Sheathbills - amazing Antarctic land birds
Most folks who visit the Antarctic focus only on the charismatic
critters like penguins, seals and whales but I am particularly
interested in sheathbills (Family Chionidae). This is the only bird
family which is endemic to the Antarctic and sub-Antarctic and
sheathbills are also the only land birds (lacking webbed feet and the
ability to feed at sea) to breed in the Antarctic and on many of the
sub-Antarctic islands.
There are two species of sheathbills:
The Black-faced Sheathbill (Chionis minor)
is found on four sub-Antarctic island groups in the southern Indian
Ocean: Marion and Prince Edward Island; the Crozet archipelago;
Keguelen; Heard Island. These populations are year-round residents on
these islands and do not migrate.
This is also known as the Lesser Sheathbill but that is a misnomer since they are about the same size as Chionis alba.
The Pink-faced Sheathbill (Chionis alba)
breeds on the Antarctic Peninsula and the islands of the South
Shetland, South Orkney, South Sandwich and South Georgia archipelagos,
and occurs as a non-breeding migrant on the coast of southern Patagonia
and on the Falkland/Malvinas islands.
This species has many other names - Snowy Sheathbill, Wattled
Sheathbill, American Sheathbill, Greater Sheathbill etc. - but
Pink-faced is the most accurate and descriptive and is now commonly
used.
Distribution map of the two sheathbill species:
Food and lifestyle of sheathbills
Because they are not seabirds, sheathbills have to be extremely opportunistic in finding food. Basically they will eat almost anything that is organic and will often try out the inorganic (ropes, Zodiacs, gumboots). Most of their food comes from penguins and, to a lesser extent, other seabirds and seals. In seabird colonies they are constantly on the lookout for unguarded eggs or small chicks which they grab and carry off. Much of their food also comes from scavenging bits of flesh off bird and seal carcasses - but being much smaller than thegiant petrels and skuas they usually just get the lean pickings off carcasses. They also eat intertidal algae and invertebrates (limpets, polychaete worms) and they can often be seen scraping algae off the rocks with their beaks. Black-faced Sheathbills also eat large quantities of terrestrial invertebrates (earthworms, insect larvae, beetles) and these foods are particularly important in the winter when there are few seabirds remaining on the islands.
Although they are food generalists, the one thing that both species of sheathbills specialize in is kleptoparasitism, i.e., robbing other animals of their food. Penguins that are feeding chicks are their main targets. The sheathbills will stand near a penguin that has come in from the sea with a belly full of food to deliver to its chick. As the penguin regurgitates the food bolus to the chick, the sheathbill will attack. It leaps against the penguins, fluttering it wings, and often thumps against the penguin chick with enough force to topple the chick over. With any luck, some of the food which the penguin was regurgitating will spill on the ground and the sheathbill will then scoop this up - either to eat or to carry to its nest to feed its own chick. In a study I did years ago on sub-Antarctic Marion Island, Black-faced Sheathbills living in colonies of Rockhopper Penguins obtained over 90% of the food delivered to its chicks by robbing the neighbouring penguins. Throughout their range, kleptoparasitism coupled with opportunistic scavenging is the key to sheathbills' survival in some very hostile environments.
Here a Black-faced Sheathbill is attacking a Rockhopper Penguin as it feeds its chick on sub-Antarctic Marion Island.
And Pink-faced Sheathbills doing the same thing with Adelie Penguins on Paulet Island in the Antarctic.
Other seabirds are occasionally kleptoparasitized too. On Marion Island one pair of Black-faced Sheathbills had a nest among nesting Sooty Albatrosses. Here is one of these sheathbills taking food from an albatross as it feeds its chick. If you look closely on the photo to the left you can see the sheathbills own chick standing at the entrance to its nest cavity, just below the albatross nests. Whenever possible sheathbills will nest right among their hosts.
This one is stealing my lunch! ........................................ and another is trying to eat my gumboot.
Publications on sheathbills
I did my PhD on Black-faced Sheathbills based on work at Marion Island in the sub-Antarctic. Here are papers published from that work:Burger, A.E. 1979. Breeding biology, moult and survival of Lesser Sheathbills Chionis minor at Marion Island. Ardea 67:1-14.[pdf version]
Burger, A.E. 1980. Sexual size dimorphism and aging characters of Lesser Sheathbills at Marion Island. Ostrich 51: 39-43.
Burger, A.E. 1980. An analysis of the displays of Lesser Sheathbills. Zeitschrift Tierpsychologie 52: 381-96.
Burger, A.E. and Millar, R.P. 1980. Seasonal changes in sexual and territorial behaviour and plasma testosterone levels in male Lesser Sheathbills Chionis minor. Zeitschrift Tierpsychologie 52: 397-406.
Burger, A.E. 1981. Food and foraging behaviour of Lesser Sheathbills Chionis minor. Ardea 69: 167-180. [pdf version]
Burger, A.E. 1981. Time budgets, energy needs and kleptoparasitism in breeding Lesser Sheathbills Chionis minor. Condor 83:106-112. (pdf version)
Burger, A.E. 1982. Foraging behaviour of Lesser Sheathbills Chionis minor exploiting invertebrates on a sub-Antarctic island. Oecologia 52: 236-245.(pdf version)
Burger, A.E. 1984. Winter territoriality in Lesser Sheathbills on breeding grounds at Marion Island. Wilson Bulletin 96:20-33.(pdf version)
Burger, A.E. 1985. Terrestrial food webs in the sub-Antarctic: island effects. Pp 582-591 in: Antarctic nutrient cycles and food webs (W.R. Siegfried, P.R. Condy and R.M. Laws, eds.). Springer-Verlag, Berlin & Heidelberg.
Burger, A. E. 1996. Chionidae. Pp. 546-555 in Handbook of the birds of the world, Vol. 3, Hoatzin to Auks (J. del Hoyo, A. Elliott and J. Sargatal, eds.). Lynx Edicions, Barcelona, Spain.