This distinctively plumaged warbler habitually holds its tail partly cocked and wings slightly drooped. Its easily recognized song is often heard from various early successional habitats in the mountains of our state. The Chestnut-sided Warbler was virtually unreported during the time of Audubon and other early American naturalists, but has increased greatly in numbers since the clearing of eastern forests in the 1800’s. This is a bird of second growth, scrub and disturbed woodlands throughout its breeding range in North America. It is one of the few neotropical migratory birds that has benefited dramatically from human activities, at least for the short term.
This Neotropical migrant is a common breeder in the mountains of North Carolina above elevations of 2,000 feet where humans have created suitable openings in the forest. Neotropical (New World) migratory birds breed during summer in temperate North America, migrating north each spring from wintering areas, then fly back south to spend the bulk of the year in Mexico, Central or South America, or the Caribbean. A bird of early successional habitats (abandoned farmland, regenerating clearcuts, burned areas), it now can be among the most abundant breeding warblers in second-growth deciduous woods. It was a seldom described bird before the lumbering operations of the 1800’s. Local increases also occurred with forest loss from chestnut blight and where farmland and pastures have turned into scrub and second-growth brushy areas. Because of a preference for disturbed habitats, it has similarly declined with the regeneration of mature forests in some regions. This bird does not seem to do well in urbanized or fragmented areas and has experienced local declines in parts of its range in the last few decades. However, for now it is still a common bird over much of its range despite being a frequent cowbird host. Rangewide, populations have undergone a slow decline from the 1960’s to present.
Local populations fluctuate due to habitat change, and breeding density differs markedly with habitat. According to the Breeding Bird Survey (BBS), this bird’s population trends were about stable from 1966-1994, but declining slightly from 1978-1988. In many northern areas, this species has declined with the expansion of agriculture and urban areas, as well as the maturation of forests.
In all plumages the underparts are mostly whitish or pale gray, with deep chestnut on the sides and flanks in spring birds, winter adults and many first-winter males. Both sexes have wing bars that are yellow or tinged yellow. Birds during the breeding season have a yellow or yellow-green crown, a black-and-yellow-streaked back and some black on the face. Males tend to be somewhat brighter colored than females, but this is highly variable. The bird has a rather long tail for a warbler and a stout bill. In fall migration this bird is more uniformly yellow-green or even lime green above, with a distinct green cap and clean white eye-ring. Immatures can be tricky to identify with a plain yellowish-green back, yellow wing bars and white underparts.
This is a bird of scrubby second-growth areas and forest edges, thus in some places in the mountains of North Carolina abundant habitat exists, while in other areas little suitable habitat exists. Before extensive felling of deciduous trees in the 1800’s, this species was likely uncommon and local. On the wintering grounds, this bird uses disturbed areas within tropical forests, forest borders, second-growth and even shaded gardens and coffee plantations. Migrating birds can be found in a variety of brushy areas and in open woodlands.
The nest site is selected by the female, and is typically in shrubby, deciduous understories close to the ground. Females build the nest, which is a loosely woven cup made of a variety of plant parts like bark strips, shredded weed stems, grasses, plant down, sedges and rootlets. Incubation is by the female alone, and the male often ventures beyond the territorial border while the female is on the nest. Many neotropical migrants attempt to mate with other birds other than their current partner, and lower population densities, as well as highly fragmented landscapes make this more difficult.
The males use two song types. Their well-known song (please, please, pleased to meetcha), belongs to the accented-ending class of songs and is used mostly before the arrival of females, and early in the nesting cycle. Its primary purpose is believed to be attracting females. Unaccented-ending songs are another class of songs used as the nesting cycle progresses and in aggressive encounters with other males. These two song classes are learned separately, and birds require visual contact with tutor males to fully develop their repertoires.
This bird feeds on mostly insects and spiders, and small amounts of fruit and seeds. It mainly forages by gleaning the undersurfaces of leaves at the lower levels, but may move up in the canopy to feed. The Chestnut-sided Warbler usually forages alone and its small size may aid in its specialization on foliage insects on both the breeding and wintering grounds. It seems to eat a lot of small flies and larvae of butterflies and moths.
This species primarily breeds in northern hardwood and mixed forests of eastern North America from southern Canada southward through the Appalachian mountains to Georgia. Disparate populations also breed from extreme southern Illinois south parts of Missouri and Arkansas. Localized breeding occurs in eastern Iowa and northern Ohio. The Chestnut-sided Warbler is localized and less common throughout the southeastern parts of its range. It winters in Central America, with high concentrations found in Costa Rica.
As a species of early successional stages, this bird is very responsive to forest management activities, but is rarely managed for directly. Shooting and trapping may be a problem on the wintering grounds. Excessive use of pesticides may be a potential threat, but may be less of a problem than for other gleaners because of this species habit of foraging on the undersides of leaves. It has been found dead at the bases of TV towers, smokestacks and building during migration. It responds favorably to some habitat changes, like areas cleared that are allowed to regenerate. This bird seems to decline in areas that have expansions of agriculture and urbanization.
Dunn and Garrett. Peterson Field Guide: Warblers. Boston: Houghton Mifflin, 1997
Price, Droege and Price. The Summer Atlas of North American Birds. London: Academic Press, 1995.