Larger than a duck, the adults have black plumage which shows iridescent green and violet tones. It occurs on reservoirs and the larger slow-flowing rivers. They feed on fish, captured whilst diving and they can remain underwater for long periods. Lacking an uropygial gland (used by birds for waterproofing feathers), they are often seen perched on branches or on the bank with wings outstretched to dry their plumage.