Skuas & Jaegers
Genus Stercorarius
A small group of parasitic birds of the gull family. Some of them are very migratory and can be found at both sides of the Equator.
The Skuas are a group of large, mostly dark brown birds, with a pale morph in only one species. The Jaegers are the smaller representatives of this group with elongated central tail feathers and different color morphs. All belong to the single genus Stercorarius.
Skuas
five species:
- Brown Skua Stercorarius lonnbergi
- South Polar Skua Stercorarius maccormicki
- Antarctic or Southern Skua Stercorarius antarcticus, two subspecies:
- S. a. antarcticus
- S. a. hamiltoni
- Chilean Skua Stercoraria chilensis
- Great Skua Stercorarius skua
The five species of skuas are more or less of the same size and appearance. The South Polar Skua has three color morphs and might show a lighter colored bill in most birds of the dark morph and blue in young birds of the pale morph. Other great skuas have dark brown bills. The Great Skua sometimes also has a bill with a pale base.
Great Skua skulls develop, like in some alcids, slowly a supraorbital ridge. See Skuas: skull and ageing
Jaegers
Three species:
- Pomarine Jaeger/Skua Stercorarius pomarinus,
- Parasitic Jaeger or Arctic Skua Stercorarius parasiticus,
- Long-tailed Jaeger/Skua Stercorarius longicaudus. Two subspecies:
- S. a. longicaudus, breeds West Palearctic to Siberia
- S. a. pallescens, breeds Greenland, northern North America and eastern Siberia
The three Jaeger species differ from the Skua by their size and by their elongated central tail feathers. The Pomarine Jaeger is the largest with the shortest central tail feathers, the Parasitic sits in the middle and the Long-tailed is the smallest with the longest tail feathers. Pomarine and Parasitic Jaeger are both polymorphic and sometime difficult to distinguish. All three species may have a dark tipped pale bill, depending on age or morph.
Evolution and taxonomy
The taxonomy and the evolutionary relationships between the two different groups of Skuas and Jaegers is complex. After the separation from the Gulls, about 10 My ago, the group diverged into two different types: the polymorphic Jaegers in the Northern Hemisphere with elongated tail feathers and a more colourful plumage and the large predominantly brownish Skuas which colonised the Southern Hemisphere. Fossil evidence suggests that the group is of northern origin.
There is much debate about whether the Skuas and Jaegers belong to one single genus Stercorarius or to two separate genera Stercorarius for the smaller species and Catharacta for the large Skuas.
The debate concerns the relation between pomarinus and skua. The Pomarine Jaeger shares some genetic and behavioural characterisitics with skua, but is more like the Jaegers in plumages. Andersson (1999) suggested that pomarinus is a sister species of skua or that early hybridization may explain the genetic similarties between the species. (Furness 1987, Olsen & Larsson 1997, Anderson 1999)
Distribution
The majority of the large Skuas are birds from the Southern Hemisphere, but some move to the North outside the breeding season and som individuals of the South Polar Skua, normally occurring along the coasts of Antarctica, the Antarctic peninsula and on the South Shetlands, may wander even up to Alaska or Greenlandand northern Indian Ocean. The Brown Skua ranges circumpolar on the Southern Hemisphere and wanders up to the tropics. The Chilean Skua is restricted to the East and West coasts of South America and moves up to the tropics. The Antarctic Skua also has a circumpolar distribution. One subspecies (hamiltoni) and occurs in the Tristan da Cunha Archipelago, New Zealand, South America and Antarctica, the othe is restricted to coastal Argentina and Terra del Fuego, Falklands. The Great Skua is the only northern representative ans lives in the North Atlantic down to north-west Africa and USA east coast.
All Jaegers have a circumpolar range in the Arctic region. Pomarine Jaegers move southward in all three oceans during the winter. The Parasitic Jaeger moves further south to South America, South Africa and Australasia. The Longtailed winters in the Pacific and Atlantic, more pelagic than the first two species.
Behaviour
All species are notorious kleptoparasitic. They pursue other seabirds until these give or throw up their prey. As a result of this habit all of them are strong flyers that can easily outrace similar sized or even much larger birds. The large Skuas predate ferociously on eggs, chicks and even adults of other seabird species.
South Polar Skua Stercorarius maccormicki
Total length: 119.2 mm, unsexed adult. 1)
Great Skua Catharacta skua. The Netherlands.
Culmen: 52.8 mm, total: 112.2 mm; unsexed adult 2)
Pomarine Jaeger Stercorarius pomarinus. Kornwerderzand, The Netherlands.
Culmen: 37.5 mm, total: 90.0 mm, 1st CY female
Parasitic Jaeger Stercorarius parasiticus. The Netherlands
Culmen: 28.4 mm, total: 75.1 mm, unsexed adult
1) Courtesy of Jacob Gonzalis Solis, Univ. of Barcelona, Spain
2) Courtesy of Kees Camphuysen, NIOZ (Royal Netherlands Institute for Sea Research)