A fairly common sea duck that feeds by diving. It nests on tundra lakes and spends the winter along coasts in bays, estuaries, and harbours. Inland records on freshwater are uncommon and usually occur during migration. It may gather in flocks of hundreds, though singles and small parties are also seen. The male is unmistakable, entirely black with a swollen knob at the base of the bill and a yellow wedge along the top. The female is dark brown with noticeably paler cheeks. In flight it appears uniformly dark and often travels in tight groups, skimming low or flying higher over the sea.