The chestnut-sided warbler (Setophaga pensylvanica) is a New World warbler. They breed in eastern North America and in southern Canada westwards to the Canadian Prairies. They also breed in the Great Lakes region and in the eastern United States.
In the summer, male chestnut-sided warblers are unmistakable in appearance. They display dark-streaked gray backs, white faces, black eyestripes and yellow crowns. Their underparts are white, with chestnut flanks, and they also have two white wing bars. The adult females resemble washed-out versions of the summer male, and in particular, the females lack the strong head pattern, and also have little to no chestnut coloring on their flanks.
Non-breeding birds of both sexes have bright yellow-green crowns, white eye-rings on a grey face, and unstreaked underparts. They also have unstreaked pale grey breasts. Their wing bars are always present in their plumages. Their lack of streaking and yellowish backs help to distinguish this species from the larger blackpoll warbler in the fall.