The mtDNA difference between the Pomarine 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 wide-spread species. The apparent capability for hybridization has led to the abolition of the separate genus Catharacta for the Southern Hemisphere and Great Skuas.
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. It is a migrant, wintering at sea in the tropical oceans. It has many harsh chattering calls and others which sounds like which-yew.
This bird feeds on lemmings and other rodents on the breeding grounds and also robs gulls, terns and even Gannets of their catches; it will also kill birds up to the size of Common Gull. Like most other skua species, it continues this piratical behaviour throughout the year, showing great agility as it harasses its victims.
Identification of this skua is complicated by its similarities to Arctic Skua and the existence of three colour phases. Pomarine Skuas are larger than Common Gulls. They are much bulkier, broader-winged and less falcon-like than Arctic Skua, but show the same wide range of plumage variation. The flight is more measured than that of the smaller species.
Light-phase adult Pomarine Skuas 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-phase adults are dark brown, and intermediate phase birds are dark with somewhat paler underparts, head and neck. All phases have the white wing flash, which appears as a diagnostic double flash on the underwing. In breeding adults of all phases, 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 Arctic Skua at a distance on plumage alone.
Cited byDeBenedictis, Paul A. (1997). "Skuas". Birding XXIX (1): 66–69.