Chestnut-sided Warbler

The Chestnut-sided Warbler is a small wood-warbler breeding in second-growth woodland, forest edge, and shrubby habitat across the northeastern and north-central United States and adjacent Canada, wintering mainly in Central America. Breeding adults show a distinctive chestnut stripe along otherwise white flanks, paired with a yellow-green crown and black facial markings, a combination unique among North American warblers.

The crown is yellow-green, contrasting with a white face marked by a black eye-line and black mustache stripe in breeding adults. Underparts are white, broken by a bold chestnut stripe running along the flanks below the wing. The back is streaked black and yellow-green, and two pale wingbars cross the closed wing. In nonbreeding plumage the chestnut flank stripe is greatly reduced or nearly absent, and the bird appears plain lime-green above and whitish below with a distinct white eye-ring.

Breeding adults show the fullest expression of the chestnut flank stripe and black facial pattern, with males typically showing the boldest markings. Nonbreeding adults and immatures are much plainer, lacking most of the facial pattern and showing only a trace of chestnut on the flanks, with a bright lime-green back and prominent white eye-ring. A single complete molt occurs after breeding, followed by a partial molt before spring migration in some individuals.

This species breeds in early successional habitat such as regenerating clearcuts, overgrown fields, and shrubby forest edge across the northeastern and north-central United States and southern Canada. It winters primarily in shrubby and forest-edge habitat of Central America.

Chestnut-sided Warblers forage actively for insects in low to mid-level shrubs and saplings, often with drooped wings and a cocked tail. The male's song is a bright, whistled phrase often rendered as "pleased, pleased, pleased to meet you." Nests are built low in dense shrubs. The species is a nocturnal migrant traveling between breeding grounds and Central American wintering areas.