Introduction

The Tennessee Warblers loud, staccato song often signals the peak of spring songbird migration in eastern North America. Described by Alexander Wilson in 1811 from a migrant specimen on the banks of Tennessee’s Cumberland River, its common name belies the fact that its breeding range is restricted almost entirely to the boreal forest zone of Canada, extending into southeastern Alaska and the extreme northern fringe of the United States. Most migrants move along the eastern seaboard east of the Mississippi Valley, crossing the Gulf of Mexico to and from wintering grounds in Central and northern South America.

Numbers of transients and breeders fluctuate markedly from year to year, often in response to periodic outbreaks of spruce budworm (Choristoneura fumiferana) caterpillars, on which the species is a well-documented specialist. Breeding-population densities recorded during budworm epidemics may exceed 500 males/100 ha, and Tennessee Warblers often rank as the most abundant breeding species in boreal forests of eastern Canada. Long-term (30-year) continental census data show nonsignificant population increases overall, and the species is probably more abundant now than it was in the nineteenth century, because of its exploitation of budworm outbreaks and use of successional habitats following commercial logging operations.

Surprisingly few detailed studies of the breeding ecology of Tennessee Warblers have been conducted, and Chapmans (1907: 85) assertion that the Tennessee Warbler awaits a biographer still rings true. The species inhabits remote areas, its nests are difficult to find, and no systematic study of marked populations has been attempted. Although most aspects of the species breeding behavior and demography remain poorly known, the wintering biology of the Tennessee Warbler is better understood. Several studies (e.g., Tramer and Kemp 1979, 1980, Morton 1980) examining its winter habitat use, foraging ecology, and social behavior have shown considerable plasticity and seasonal variation. A. Skutch (in Bent 1953) suggested that the species would be more appropriately named coffee warbler, because of the strong affinity of wintering individuals for coffee plantations in Central America. Recent studies have affirmed the importance of shade coffee plantations for Tennessee Warblers, especially during the winter dry season (Greenberg and Bichier in press). The species regularly migrates southward during the main period of its Prebasic flight-feather molt, a phenomenon that is exceptional among North American passerines and merits further study.