Parasitic Jaegers are well named for their habit of forcing other seabirds to disgorge their food, which the jaegers deftly swoop down to retrieve. Their acrobatics and aggressiveness in pursuing their targets have evoked both admiration and defilement by human observers. In the northeastern Atlantic, and possibly also in the Aleutian Islands, Parasitic Jaegers obtain most of their food by stealing from colonial seabirds. Nevertheless, in most of their circumpolar breeding range, “kleptoparasitism” is not the main way of life for Parasitic Jaegers. Throughout the tundra regions of the Arctic, they defend large territories within which they hunt for birds, mammals, and eggs. They are the most important predator of small birds in the Arctic and among the two or three most important predators of birds' eggs. Because they are so effective at such diverse forms of predation and parasitism, they do not require dense populations of lemmings (Lemmus, Dichrostonyx) for successful reproduction. In this respect, their ecology during the breeding season differs from that of most other arctic predators, including the smaller Long-tailed Jaeger (Stercorarius longicaudus) and the larger Pomarine Jaeger (S. pomarinus).
During migration and winter in the southern hemisphere, Parasitic Jaegers depend primarily on food obtained by kleptoparasitism. Some parasitize colonial seabirds feeding their young in the southern hemisphere and thus take advantage of nesting seabirds year round. On their way southward in autumn, they often accompany migrating terns (Sterna) and thus remain near shore more often than do other jaegers. Consequently, of the three species of jaegers, they are the ones most often seen in southern Canada and the lower United States.
Field identification of jaegers is difficult, especially when they are not in their characteristic breeding plumages. The difficulty arises in part because Parasitic Jaegers overlap the other two species in size. In addition, all three species have complex variation in their plumages. Because molts occur primarily at sea in the southern hemisphere, winter specimens of Parasitic Jaeger are not well represented in museums.
In all plumages, Parasitic Jaegers have two contrasting morphs, dark and light, as well as intermediates. In breeding populations in the north Atlantic and the north Pacific, the frequencies of these morphs vary with latitude. Studies of banded birds for more than 30 years on Fair Isle, northern Scotland, have yielded detailed information on demography and sexual selection of the morphs in this population (O'Donald 1983). In addition, many studies in the northeast Atlantic have evaluated the efficiency of the two morphs in kleptoparasitism (Furness 1987). As a consequence of this work, Parasitic Jaegers are among the best studied of all seabirds. Nevertheless, away from the kleptoparasitic populations in the northeast Atlantic, they remain little known. It is the scarcest and least studied of the three jaegers in the Arctic, and almost nothing is known of its life during winter in the southern hemisphere.