Wiley, R. H., and D. S. Lee. 1999. Parasitic Jaeger (Stercorarius
parasiticus). In The Birds of North America, No. 445 (A.
Poole and F. Gill, eds.). The Birds of North America, Philadelphia, PA.
arasitic 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, Dicrostonyx) 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 beacause 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 [see photo on front page!]. 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 the 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
remian 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.