Pomarine Jaeger
Stercorarius pomarinus
Bulky gull-like seabird; the largest and most barrel chested of the three jaegers. Breeds on Arctic tundra, spends the rest of the year at sea. Breeding adults are unique with elongated spoon-shaped central tail feathers. Also note blackish-brown upperparts, messy black cap that extends below the bill, and extensive white flash on upper and underside of outerwing. Rare dark morph adult is entirely blackish-brown but still has white wing flashes. Immature birds are variable, but in general, they are barred with brown above and have bold black-and-white bars on the underwing and undertail coverts. Difficult to separate from other jaegers, especially Parasitic. At all ages, note powerful direct flight and broad wings. Typically shows white shafts on six to eight outer wing feathers (more than other jaegers) and more obvious white flash on underwing. Bill is also thicker than other jaegers. All jaegers chase other birds and steal their food.
Identification
This species ranges from 46 to 67 cm (18 to 26 in) in length, 110 to 138 cm (43 to 54 in) in wingspan and 540–920 g (1.19–2.03 lb) in weight. The upper limit of the length includes the elongated tail streamer of breeding adults, which is about 10 cm (3.9 in) in length. Identification of this jaeger is complicated by its similarities to parasitic jaeger and the existence of three morphs. Pomarine jaegers are larger than common gulls. They are much bulkier, broader-winged and less falcon-like than the parasitic jaeger, but show the same wide range of plumage variation. The flight is more measured than that of the smaller species. It has many harsh chattering calls and others which sounds like which-yew.<p>Light-morph adult pomarine jaegers have a brown back, mainly white underparts and dark primary wing feathers with a white "flash". The head and neck are yellowish-white with a black cap. Dark morph adults are dark brown, and intermediate morph birds are dark with somewhat paler underparts, head and neck. All morphs have the white wing flash, which appears as a diagnostic double flash on the underwing. In breeding adults of all morphs, the two central tail feathers are much longer than the others, spoon-shaped, and twisted from the horizontal. Juveniles are even more problematic to identify, and are difficult to separate from parasitic jaegers at a distance on plumage alone.
Taxonomy
Its relationships are not fully resolved; its mitochondrial DNA is most similar to the great skua, but from morphology and behavior, it is closer to the lesser skuas (such as the parasitic jaeger). The most likely explanation is extensive hybridization between the great and one species of lesser skuas, which resulted in a hybrid population that eventually evolved into a distinct species, the pomarine jaeger; or alternatively between the pomarine and a species of Southern Hemisphere skua, with the great skua being the hybrid offspring, perhaps appearing as recently as the 15th century. Judging from characteristics of the skeleton and behavior, the former seems more likely, as the pomarine jaeger shares several similarities with the "Catharacta" skuas, while the great skua does not seem much different from its Southern Hemisphere relatives.<p>The mtDNA difference between the pomarine jaeger and the great skua is one of the smallest between any two vertebrate species yet analyzed, being less than the variation found between different individuals of widespread species. The apparent capability for hybridization has led to the abolition of the separate genus Catharacta for the Southern Hemisphere and great skuas.
- Scientific name
- Stercorarius pomarinus
- Authority
- (Temminck, 1815)
- Order
- Charadriiformes
- Family
- Stercorariidae
- Genus
- Stercorarius
Range and habitat
- Breeding range
- NA, PAL : Holarctic high Arctic tundra: erratically off sw Greenland, Svalbard (nesting unconfirmed; n of Norway), nw to ne Arctic Russia (including many high Arctic islands), high Arctic n Alaska and sw Alaska mainland, and Arctic Canada to Baffin I. (ne Canada)
- Non-breeding range
- to temperate to tropical coasts and offshore waters worldwide
- Migration
- Complete migrants
Behavior
This species breeds in the far north of Eurasia and North America. It nests on Arctic tundra and islands, laying 2–3 olive-brown eggs in grass lined depressions. Like other skuas, it will fly at the head of a human or other intruder approaching its nest. Although it cannot inflict serious damage, the experience is frightening and painful.<p>This bird feeds on fish, carrion, scraps, smaller birds up to the size of common gull and rodents, especially lemmings. It robs gulls, terns and even gannets of their catches. Like most other skua species, it continues this piratical behaviour throughout the year, showing great agility as it harasses its victims. Only the Great Black Backed Gull, White-Tailed Eagle and Golden Eagle are known to take adult, healthy pomarine jaeger.
Conservation
- Population trend
- stable
Photos
Distribution
Bird Calls
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