About Stercorarius parasiticus (Linnaeus, 1758)
Identification of Stercorarius parasiticus (Linnaeus, 1758), commonly called the parasitic jaeger, is complicated by similarities to the long-tailed jaeger and pomarine jaeger, as well as by the existence of three distinct colour morphs. This species is small for a skua: adult parasitic jaegers measure 41–48 cm (16–19 in) in total length, have a wingspan of 107–125 cm (42–49 in), and weigh 300–650 g (11 oz – 1 lb 7 oz). The elongated tail streamer of a breeding adult makes up about 7 cm (3 in) of the species' total length. Light-morph adults have a brown back, mostly white underparts, and dark primary wing feathers marked with a white "flash". Their head and neck are yellowish-white with a black cap, and they have a pointed projection on their central tail feathers. Dark-morph adults are entirely dark brown, while intermediate-phase birds are dark overall with somewhat paler underparts, head, and neck. All three morphs have the characteristic white wing flash. Identifying juvenile parasitic jaegers is even more challenging, and it can be difficult to distinguish them from juvenile long-tailed jaegers. Compared to long-tailed jaegers, parasitic jaegers are bulkier, have shorter wings, and have a less tern-like overall shape. They are usually warmer in tone, with browner coloring instead of grey. Like long-tailed jaegers, parasitic jaegers show the same wide range of plumage variation, and their flight is more falcon-like. The parasitic jaeger is the most common of the three jaeger species observed from shore. The species' typical call is a nasal mewing sound, repeated a few times during display. Its alarm call is a shorter vocalization. This species breeds in northern Eurasia and North America. In the United Kingdom, significant breeding populations extend as far south as northern Scotland, occurring in Shetland, Orkney, the Outer Hebrides, Sutherland, Caithness, and some islands in Argyll. In North America, breeding populations are found in Alaska, Yukon, the Northwest Territories, Nunavut, the coast of Hudson Bay, and parts of Northern Quebec and Nunatsiavut. The parasitic jaeger is a migratory species that winters at sea in the tropics and southern oceans. While most of its migration occurs over open sea, spring overland migration takes place in the Canning River Valley, Alaska. For Eurasian populations, fall overland migration occurs from northern Russia to the Persian Gulf, and for North American populations, fall overland migration occurs over the Great Lakes, particularly Lake Ontario.
