Chestnut-sided Warbler

How to find it

Check shade-coffee farms, scrubby edges, regrowth and mature wet forest. Forest edges, openings, partly cleared second growth and shaded gardens are all worth a slow look. Quiet pishing sounds often bring this warbler closer. Work through clearings and road edges, scanning the fine branches of saplings. Watch fruiting and flowering trees too, where wintering warblers come to feed, and listen for the sharp chip.

How to recognise it

This is a tiny bird, about 11.5 cm long and weighing about 9 g.

In every plumage it has two yellowish wing bars. In fall and winter, when visitors see it, it shows a white eye ring and a gray face against a bright yellow-green crown. Winter birds are bright olive-green above and pale gray-white below. Most show a thick chestnut stripe along the flanks, though it can be faint or missing, especially on females.

In spring, before they leave, breeding-plumage males look quite different: a yellow crown, a black line through the face, white cheeks and chestnut sides. Females look similar but duller. It often cocks its tail up, like a wren or gnatcatcher, and the cocked tail is visible from a distance, which helps pick it out from other warblers in scrubby habitat.

What it sounds like

Recording: Sean Frey, CC BY 4.0, source, read 4 Nov 2026.

Behaviour and diet

In winter it is usually solitary and territorial, yet wintering birds often join mixed-species flocks of migrants and resident birds. It hops quickly through foliage, and most feeding is gleaning insects from leaves. It forages actively in shrubs and small trees and sometimes snatches insects in mid-air. It catches small insects, caterpillars and spiders, and will chase flying prey. It also eats berries.

Where it lives

It ranges from the lowlands up to about 1850 m, sometimes higher, and is a common winter resident up to about 1500 m on both slopes. It uses the middle and upper levels of forest, and in its winter range it mostly stays in the mid to upper canopy of mature rainforest. It drops to shrub level at forest edges, openings and second growth. In Costa Rica it also uses coffee plantations, riverside areas, partly cleared second growth and shaded gardens. It is found in every conservation area listed: Amistad Caribe, Arenal, Cordillera Volcanica Central, Guanacaste, Osa and Pacifico.

When to see it

Fall arrival runs from mid-September, rarely late August, to early November. Spring passage is mostly along the Caribbean lowlands, from early April to mid-May, and it can be seen in the country from early September to mid-May.

Similar birds

Its eye ring and wing bars make it a frequent false alarm, turning up in mixed flocks, away from flocks, in forest shade and in second growth, just when you think you have found something rarer. In fall and winter, the lack of streaking on the sides and the yellowish-green back help separate it from the larger Blackpoll Warbler.

Names and lore in Costa Rica

Costa Rica's official Spanish name is Reinita de Costillas Castañas. Costa Rican sources also write it as reinita de costado castañas or reinita flanquicastaña. Elsewhere other names are used: reinita de Pensilvania in Colombia, reinita flanquicastaña in Panama and Nicaragua, and chipe flancos castaños in Mexico.

Status and conservation

The AOCR list marks it as a migrant (M): a common winter resident in Costa Rica, though uncommon in the dry northwest.

Photography tips

The cocked-up tail is visible from a distance, so look for it in scrubby habitat. Fruiting trees are a good bet: on a winter trip, wintering warblers were seen visiting fruit at Cinchona, Bosque de Paz and Arenal.