The Bay-breasted Warbler is one of the richer-toned warblers of spring migration, the breeding male marked with warm chestnut ("bay") across the crown, throat, and sides, set against a dark face and a creamy neck patch. Fall birds are transformed into plain, greenish, faintly streaked warblers that are notoriously difficult to distinguish from Blackpoll and other drab autumn migrants — a classic "confusing fall warbler." It forages rather deliberately in the canopy, moving more slowly than many of its quick, flitting relatives.
Like the Cape May and Tennessee Warblers, the Bay-breasted is a spruce budworm specialist, and its numbers rise and fall dramatically with outbreaks of that caterpillar in the boreal forest, where it breeds. In Michigan it is a passage migrant, moving through in spring and fall on its way between the northern spruce woods and its wintering grounds in Central and South America. Spotting a crisp chestnut-sided male in a May treetop is one of the rewards of the warbler wave.
Feeds heavily on caterpillars, especially spruce budworm during outbreaks, along with other insects gleaned deliberately from foliage in the canopy. Its breeding numbers track budworm abundance, and it takes some berries and fruit on migration and in winter.
It specializes on spruce budworm caterpillars in the boreal forest, so its numbers rise sharply during budworm outbreaks and fall when the caterpillars are scarce, much like the Cape May and Tennessee Warblers.
It is difficult — both are plain, greenish, streaky fall warblers. Bay-breasted often shows a warm buff wash on the flanks and dark legs, while Blackpoll tends to be crisper with streaked underparts and paler legs and feet. This is a classic identification challenge.
As a migrant in spring and fall, moving between its northern spruce-forest breeding grounds and tropical wintering areas. It does not breed in the state.
Fall birds lose the chestnut and turn plain olive above with dingy buff flanks and dark legs, which is how they separate from the similar fall Blackpoll's pale legs. Immatures are the plainest of all.
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