Sheathbills vary in size from 13.4 to 16.1 inches (34 to 41 centimeters) in length and from 16 to 27 ounces (450 to 760 grams) in weight. They have plump bodies, short legs, and short conical, cone-shaped, bills with a horny sheath covering the upper bill. Sheathbills also have unusual featherless patches on the face covered with wart-like bumps called caruncles (KAR-un-kulz). The rest of the sheathbill body is covered with white feathers. Pale-faced sheathbills have a greenish sheath on the bills and pink caruncles, whereas black-faced sheathbills have black bill sheaths and black caruncles. Males are larger than females in the sheathbills, generally weighing about 15 percent more. Males also have larger bills and larger sheaths. Adult birds have more caruncles on their faces than younger birds. Older birds also have spurs on their wings which they use for defense and in fights.
Sheathbills are found along the Antarctic Peninsula, on islands of the subantarctic, and in the southern parts of South America.
Sheathbills are found primarily on coastal plains and in coastal wetlands. They are generally found in the vicinity of large seabird colonies. During the nonbreeding season, sheathbills may be found in meadows, bogs, and ice floes, sometimes as far as 0.6 miles (1 kilometer) inland.
Sheathbills are omnivores, they eat substantial amounts of both plant and animal matter. Because they live in harsh environments where conditions often change quickly, sheathbills are opportunistic feeders that are able to take advantage of whatever food becomes available. In most areas, sheathbills feed largely on the eggs, chicks, and even excrement of penguins and other seabirds. They also try to intercept adult seabirds returning to feed their chicks and either steal the food outright, or jostle the adults enough that some of the food is spilled. Food obtained in this way is critical to sheathbill survival, but does not have a very large impact on the seabirds. Sheathbills will also eat carrion, dead and decaying flesh, usually dead seal pups, as well as seal placentas, the organ that attaches to the uterus during pregnancy. When foods derived from seabirds and seals are unavailable, sheathbills survive by eating seaweed and invertebrates.
BEHAVIOR AND REPRODUCTION
Sheathbills are monogamous (muh-NAH-guh-mus), a single male mates with a single female during the breeding season. Sheathbill pairs are territorial, and defend nesting and feeding areas within seabird colonies from other pairs of sheathbills. Territorial disputes are resolved by calling, displays, chases, and sometimes, actual battles.
The timing of the breeding season varies among populations of sheathbills, since breeding usually occurs whenever local seabirds are breeding. This strategy allows for plentiful food resources to be available when sheathbill chicks hatch. Females lay one to three eggs at a time, usually in November or December. Eggs are laid in crude nests built from feathers, bones, shells, rocks, and plant material. Nests are usually built in small caves or cracks, usually in rocky areas. However, some sheathbills will nest in the abandoned burrows of other species. Sheathbill eggs are white, flecked with brown or gray, and somewhat pearshaped. Chicks hatch after twenty-eight to thirty-two days, and are partially covered with feathers when they hatch. Unlike the parents, chicks are brownish in color. Chicks stay near the nest for one to three weeks after hatching and are fed by their parents. The primary cause of death for chicks is starvation, although some chicks are also eaten by predators, animals that hunt them for food.
SHEATHBILLS AND PEOPLE
Because of their range, sheathbills have little contact with people. However, near Antarctic research stations, they have been known to eat food scraps and human excrement. They sometimes also nest in abandoned stations.
Neither of the two species of sheathbills is currently considered threatened. However, some predators brought by humans, including cats and mice, sometimes eat sheathbill chicks or eggs.