Any of about 20 species (family Hydrobatidae) of petrels that vary from 5 to 10 in. (13–25 cm) long. All are dark gray or brown, sometimes lighter below, often with a white rump. The relatively short wings are rounded at the tips. The toes are webbed, except for the small hind toe; the tail is square, forked, or wedge-shaped. Most species breeding in southern oceans walk over the water with wings spread, picking up minute marine organisms. Most northern species swoop over the water like tiny terns, occasionally alighting on the surface.