5.0 inches long. The Chestnut-sided Warbler is the only warbler with pure white underparts in all seasons. The undertail coverts are white and there are large white tail spots. This bird shows a marked difference between spring and fall plumage. In spring the male has chestnut streaks on the flanks, two pale wing bars, a bright yellow crown, a black eye stripe and mustache, and a white cheek. The back has black streaks. The female is similar but more subdued. Fall birds have a lime green crown and back, yellow wing bars, a plain face with a bold white eye ring, and little to no chestnut on the flanks. Immature birds are similar to fall adults.