Common in wetlands from marshy and reed-fringed lakes to open reservoirs; locally on the sea in winter, especially when lakes freeze. Often in flocks in winter; feeds mainly by diving, but often spends much of day sleeping. Boldly patterned male distinctive: black overall with white flanks, floppy crest. Female dark brown overall with short crest, paler flanks, staring golden-yellow eyes, and often a messy white patch at base of bill. In flight, both sexes show bold white wing stripe.