Sheathbills (Chionidae)

(Chionidae)

Class: Aves

Order: Charadriiformes

Suborder: Charadrii

Family: Chionidae

Thumbnail description
Medium-sized, chunky birds with dove-like or fowl-like body form, uniform white plumage, short legs, facial caruncles or warts, and compressed bill with characteristic horny sheath

Size
13.4–16.1 in (34–41 cm); 1.0–1.72 lb (450–760 g); wingspan 29.1–31.5 in (74–80 cm)

Number of genera, species
1 genus; 2 species

Habitat
Coastal plains, rocky and sandy intertidal zones, occasionally ice floes

Conservation status
Not threatened

Distribution
Antarctic Peninsula, subantarctic islands, mainland and adjacent islands of southern South America

Evolution and systematics

Sheathbills are a distinct and peculiar group of birds in terms of their morphology, behavior, and distribution. There is general agreement that sheathbills are members of the order Charadriiformes, a diverse group containing auks (Alcidae), gulls (Laridae), and waders or shorebirds (Charadrii). However, the evolutionary relationships among sheathbills and other charadriiform families remains unresolved and is a continued source of contention among taxonomists. Results from morphological, behavioral, and biochemical approaches are varied and remain ambiguous. The DNA-DNA hybridization studies of Sibley and Ahlquist, published in the 1990s, offered a potential breakthrough and classified sheathbills as a sister-group to the plover-like birds (Charadriidae) and thickknees (Burhinidae). However, these studies have received sufficient criticism that their validity is questioned. It is probable that genetic comparisons involving mitochondrial and nuclear DNA will ultimately resolve the issue.

Sheathbills occur in areas where fossil bird evidence is scarce and it is difficult to determine a precise estimate for their origin. They likely radiated in the Miocene epoch or later (over 30 million years ago) from an ancestor that colonized Antarctica from more temperate regions. The close association of sheathbills with seabird and seal colonies has undoubtedly been a major force in their evolution.

Sheathbills consist of a single genus with two recognized species, black-faced sheathbills (Chionis minor) and pale-faced sheathbills (C. alba). Their ranges do not overlap and they are further differentiated based on migratory behavior and physical characteristics. Within black-faced sheathbills there are four recognized subspecies based on geographical distribution on isolated archipelagos and islands.

Physical characteristics

Sheathbills are easily recognized by their stocky appearance, uniform white plumage, and characteristic sheathed bill. The bill is conical and the horny sheath, from which the species derives its name, covers a proximal segment of the upper bill. Adding to their distinct appearance is a bare portion of the face that is partially covered with wart-like caruncles. In pale-faced sheathbills, the sheath is greenish and the caruncles are pink, whereas all facial features are black in blackfaced sheathbills. Sheathbills are well adapted for life in a harsh South Pole environment and possess short, stout legs, an insulating coat of dense gray down, and a thick layer of subcutaneous fat. Indicative of their terrestrial habits, sheathbills are one of few Antarctic and subantarctic birds lacking webbed feet.

Sexes are similar in appearance but dimorphic in size; males are generally 15% heavier than females and have larger bills and sheaths. Adult birds can be differentiated from younger individuals based on bill sheath size and extent of facial caruncles. Older birds also develop carpal spurs that are used in aggressive encounters and tend to have deeper and harsher voices.

Distribution

Sheathbills occur at sites along the Antarctic Peninsula, at various archipelagos and isolated islands of the subantarctic, and as far north as the southern regions of South America. In part, distribution of these birds is restricted to sites with penguin and other seabird colonies, which sheathbills depend on as a source of food for much of the annual cycle.

The two sheathbill species do not overlap in range. Palefaced sheathbills are partially migratory and are found along the Antarctic peninsula, subantarctic islands of the Atlantic Ocean, and certain regions of southern South America. The four subspecies of black-faced sheathbills are sedentary birds with an allopatric distribution (mutually exclusive geographic areas) among archipelagos and isolated subantarctic islands of the Indian Ocean.

Habitat

Sheathbills are predominantly terrestrial birds that inhabit areas of coastal plains and adjacent intertidal zones. For much of the year sheathbills occur in seabird colonies and focus activities in these areas. An exception is some black-faced sheathbills of the Kerguelen Islands, where birds without access to seabird colonies spend more time in intertidal zones. Outside the breeding season, sheathbills frequent rocky and sandy intertidal zones, wet meadows and bogs as far as 0.6 mi (1 km) inland, and even ice floes.

Behavior

Sheathbill activities throughout the annual cycle vary depending on the occurrence of seabirds and seals to exploit. For example, some black-faced sheathbills at Marion Island breed in colonies of migratory crested penguins (Eudyptes). These sheathbills are forced to shift territories during the nonbreeding season to intertidal areas or colonies of king penguins (Aptenodytes patavonicus). In contrast, most pale-faced sheathbills are migratory and leave breeding grounds with other seabirds.

Feeding ecology and diet

Sheathbills are omnivores. Their diet includes a wide range of animal and plant materials. Their opportunistic foraging strategies are likely a response to the harsh and variable environments that they inhabit. Colonies of seabirds, particularly penguins, are the most prominent food source for sheathbills. At these colonies sheathbills are predators and scavengers that feed on eggs, chicks, and even excrement. Sheathbills also kleptoparasitize seabirds returning to colonies to provision chicks. They harass or jostle birds as adults regurgitate a bolus to their begging chicks and then scavenge food spilled during the transfer. Research indicates that this is an important food source for breeding sheathbills, but it has little overall impact on seabirds.

During spring, sheathbills also scavenge dead pups, placentas, and seal milk. In cases where seabirds and seals are absent, sheathbills consume large amounts of seaweed and various invertebrates. For nonmigratory populations, the intertidal zone is an important foraging area during winter.

Reproductive biology

Sheathbills are monogamous birds that maintain long-term pair bonds. In most cases, pairs defend nesting and feeding territories in seabird colonies. Territorial defense involves displays, vocalizations, chasing, and occasional physical battles.

Sheathbills first breed at three to five years of age and annual reproduction is usually synchronized with breeding activities of associated seabird colonies. This strategy provides for maximum food availability during the energetically costly breeding period. As a result, nesting dates of sheathbills are somewhat variable, with a clutch of one to three eggs laid in late November or December. Eggs are pear-shaped and white with flecks of gray or brown. Nest sites are located in small caves and cracks in rocky areas. In some cases, birds occupy abandoned burrows excavated by other species. Locations of nest sites provide shelter from winds and precipitation and cover from predatory skuas (Stercorariidae). Crude nests are constructed from feathers, pebbles, bones, shells, lichens, grasses, and seaweed.

Chicks hatch asynchronously after an incubation period of 28–32 days and are semi-precocial and nidiculous, partially covered with brown down. Both adults participate in parental care duties as young remain at of near nests for one to three weeks and then fledge approximately 50–60 days following hatching. Studies have found that survival rate is higher for the first chick hatched in a nest and overall nest success ranges from 1.1 to 1.9 fledglings per nest. Starvation is the primary cause of mortality in young sheathbills.

Conservation status

Neither sheathbill species is listed as threatened or endangered. External threats to population persistence are limited due to the remote and harsh areas occupied. Introduced nonnative vertebrates, such as feral cats and mice, prey on chicks and eggs and deplete invertebrate food sources.

Significance to humans

Sheathbills have little contact with humans. At research bases they will feed on discarded food scraps and human excrement, and abandoned stations are sometimes used as artificial nesting sites.

Species accounts

Resources

del Hoyo, J., A. Elliott, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 3, Hoatzin to Auks. Barcelona: Lynx Edicions, 1996.

Feduccia, A. The Origin and Evolution of Birds. New Haven: Yale University Press, 1996.

Parmelee, D.F. Antarctic Birds: Ecological and Behavioral Approaches. Minneapolis: University of Minnesota Press, 1992.

Bried, J., and P. Jouventin. "Morphological and Vocal Variation among Subspecies of the Black-Faced Sheathbill." Condor 99 (1997): 818–825.

Burger, A.E. "Time Budgets, Energy Needs and Kleptoparasitism in Breeding Lesser Sheathbills (Chionis minor)." Condor 83 (1981): 106–112.

Huyser, O., P.G. Ryan, and J. Cooper. "Changes in Population Size, Habitat Use and Breeding Biology of Lesser Sheathbills (Chionis minor) at Marion Island: Impacts of Cats, Mice, and Climate Change?" Biological Conservation 92 (2000): 299–310.

Jouventin, P.J., J. Bried, and E. Ausilio. "Life-History Variations of the Lesser Sheathbill Chionis minor in Contrasting Habitats." Ibis 128 (1996): 732–741.

[Article by: Peter Martin Sanzenbacher, MS]