Tiny waterbird, the size of an elongated grapefruit. In nonbreeding plumage, they are black above and white below with a black stripe on the side of the neck and a white comma behind eye. Short, stout bill. Horizontal posture like a football floating on the water. Dives frequently when feeding. In winter, only likely to be seen from land following the passage of a storm, otherwise stays well offshore. Found singly or in flocks. Much smaller than guillemots or puffins, with fast wingbeats and low, twisting flight that recalls a flock of sandpipers. Breeds in large numbers in Greenland and Europe; rare at alcid breeding colonies in Alaska. Birds in breeding plumage have an all-black head and chest.