Stercorariidae: Skuas
Skuas are gull-like seabirds that specialize in piracy, pursuing and attacking weaker seabirds to steal their prey. Most species also persecute other birds in the breeding season, raiding their nests for eggs and chicks. Many skuas are easily mistaken for immature gulls, nearly identical in their general appearance, but more powerfully built, and armed with stronger, sharper claws and bills. Despite their close resemblance to gulls, skuas are more closely related to alcids (i.e., Alcidae, which includes puffins).
The family is composed of two distinct groups: the large, barrel-chested, short-tailed, mostly brownish skuas, and the comparatively slim, long-tailed, often-bicolored jaegers. The jaegers breed on arctic tundra and the skuas breed along seacoasts of northern Europe and the Southern Ocean. Both groups are largely pelagic outside the breeding season.
Taxonomy
The internal relationships of the Stercorariidae are fairly well-understood. The Pomarine Jaeger (Stercorarius pomarinus) is the central figure in the family tree: according to genetic analyses, it is more closely related to the large brown skuas than it is to the two more similar-looking jaeger species.
The large brown skuas have often been assigned to a separate genus, Catharacta, and have been classified as anywhere from one to seven species. Considering their close relationship to the Pomarine Jaeger, a single, combined genus to include them all seems more appropriate.
The most difficult taxonomic question remaining for the Stercorariidae is the appropriate status of the four forms of Brown Skua (Stercorarius antarcticus) either as subspecies or as somewhere between two and four species. Depending on the resolution of that question, the family comprises somewhere from 7 to 10 species.
Parasitic Jaeger (Stercorarius parasiticus)
Long-tailed Jaeger (Stercorarius longicaudus)
Pomarine Jaeger (Stercorarius pomarinus)
Great Skua (Stercorarius skua)
South Polar Skua (Stercorarius maccormicki)
Chilean Skua (Stercorarius chilensis)
Brown Skua (Stercorarius antarcticus)
“Chatham Skua” (S. a. ssp. nova)
“Subtropical Skua” (S. a. hamiltoni)
“Falkland Skua” (S. a. antarcticus)
“Subantarctic Skua” (S. a. lonnbergi)
References
Boyd, J.H., 2019. Taxonomy in Flux: Gruae II: Charadriiformes. http://jboyd.net/Taxo/List9a.html. (Posted August 2, 2019. Accessed May 9, 2021.)
Harrison, P. 1983. Seabirds: An Identification Guide. Houghton Mifflin, Boston.
Howell, S.N.G., and K. Zufelt. 2019. Oceanic Birds of the World. Princeton University Press.
Roberson, D. 2004. Bird Families of the World: Skuas & Jaegers, Stercorariidae, http://creagrus.home.montereybay.com/skuas.html. (Posted October 31, 2004. Accessed May 9, 2021.)