The Chestnut-sided WarblerSetophaga 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 USA.
These birds are migratory, wintering in Central America south to northern Colombia, with an unconfirmed sighting from as far south as Ecuador; they are also very rare vagrants to western Europe. They arrive in their breeding range in May and depart by mid-September.
This species is a moderately-sized New World warbler. Despite having very different plumage, it is thought to be closely related to the widespread Yellow Warbler.
In the summer, male Chestnut-sided Warblers are unmistakable in appearance. They display dark-streaked gray backs, white faces, black eye stripes and greenish 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.
The Chestnut-sided Warbler has benefited from the clearing of mature forests. They make use of the abundant second growth habitats. In the tropics where they winter however, the species occurs mostly in mature tropical rainforests. Their cup-shaped nests are placed in a low bush, which is usually located in young deciduous woodland or scrub. This bird's numbers have increased as second growth forest became more common in the east in the late 19th century; their numbers have declined slightly since then.