Omnivorous and opportunistic. The Snowy Sheathbill will take any seasonally available food. For the summer, regurgitated krill (primarily euphausiid shrimp) obtained by direct interference with penguins feeding chicks, is the primary food, but penguin and cormorant eggs, excrement, and, to a lesser extent, young chicks are also taken (Jones 1963). Sheathbills pick tiny scraps of blubber and flesh off the skin and skeletons of carcasses discarded by larger predatory and scavenging birds; if larger birds are present, sheathbills stealthily dash in and tear at the carcass (Parmelee 1980). Sheathbills also forage on both carcasses of pups and the placentas of southern elephant seals (Mirounga leonina; Burger 1981, Favero 1996) and of Weddell seals (Leptonychotes weddelli; Jones 1963). They also consume, in smaller amounts, milk from nursing cow seals, and on seal blood and feces (Favero 1996). Sheathbills also have been observed attempting to devour the umbilical cord while it is still attached to the baby seals (Murphy 1936, Parmelee 1992). They also eat human refuse from refuse heaps (Shaw 1986).
During the spring and summer at Signy Island (South Orkney Islands), sheathbills forage along the intertidal areas, when limpets are important components of the diet (Jones 1963). From the same areas 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 (Jones 1963).
In the winter, the Snowy Sheathbill will remain far south only if there is a constant refuse heap at a research station (Watson 1975).
Sheathbills are described by Parmalee (1980) as "shifty birds ... forever fitting about near their food, as thought the feeding had to be consummated in bits and dabs with split-second timing. This behavior enables them to dart in and snip flesh from the open wound of a living fur seal that has been bitten by another in the battle for mates. The sheathbills often enlarge the wound until healing seems improbable."