INTRODUCTION
O f the three species of Stercorarius, the Long-tailed Jaeger is the smallest and most graceful in flight. Transequatorial migration allows this species to forage in regions with high productivity and extended day lengths throughout the year. During the breeding season, it feeds over arctic tundra; as a migrant and through- out the northern winter, it is highly pelagic, spending its time over open seas. Even though this jaeger spends more than three-fourths of its life at sea, nearly all information about its biology comes from its arctic breeding grounds. The species is seldom seen in North America south of Canada and Alaska, and much of the published information not directly related to its breeding biology focuses on vagrancy or identification. In the Arctic this species is the most widely distributed and abundant jaeger. Breeding the farthest north of any jaeger, probably as far north as any bird, during the summer it frequents dry tundra, sometimes even barrens, on low slopes and ridges. Its diet in this habitat consists primarily of		lemmings and voles, so this bird is vulnerable to cycles in the densities of these rodents. The Long-tailed Jaeger is supremely adapted for predation on these cyclic rodents. Because it depends on rodents only for breeding, it can survive crashes in rodent numbers without the irruptions and high mortality experienced by other arctic predators. In years with low densities of lemmings, the Long-tailed Jaeger simply does not breed and returns to sea. In effect it "skims the cream" from the population cycles of lemmings. Migrant Long-tailed Jaegers are usually encountered singly. Most individuals migrate over the open ocean. Jaegers in general, and the two smaller species in particular, are difficult to separate. The challenges of identification result from the conservation plumages of the genus and from individual, age-related, and polymorphic variation. Many birds seen away from nesting areas are not in the distinctive adult breeding plumage and thus lack the characteristic central tail-feathers and other features of this plumage.