sheathbill, (family Chionididae), either of two species of white stout-billed Antarctic shorebirds making up genus Chionis (order Charadriiformes), the only bird family confined to south polar regions. It is named for the rough, horny sheath around the base of its bill shielding its nostrils. The short, stout bill has pimply skin at the base; the eyes are pink rimmed; and the short, thick legs and unwebbed feet are blue gray. Sheathbills combine the habits of gulls and shorebirds, to which they are related. Living around the borders of the Antarctic, they are the only birds there without webbed feet.
Sheathbills may be seen at sea hundreds of miles from land but are usually encountered in small parties along shore. When not persecuted, they become tame and inquisitive, walking among expedition camps like chickens. The sheathbill is an aggressive predator on the eggs and the young of penguins, petrels, and terns; it also scavenges the feces and afterbirths of seals and the offal around whaling stations. Outside the breeding season, sheathbills eat intertidal creatures and algae. Sociable, though sometimes quarrelsome, birds, they bow to one another in courtship and—when quarreling—fight with sharp shin spurs. Two or three off-white eggs are laid in December in an untidy nest of litter hidden in a rock crevice. Usually only one chick survives. The young take up to nine weeks to fledge. The pure-white snowy sheathbill (C. alba), 40 cm (16 inches) long, has a yellow bill. The lesser sheathbill (C. minor) is black-billed and is about 38 cm (15 inches) long.