Stercorario mezzano

Identificazione

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.

Tassonomia

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.

Nome scientifico

Stercorarius pomarinus

Autorità

(Temminck, 1815)

Ordine

Caradriformi

Famiglia

Stercoraridi

Genere

Stercorarius

Areale e 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.

Areale riproduttivo

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)

Areale non riproduttivo

to temperate to tropical coasts and offshore waters worldwide

Migrazione

Migratori completi

Comportamento

Comportamento migratorio: Migratori completi.

Conservazione

Tendenza della popolazione: stabile.

Tendenza della popolazione

stabile