The Pomarine Jaeger, the largest of the three species of jaegers, is a powerful, heavily built predator. An adult in full plumage is a magnificent sight, with its twisted central rectrices often much more elongated than shown in illustrations. Like other jaegers, the Pomarine Jaeger nests in arctic tundra and spends the rest of its life at sea. The name “pomarine” is based on the scientific name, proposed in 1815 by C. J. Temminck, from Greek roots meaning “lid-nosed,” a reference to the saddle-like sheath covering the base of the upper bill in all species of jaegers ( Lockwood 1984 ).
Pomarine Jaegers are perhaps unique among birds in their dependence on a single species of prey for successful reproduction. Suitable conditions for nesting usually occur in low-lying wet tundra near arctic coasts where there are periodic irruptions of brown lemmings (Lemmus trimucronatus) or, in northern Russia, the very similar L. sibiricus . Successful reproduction occurs only in one of every three or four years in a typical lemming cycle, and only in areas where lemming populations reach high biomass. It is possible that most production of young occurs episodically at only a few places in the Arctic.
Because breeding is so variable at any one location, some investigators have speculated that this species is nomadic, breeding opportunistically wherever lemmings occur in sufficient numbers in a particular summer. In many summers, however, there are apparently no such opportunities over large areas of the Arctic and most Pomarine Jaegers leave the Arctic almost immediately.
When not breeding, individuals spend their lives at sea, often far from land, in tropical and subtropical oceans. There they forage primarily by scavenging and by predation on small seabirds. They also practice kleptoparasitism, stealing food from other birds, but less often than do Parasitic Jaegers (Stercorarius parasiticus). The Pomarine's methods of parasitism are less elegant than those of the smaller jaegers; they often rely on surprise and brute force to tackle shearwaters and gulls feeding on the water rather than on acrobatics in flight.
The distinctive twisted rectrices of breeding Pomarine Jaegers are often lost soon after birds leave the Arctic. Many birds seen near North American coasts in autumn and winter are adults in Basic plumage, which closely resemble immatures. Identification of the three species of jaegers in these plumages presents a continuing challenge, as a result of general similarities among species and great variation within each species. Like the Parasitic Jaeger, Pomarine adults as well as immatures have dark and light morphs, although in the Pomarine Jaeger dark morphs are always scarce (<15% of birds in all populations).
The highly specialized reproductive ecology of the Pomarine Jaeger makes it one of the least studied birds of the Arctic, and its exceptionally localized and episodic reproduction make it one of the most potentially vulnerable to human disturbance. Intensive studies of breeding biology have been restricted to northern Alaska and a total of no more than 8 seasons ( Pitelka et al. 1955b , Maher Maher 1970a , Maher 1974c , Andersson Andersson 1973a , Andersson 1973b ).