The Common Loon (Gavia immer) or Great Northern Diver, is a large member of the loon family with adults averaging 28–36 inches in length and a 48-58 inch wingspan.

Breeding adults of the species have a black head, white underparts and a checkered black-and-white mantle. Plumage for the non-breeding season is much drabber, with a white chin and fore neck. The bill is grey or whitish and held horizontally (Bill color and angle distinguish this species from the similar White-billed Diver.).