thumb|right|Male, [[Ottawa, Ontario]]

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.

Taxonomy

The chestnut-sided warbler was formally described in 1766 by the Swedish naturalist Carl Linnaeus in the twelfth edition of his Systema Naturae under the binomial name Motacilla pensylvanica. Linnaeus based his account on the "Red-throated fly-catcher" that had been described and illustrated in 1760 by the English naturalist George Edwards in his book Gleanings of Natural History. Edwards had received specimens from the American naturalist William Bartram that had been collected in Pennsylvania. The chestnut-sided warbler is now one of more than 30 species placed in the genus Setophaga that was introduced in 1827 by the English naturalist William Swainson. The genus name Setophaga combines the Ancient Greek σης/sēs, σητος/sētos meaning "moth" with -φαγος/-phagos meaning "-eating". The species is monotypic: no subspecies are recognised.

The chestnut-sided warbler used to be in the genus Dendroica; however, it was merged with the genus Setophaga in 2011. The change resulted with Setophaga having the most species in the family of New World warblers.

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.

Distribution and habitat

The chestnut-sided warbler breeds in Eastern North America, from Saskatchewan to the Maritimes. They are usually found south of the 50°N, with the north-west Georgia as the most southern extent of their breeding range. with a confirmed sighting from as far south as Ecuador. While they mainly spend their non-breeding period in Central America, with the highest numbers in Costa Rica, the chestnut-sided warbler does winter in southern parts of Mexico and some north-western parts of South America. Their numbers increase as soon as a year after clear-cuts, peaking seven to eight years after the disturbance.

In the tropics where they winter, the species occurs mostly in mature tropical rainforests where they tend to stick to the mid- to upper- canopy. They are known to use coffee plantations and riparian zones in Costa Rica. Some males sing only unaccented songs, and they are less successful at securing mates than males that sing both songs. Their calls are harsh chips. Despite the fact that songs for courtship do not vary across small distances, songs for aggression are highly localized, a possible explanation being that female Chestnut-Sided Warblers disperse over long distances.

thumb|Eating [[rough prominent caterpillar]]

Food and feeding

Chestnut-sided warblers are primarily insectivorous. Chestnut-sided warblers forage alone. This species is frequently parasitized by brown-headed cowbirds (Molothrus ater). These birds lay 3–5 eggs that are creamy white or greenish with brown speckles in color. Usually, couples will only have one clutch; however, they might have a second clutch if the first one fails.

Hybridization

On May 25, 2014, the Long Point Bird Observatory (Ontario, Canada) recorded a new hybrid: a chestnut-sided warbler hybrid with a magnolia warbler (Setophaga magnolia). The individual was caught in a mist-net at the Long Point's research station Old Cut. Genetic and morphological evidence points to the mother being a chestnut-sided warbler and the father being a magnolia warbler.

A Brewster's warbler with two small brown spots on its throat was documented in Pennsylvania in 2018. Brewster's warblers are a hybrid of golden-winged warbler (Vermivora chrysoptera) and blue-winged warbler (Vermivora cyanoptera). The chestnut spots and its class pointed to the individual having a chestnut-sided warbler parent, making it a three-species hybrid. It was later confirmed through genetic tests that the father was a chestnut-sided warbler, and the mother was a Brewster's warbler. The extremely rare hybrid was named Burket's warbler after the person who first made the observation of the individual.

Billy Weber uploaded his observation of a chestnut-sided warbler and black-throated blue warbler (Setophaga caerulescens) hybrid to ebird in 2019. The hybrid, located in Pennsylvania, visually looked like a male black-throated blue warbler with brown sides.

The chestnut-sided warbler might have hybridized on two other accounts. the Handbook on Avian Hybrids of the World highlights a black-and-white warbler (Mniotilta varia) hybrid and a blackburnian warbler (Setophaga fusca) hybrid who might have had a chestnut-sided warbler parent. However, other species were also suggested to have contributed to creating the hybrids, and no genetic tests were performed to confirm one way or another.

Population

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. The Breeding Bird Survey BBS in New England has shown an overall decrease in population from 1966 to 2015, but an increase in Pennsylvania. Local population trends are most likely tied to habitat changes.