Parasitic Jaeger
The distinguishing behavior of the Parasitic Jaeger, and the source of its common name, is its relentlessly aggressive practice of “kleptoparasitism” — stealing food from other birds. The species is also notable for its impressive long-distance migration, efficient cooperation in foraging, and attentive care of mates and young. Though its foraging habits have been well studied, the remoteness of its wintering grounds and the difficulty of distinguishing the species from other jaegers have left many aspects of its behavior, especially in winter, unknown to science.

The breeding range of the species includes the coasts of Alaska, as well as coastal and inland tundra regions of northern Canada. Parasitic Jaegers also breed on the coasts of Greenland, Iceland, Scandinavia, and northern Russia. Parasitic Jaegers spend the boreal winter in the temperate and tropical oceans of the southern hemisphere. Unlike other species of jaeger, which migrate along the Arctic coastline, Parasitic Jaegers that breed in northern Canada are thought to migrate over land to the Atlantic and Pacific coasts of North America. Between the northern and southern oceans, Parasitic Jaegers migrate over the seas, closely tracking the movements of terns and gulls.
Parasitic Jaeger Range Map

Migrant Parasitic Jaegers engage in kleptoparasitism on the open sea during fall, winter, and early spring. Parasitic Jaegers that breed in the northeast Atlantic also do so in spring and summer. Parasitic Jaegers pursue and intercept their targets, which include terns, gulls, murres, and puffins, and force them to release the small fish or other food that they have gathered in their bills or crops. Attacks may involve pursuit from behind, swoops from below, or steep dives from above. Parasitic Jaegers also sometimes work in tandem to steal food; working as a group increases the likelihood that the target will drop prey, but tends to decrease the success rate per individual jaeger. Parasitic Jaegers in migration on the St. Lawrence River themselves sometimes lose food to kleptoparasitism by Herring Gulls.

Parasitic Jaegers breeding in northern Canada, on the other hand, do not commonly steal their food. Instead, these Parasitic Jaegers hunt small and medium-sized birds (including longspurs, sandpipers, and juvenile ducks and ptarmigans), birds' eggs (including those of gulls, geese, loons, and alcids), insects, fish, rodents, and carrion. Paired Parasitic Jaegers cooperate in hunting birds, with one jaeger chasing a bird near the ground while its mate flies above to intercept dodges, and with each helping to hold the slain prey while the other pulls off meat. Parasitic Jaeger pairs also work together to take eggs, alternating in approaching nests and distracting the incubating bird, while the other makes off with an egg.

Nests are shallow depressions in the ground, often in meadows or on small rises in wet tundra areas. Clutches usually consist of two eggs. Both parents share duties in incubation, nest defense, and brooding. Young Parasitic Jaegers leave the nest after about four weeks. Although young birds remain with their parents for up to three weeks after fledging, the young are believed to be on their own for their first fall migration.

Description: Parasitic Jaegers are medium-sized oceanic seabirds, similar to gulls in proportions and general shape. The species is rarely observed on land outside of the breeding season. They closely resemble the other two North American species of jaeger, Long-tailed Jaeger ( Stercorarius longicaudus ) and Pomarine Jaeger ( S. pomarinus ) in all plumages. Especially among nonbreeding birds, distinguishing among these species is one of the more difficult challenges of bird identification.

Parasitic Jaegers are intermediate in size between Long-tailed and Pomarine jaegers, though the mass of individual Parasitic Jaegers does overlap with the mass ranges of the other species. Parasitic Jaegers measure about 16 inches in length, reaching up to 20 inches in the breeding season, when adults have elongated central tail feathers. Females are larger than males.

Parasitic Jaegers show a range of color morphs, from entirely dark (with only a “window” of white visible on the underside of the primaries) to mostly light. Lighter breeding adult Parasitic Jaegers have solid brown upperparts, a lighter brown breastband, cream-colored belly and nape, and a dark brown cap. Nonbreeding light adults show some faint barring on their tails and upperparts.

Juvenile Parasitic Jaegers also show wide variation between light and dark morphs. Intermediate and lighter juveniles show barring, ranging from deep cinnamon to whitish, on both their upperparts and underparts.

Voice: Parasitic Jaegers issue several calls, including a series of repeated bisyllabic notes.


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