Chestnut-sided Warblers nest in young (early successional) deciduous habitats and thickets, in places regenerating after logging, fire, flooding, or storm damage. Because such habitats are ephemeral in nature, this species is usually present for fewer than 10 years in any given patch, except in habitats that are stunted for other reasons, such as oak forests at higher elevations in the Appalachians. Breeding birds move into regenerating areas quickly, often just a few years after disturbance, and even small patches of habitat can support a nesting pair. They tend not to nest in urban or suburban areas. Migrants can be found in virtually any wooded or shrubby habitat, including mature forests. Wintering birds in Central America inhabit shade-coffee plantations, scrubby edge habitats, successional habitats, and mature wet forest.Back to top
Chestnut-sided Warblers eat mostly insects, especially caterpillars and fly larvae, as well as spiders, locusts, cankerworms, and leafhoppers. They feed mostly in deciduous habitats, where these warblers pick or glean caterpillars and other prey from the undersides of leaves. To find prey, the birds hop quickly from perch to perch to change their perspective. They tend to stay in the foliage rather than near trunks or large branches. Seeds and fruit make up a small part of the diet. On the wintering grounds they eat fruits of plants in the melastome, mahogany, and other tropical families.Back to top
Nests are fairly low to the ground, in deciduous trees and shrubs of the understory, usually no more than 6 feet high. Some very low nests have been found in blackberry, alder, multiflora rose, and ferns. Most nests are set in the crotch of several small branches.
The nest is a cup of bark, weeds, grasses, and plant down, lined with fine grasses, hair, sedges, and rootlets, bound with spider silk. Nests average 2.8 inches in diameter and 2.6 inches in height, with the interior cup 2 inches across and 1.5 inches deep.
|Clutch Size:||3-5 eggs|
|Number of Broods:||1-2 broods|
|Egg Length:||0.6-0.7 in (1.554-1.832 cm)|
|Egg Width:||0.5-0.5 in (1.215-1.31 cm)|
|Incubation Period:||11-12 days|
|Nestling Period:||10-11 days|
Creamy white or greenish with brown speckles.
|Condition at Hatching:|
Helpless with sparse down.
Chestnut-sided Warblers form monogamous pair bonds. Courtship displays involve males raising and spreading their tail and wings, then vibrating the flight feathers and crown feathers while raising and lowering them. The male guards his mate as she constructs the nest and occasionally follows or chases intruders in the territory. Aggressive encounters between males involve straightening or lowering the tail (which they typically hold upward when foraging) and fluffing out their chestnut flank feathers, as well as calling. Males also chase, and are chased by, Yellow Warblers in nearby territories, while females chase other females that come too close after the eggs hatch. Wintering Chestnut-sided Warblers can be highly territorial against others of their species (from studies on Panama’s Barro Colorado Island); but elsewhere, particularly at fruiting plants, they appear to tolerate other Chestnut-sideds. In some parts of the winter range, they routinely join mixed species flocks, but in other areas they are solitary.Back to top
Chestnut-sided Warbler populations declined by about 1.2% per year between 1966 and 2015, indicating a cumulative decline of about 45% over that period, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 18 million. The species rates a 12 out of 20 on the Continental Concern Score, indicating it is a species of low conservation concern. Numbers have declined in part due to loss of habitat, especially in parts of the East where forests have matured. Chestnut-sided Warblers, like most songbirds that migrate nocturnally, often strike buildings and other structures and are killed.Back to top
Byers, Bruce E., Michael Richardson and Daniel W. Brauning. (2013). Chestnut-sided Warbler (Setophaga pensylvanica), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.
Dunn, J. L., and K. L. Garrett (1997). A Field Guide to the Warblers of North America. Houghton Mifflin Company, Boston, MA, USA.
Lutmerding, J. A. and A. S. Love. (2019). Longevity records of North American birds. Version 1019 Patuxent Wildlife Research Center, Bird Banding Laboratory 2019.
Partners in Flight (2019). Avian Conservation Assessment Database, version 2019.
Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2017). The North American Breeding Bird Survey, Results and Analysis 1966–2015. Version 2.07.2017. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.
Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.
Stephenson, T. and S. Whittle (2013). The Warbler Guide. Princeton University Press, New Jersey, USA.