod, flounders, sea trout and herring.

The tremolo call, sometimes referred to as loon laughter, is an eerie wailing, a symbol of the Canadian wilderness, and often used as atmosphere in horror films.

Head of a Great Northern Diver

This diver is well-known in Canada, appearing on the "loonie" coin, and is also the provincial bird of Ontario and the state bird of Minnesota. Native tribes of British Columbia believed that an excess of calls from this bird predicted rain, and even brought it.

Folk names include big loon, black-billed loon, call-up-a-storm, ember-goose, greenhead, guinea duck, imber diver, ring-necked loon, and walloon.

These birds have disappeared from some lakes in eastern North America due to the effects of acid rain and pollution. Artificial floating nesting platforms have been provided for loons in some lakes to reduce the impact of changing water levels due to dams and other human activities.

Gavia is Latin for "sea smew" (although divers are not Smew). The specific meaning of immer either is:

1. related to Swedish immer and emmer, the gray or blackened ashes of a fire, referring to its dark plumage; or
2. Latin immergo, to immerse, and immersus, submerged.

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• Loons