24 October

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 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 no significant 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.