Skua

Skuas
Pomarine skua
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Charadriiformes
Family:	Stercorariidae Gray, 1871
Genus:	Stercorarius Brisson, 1760
Species
See text.

The skuas /ˈskjuːə/ are a group of seabirds with about seven species forming the family Stercorariidae and the genus Stercorarius. The three smaller skuas are called jaegers in the Americas.

The English word "skua" comes from the Faroese name skúgvur [ˈskɪkvʊər] for the great skua, with the island of Skúvoy renowned for its colony of that bird. The general Faroese term for skuas is kjógvi [ˈtʃɛkvə]. The word "jaeger" is derived from the German word Jäger, meaning "hunter".

Skuas nest on the ground in temperate and Arctic regions, and are long-distance migrants. They have even been sighted at the South Pole.^[1]

Biology and habits

Outside the breeding season, skuas take fish, offal, and carrion. Many are partial kleptoparasites, comprising up to 95% of the feeding methods of wintering birds, by chasing gulls, terns and other seabirds to steal their catches, regardless of the size of the species attacked (up to 3 times heavier than the attacking skua). The larger species, such as the great skua, also regularly kill and eat adult birds, such as puffins and gulls, and have been recorded as killing birds as large as a grey heron.^[2] On the breeding grounds, the three, more slender northern breeding species commonly eat lemmings. Those species that breed in the southern oceans, largely feed on fish that can be caught near their colonies. The eggs and young of other birds are an important food source for most skua species during the nesting season.^[3] In the southern oceans and Antarctica region, some skua species (especially the south polar skua) will readily scavenge the carcasses at breeding colonies of penguins and pinnipeds, sometimes taking live penguin chicks. In these areas, skuas seem to defer to the considerably larger giant petrels.^[4]

Two skuas and a giant petrel fighting over a dead Antarctic fur seal

They are medium to large birds, typically with grey or brown plumage, often with white markings on the wings.^[3] The skuas range in size from the long-tailed skua, Stercorarius longicauda, at 310 grams (0.68 pounds), to the brown skua, Stercorarius antarcticus, at 1.63 kg (3.6 lb). On average, a skua is about 56 cm (22 in) long, and 121 cm (48 in) across the wings. They have longish bills with a hooked tip, and webbed feet with sharp claws. They look like large dark gulls, but have a fleshy cere above the upper mandible. The skuas are strong, acrobatic fliers. They are generally aggressive in disposition. Potential predators who go near their nest will be quickly dived at by the parent bird, which usually targets the head of the intruder - a practice known as 'dive bombing'.^[5]

Taxonomy

Skuas are related to gulls, waders, auks, and skimmers. In the three smaller species, all nesting exclusively in the Holarctic, breeding adults have the two central tail feathers obviously elongated, and at least some adults have white on the underparts and pale yellow on the neck. These characteristics are not shared by the larger species, all native to the Southern Hemisphere except for the great skua. Therefore, the skuas are often split into two genera, with only the smaller species retained in Stercorarius, and the large species placed in Catharacta. However, based on genetics, behavior, and feather lice, the overall relationship among the species is best expressed by placing all in a single genus.^[6] The pomarine and great skuas' mitochondrial DNA (inherited from the mother) is in fact more closely related to each other than it is to either Arctic or long-tailed skuas, or to the Southern Hemisphere species.^[7] Thus, hybridization must have played a considerable role in the evolution of the diversity of Northern Hemisphere skuas.

Species

Skua species
Common and binomial names	Image	Description	Range
Long-tailed skua or long-tailed jaeger (Stercorarius longicaudus)
Parasitic skua or Arctic jaeger (Stercorarius parasiticus)
Pomarine skua or pomarine jaeger (Stercorarius pomarinus)
Chilean skua (Stercorarius chilensis)
South polar skua (Stercorarius maccormicki)
Brown skua (Stercorarius antarcticus)
Great skua (Stercorarius skua)

Skua in Antarctica

A skua nestling, with egg tooth present

References

1. Mark Sabbatini, "Non-human life form seen at Pole", The Antarctic Sun, 5 January 2003.
2. Scottish Ornithologists' Club
3. Harrison, Colin J.O. (1991). Forshaw, Joseph, ed. Encyclopaedia of Animals: Birds. London: Merehurst Press. p. 109. ISBN 1-85391-186-0.
4. http://carnivoraforum.com/index.cgi?board=interspecific&action=print&thread=390
5. "Scottish Wildlife Trust builds £50,000 loo on Handa". BBC News. 12 March 2012.
6. American Ornithologists' Union (2000). Forty-second supplement to the American Ornithologists' Union Check-list of North American Birds. The Auk 117(3):847–858.
7. Cohen, Baker, Belchschmidt, Dittmann, Furness, Gerwin, Helbig, de Korte, Marshall, Palma, Peter, Ramli, Siebold, Willcox, Wilson and Zink (1997). Enigmatic phylogeny of skuas. Proc. Biol. Sci. 264(1379):181–190.

• Harrison, Peter Seabirds: An Identification Guide ISBN 0-7470-1410-8

External links

• Skua videos on the Internet Bird Collection