Tennessee Warbler

The Tennessee Warbler is one of the “budworm warblers,” species which have been shown to respond numerically to outbreaks of spruce budworm, a conifer-feeding moth. They were far more abundant in the 1970s and 1980s but declined as the last outbreak subsided. Budworm follows a roughly 40-year cycle, and Tennessee Warblers are becoming increasingly frequent in New Hampshire in response to an ongoing outbreak in Quebec. At present this increase is most evident during migration, but the number of summer records has also been increasing slowly since 2015.

Look for breeding Tennessee Warblers in spruce and tamarack bogs, and sometimes in other open wetlands with taller trees and shrubs. The nest is built on the ground, usually in an elevated area such as a moss hummock or the base of a tree or shrub. Here they lay 5-6 eggs, although clutches as high as eight have been documented during spruce budworm outbreaks. Like most warblers, these are incubated for 11-13 days and the young depart the nest another 11-13 days later.

Tennessee Warblers are unusual in that some individuals depart their nesting areas before starting to molt into winter plumage. This is probably the explanation for a well-documented phenomenon of birds seen well south of the breeding range in July. More detailed study is needed to understand exactly how this “molt-migration” operates, but some birds that exhibit the behavior are probably failed breeders. Irrespective of when they migrate, most Tennessee Warblers make their way to Central America. Here, like Cape May Warbler in the Caribbean, they spend a lot of time eating nectar and fruit in addition to insects.