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Gregory R. Mann, Ph.D. {ret.}

Snowy Sheathbill

“Chionis albus”

Omnivorous and opportunistic, the Snowy Sheathbill will take any seasonally available food. For the summer, regurgitated krill obtained by direct interference with penguins feeding chicks is the primary food but penguin & cormorant eggs, excrement and to a lesser extent, young chicks are also taken. Snowy Sheathbills pick tiny scraps of blubber and flesh off the skin & skeletons of carcasses discarded by larger predatory and scavenging birds; if larger birds are present, Snowy Sheathbills stealthily dash in and tear at the carcass. Snowy Sheathbills also forage on both carcasses of pups and the placentas of Southern Elephant Seals & Weddell Seals. They also consume in smaller amounts, milk from nursing seals and on seal blood & feces. Snowy Sheathbills also have been observed attempting to devour the umbilical cord while it is still attached to the baby seals. They also eat human refuse from refuse heaps. During the spring & summer at the South Orkney Islands, Snowy Sheathbills forage along the inter-tidal areas, when limpets are important components of the diet. From the same areas, Snowy Sheathbills also forage on algae; the algae most frequently consumed is a green-filamentous species and is an important part of the diet in late summer and in fall. In the winter, the Snowy Sheathbill will remain far south only if there is a constant refuse heap at a research station. Snowy Sheathbills are described by as “shifty birds”, forever fitting about near their food as thought the feeding had to be consummated in bits & dabs with split-second timing. This behavior enables them to dart in and snip flesh from the open wound of a living pinniped that has been bitten by another in the battle for mates. The Snowy Sheathbills often enlarge the wound until healing seems improbable.

At an artificially concentrated supply of food, wintering Snowy Sheathbills at the South Orkey Islands formed a dominance hierarchy in which birds with larger bills were dominant over smaller-billed individuals. The population at Signy Island included many known-age, older birds with large bills but none of the individuals in the study were sexed; therefore, it was not possible to separate the effects of age and sex on dominance behavior. Because the Snowy Sheathbill is smaller than other predators & scavengers in the Antarctic region such as Giant Petrels, Skuas & Kelp Gulls, they are limited to eating the smallest chicks of penguins and cormorants. However, their small size also lets them move freely among breeding penguins than larger birds, helping them to avoid antagonizing the penguins. Unguarded eggs and small chicks are easily taken by these skilled thieves. Wing-beats of the Snowy Sheathbill are strong & rapid and the tail is spread in flight. On the ground, Snowy Sheathbills typically walk often rather fast and restlessly across open ground with pigeon-like bobbing of head. It is very tame, on occasion even permitting capture by hand or short-handled nets. The natural response of these birds is often simply to gather around anyone who takes them by surprise, rather than to be startled and to fly away. If a stone is thrown at them, they sometimes merely look at it as though amused and watch it roll. During the summer, Snowy Sheathbills bathe frequently at the shallow edges of cold coves. They are averse to deep water and test it convincingly before feeling quite content. However, they can swim well when necessary despite the fact that their feet are unwebbed. In winter, Snowy Sheathbills react to low-temperature in several different ways. At the South Orkney Islands, they hide in natural caverns of the rocks up on the slopes of the hills for the duration of the short spells of extreme cold to which these islands are subjected. They also puff out their feathers during the cold to increase their insulation, causing themselves to look like white balls of fluff. They are practically blubber covered like a seal and the layer of the fat over the abdomen is a centimeter or more in thickness. They also tuck up their feet alternately among the feathers of the belly and hop about one-legged, even while feeding. Snowy Sheathbills have been observed going about their usual activities when the thermometer was at -42.2 Cº. It was determined that the body temperature for these seabirds at the time was +40.2 Cº. Snowy Sheathbills are easy to catch by hand; their immediate response to capture is to defecate.

Breeding adults establish strongly defended territories. The majority of these foraging territories include parts of a penguin or cormorant colony. The nest is placed within the feeding territory if there is suitable terrain. The preferred hosts are Adélie & Chinstrap Penguins. On the South Orkney Islands, the territory is often quite small if located within a penguin colony, but otherwise it may be 200-2,000 meters. Both sexes of Snowy Sheathbills will aggressively will defend their breeding territory, though the male is more active and aggressive. Intruders are warned off by loud calls and a “forward” threat posture that is similar to that of gulls. If this does not work, the Snowy Sheathbill chases the intruder off by running, flying or a flapping run. Incubating Snowy Sheathbills usually run from their nests when approached by humans, but then scamper close by while vocalizing in a highly agitated manner before quickly returning to their eggs or chicks at the first opportunity. Snowy Sheathbills that are reluctant to leave the nest, strike out with their bills menacingly. Paired individuals frequently bow to each other while uttering sharp, rapid, staccato calls before entering the nest cavity or during other activities. This display is thought to maintain the pair-bond and is not antagonistic. It was called the “bowing ceremony”. It is used as a greeting display or given in response to intruders or other disturbances. “Chases” are frequent and occasionally ended in fierce combat, especially between males in the presence of a mate. Snowy Sheathbills show strong fidelity to their net sites, territories and previous mates. Both sexes defend the territory, but males are more active and aggressive. The Snowy Sheathbill is both sociable & quarrelsome among themselves. Sometimes they seem to enjoy each others company and they will play together by chasing one another around a rock and back again. Upon the first sign of competition in feeding however, they “stand up to each other and tilt or stretch up on tiptoe, open their greenish bills and utter angry cries”. They are fearless & impudent toward other animals. They walk about on the backs of Southern Elephants Seals without much response. The one creature which they evidently stand in awe is the Skua and with this bird alone, they avoid rivalry around a pinniped carcass. On the South Orkney Islands, Skuas were observed diving at Snowy Sheathbill chicks, but no predation events were witnessed. In fact, other Sheathbills were suspected of taking eggs from poorly-guarded Snowy Sheathbill nests. The timing of breeding is closely tied to the penguins and other seabirds which the Snowy Sheathbills depend on. Sheathbills begin to occupy their territories in November on the South Orkney Islands. Ovogenesis occurs when eggs are available from penguins and when the Snowy Sheathbill eggs hatch the penguins are feeding their chicks. The nests of the Snowy Sheathbill are cups with a diameter of 22-25 centimeters and are on top of untidy, smelly piles of tussock grass, moss, algae, feathers or old bones. Snowy Sheathbills prefer to nest in crevices between rocks or in cavities below rocks but may use other sites, such as in Giant Petrel burrows away from view. Both sexes participate in nest construction, although the male is more active in collecting nest material and the female in arranging the nest material.

Snowy Sheathbills lay their eggs in December. The clutch size is usually two or three eggs, but ranges from 1 to 4. The eggs are pyriform or pear-shaped, with a creamy-white base color speckled with gray or brown. The eggs are laid at intervals of 1-4 days. Full incubation begins after the clutch is complete and lasts 26-30 days or 28-32 days. Both sexes incubate.  The survival rate of the eggs ranges from 60-84% and failure can be caused by infertility, predation by Skuas or other Snowy Sheathbills, flooding of nests, accidental breakage or eggs rolling out of nests. The hatching is asynchronous. First-hatched chicks therefore have a better chance of survival than younger ones and are always heavier. Most Sheathbills successfully raise 1 chick per season and the type of penguin exploited does not affect breeding success. When the chicks hatch, they are semi-precocial and nidicolous with dense brown down. This down is replaced by mottled gray mesoptile down after 1 to 2 weeks. White contour plumage appears after day 12 and by day 50, covers the chick. Chicks can move in the nest within a few hours of hatching, but they are still brooded by the parent continuously for the first 2 weeks and less after that.  A month after hatching, the chicks wander away from the nest and 55-60 days later they forage on their own, but still follow their parents for food for up to 6 months. The parents deliver food to the nest, but they do not regurgitate it. Males & females contribute a similar amount of time to brooding and feeding chicks. The age at first breeding is 3 to 5 years, but 3 year old Snowy Sheathbills generally are unsuccessful.

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