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The Common Loon or Great Northern Diver (Gavia immer) is a large member of the loon, or diver, family of birds. Breeding adults have a plumage that includes a broad black head and neck with a greenish, purplish, or bluish sheen, blackish or blackish-grey upper parts, and pure white underparts except some black on the under tail coverts and vent.

Non-breeding adults are brownish with a dark neck and head marked with dark grey-brown. Their upper parts are dark brownish-grey with an unclear pattern of squares on the shoulders, and the underparts, lower face, chin, and throat are whitish. The sexes look alike, though males are significantly heavier than females. During the breeding season, loons live on lakes and other waterways in Canada; the northern United States (including Alaska); and southern parts of Greenland and Iceland. Small numbers breed on Svalbard and sporadically elsewhere in Arctic Eurasia. Common loons winter on both coasts of the US as far south as Mexico, and on the Atlantic coast of Europe.

The European name "diver" comes from the bird's practice of catching fish by diving. The North American name "loon" was first recorded in this sense in New Englands Prospect (1634) by William Wood (1580-1639); "The Loone is an ill shap'd thing like a Cormorant". It may be derived from Old Norse lómr, as are modern Swedish and Danish lom, in each case referring to the distinctive call.

Adult non-breeding plumage is brownish with a dark neck and head marked with dark grey-brown. The eyes are surrounded with white, and the eyelids are pale. The bill is mostly pale grey, with a dark culmen and tip, but in early spring the tip may turn whitish. The underparts, lower face, chin, and throat are also whitish. The fore neck is whitish, usually forming wedge-shaped notch in dark neck-sides, and may sometimes reveal a shadowy trace of the neck ring or a pale collar.

The common loon is an expert fisher, catching its prey underwater by diving as deep as 200 ft. With its large webbed feet, the common loon is an efficient underwater pursuit predator and adroit diver. It needs a long run-up distance to gain momentum for flight take-off, and is ungainly on land, sliding on its belly and pushing itself forward with its legs.

The common loon produces a variety of vocalizations, the most common of which are the tremolo, the yodel, the wail, and the hoot. Each of these calls communicates a distinct message.The tremolo call—sometimes called the "laughing" call—is characterized by its short, wavering quality. It often uses this call to signal distress or alarm caused by territorial disputes or perceived threats. It emits a tremulous series of up to 10 rather high notes (hu)-heheheheheheha.

Common Loon

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