Foraging behavior of the Snowy Sheathbill is discussed under Food.
At an artificially concentrated supply of food, wintering Snowy Sheathbills at Signy Islan, South Orkey Islands, formed a dominance hierarchy, in which birds with larger bills were dominant over smaller-billed individuals (Shaw 1986). In the Black-faced Sheathbill (Chionus niger), bill size increases with age, and is greater in males than in females (Burger 1980). 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 behaviour" (Shaw 1986).
Because the Snowy Sheathbill is smaller than other predators and scavengers in the Antarctic region (such as giant-petrels Macronectes, skuas Stercorarius, and Kelp Gulls Larus dominicanus), 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.
Wingbeats of the Snowy Sheathbill are strong and rapid, and the tail is spread in flight (Watson 1975). On the ground sheathbill typically walk (often rather fast and restlessly) across open ground with pigeon-like bobbing of head (Watson 1975).
It is very tame, on occasion even permitting capture by hand or short-handled nets (Watson 1975). 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 (Murphy 1936).
During the summer sheathbills bathe frequently at the shallow edges of cold coves. They are averse to deep water, and test it mincingly before feeling quite content. However, they can swim well when necessary, despite the fact that their feet are unwebbed (Murphy 1936).
In winter, Snowy Sheathbills react to low temperature in several different ways. At the South Orkneys 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 (Murphy 1936). They also tuck up their feet alternately among the feathers of the belly, and hop about one-leggedly, even while feeding. Sheathbills have been observed going about their usual activities when the thermometer was at -42.2º C. It was determined that the body temperature for these birds at the time was +40.2º C.
Sheathbills are easy to catch by hand; their immediate response to capture is to defecate (Murphy 1936).
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 Adelie (Pygoscelis adeliae) and Chinstrap (Pygoscelis antarctica) penguins (Forster 1996). On Signy Island, South Orkney Islands, the territory "is often quite small" if located within a penguin colony, but otherwise it may be 200-2000 m2 (Jones 1963). Both sexes of sheathbills will aggressively will defend their breeding territory, though the male is more active and aggressive. Intrudors are warned off by loud calls and a "Forward" threat posture that is similar to that of gulls. If this does not work, the sheathbill chases the intruder off by running, flying, or a flapping run (Forster 1996).
Socially Monogamous (Parmelee 1992).
Incubating 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. Sheathbills that are reluctant to leave the nest strike out with their bills menacingly (Parmelee 1992).
Paired individuals frequently bow to each other while uttering sharp (Parmelee 1992), rapid (Forster 1996), staccato calls before entering the nest cavity, or during other activities (Parmelee 1992). This display is thought to maintain the pair-bond, and is not agonistic. It was called the "bowing ceremony" by Jones (1963) (Parmelee 1992). It is used as a greeting display or given in response to intrudors or other disturbances (Forster 1996).
"Chases" are frequent and occasionally ended in fierce combat, especially between males in the presence of a mate (Parmelee 1992).
Sheathbills show strong fidelity to their net sites, territories, and previous mates. Divorce is rare unless there is unsuccessful breeding (Forster 1996). Both sexes defend the territory , but males are more active and aggressive (Forster 1996).
Social and interspecific behavior
The Snowy Sheathbill is both sociable and 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" (Murphy 1936).
They are fearless and impudent toward other animals. They walk about on the backs of 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 seal carcass" (Murphy 1936).
Little information. On Signy Island, South Orkney Islands, Brown Skuas (Stercorarius antarcticus) were observed diving at sheathbill chicks, but no predation events were witnessed (Jones 1963). In fact, other sheathbills were suspected of taking eggs from poorly-guarded sheathbill nests (Jones 1963).