Chestnut-sided Warbler

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 total, this species measures from 10 to 14 cm (3.9 to 5.5 in) in length and spans 16 to 21 cm (6.3 to 8.3 in) across the wings. Body weight ranges from 8 to 13.1 g (0.28 to 0.46 oz). Among standard measurements, the wing chord is 5.7 to 6.8 cm (2.2 to 2.7 in), the tail is 4.2 to 5.8 cm (1.7 to 2.3 in), the bill is 0.9 to 1 cm (0.35 to 0.39 in) and the tarsus is 1.7 to 1.9 cm (0.67 to 0.75 in). 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.

The chestnut-sided warbler breeds in Eastern North America, from Saskatchewan to the Maritimes. Although an observation of a male in breeding plumage was recorded as far West as British Columbia. They are usually found south of the 50°N, with the north-west Georgia as the most southern extent of their breeding range. They overwinter in Central America to northern Colombia, 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. The chestnut-sided warbler has benefited from the clearing of mature forests in their breeding ranges. They make use of the abundant second growth habitats, forests after clearcuttings and/or fires. 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.

Migration. These birds are Neotropical migrants, and are very rare vagrants to western Europe. …

Chestnut-sided warblers are primarily insectivorous. They feed primarily on lepidopteran and dipteran larvae, and will sometimes eat spiders, seeds, and fruits. They forage actively in shrubs and small trees, and sometimes will attempt to catch insects in mid-air. Most foraging consists of gleaning insects from foliage. They will include berries in their winter diets, such as those of Cymbopetalum mayanum; such trees can be used to attract wintering birds into gardens and parks. Chestnut-sided warblers forage alone.