Wilson's Warbler Life History

Habitat

Habitat ScrubWilson’s Warblers breed across Canada and the western United States in willow, alder, and shrubby thickets near streams up to about 11,500 feet. They also use forest edges or forest openings with a dense understory of flowering plants such as aspen stands. In the Pacific Northwest they breed in young conifer, maple, and alder forests as well as scrubby patches of coyotebush and blackberry. Wilson’s Warblers key in on forests and scrubby areas along streams to fatten up during migration. During the nonbreeding season they use many types of habitats from lowland thickets near streams to high-elevation cloud forests in Mexico and Central America. They are one of a few migrant warblers that use high-elevation treeless plains (paramo) in Costa Rica. Back to top

Food

Food InsectsWilson’s Warblers pick larval insects, spiders, beetles, and caterpillars off leaves and twigs. They also hover or sally (fly out and back) to grab flies, bees, mayflies, aphids, and other insects from leaves or in the air. They do most of their foraging in the understory in willows, alders, or other shrubs. On the wintering grounds they also drink "honeydew"—a sweet liquid excreted by scale insects as they feed on oak trees. Wilson’s Warblers hover near the insects and drink the sugary water for a few quick calories. Back to top

Nesting

Nest Placement

Nest GroundFemale Wilson’s Warblers tuck their nests in small depressions on the ground typically at the base of a tree sapling, willow stem, flowering plant, dense clump of grass, or log so that it is well hidden. Although most Wilson’s Warblers nest on the ground, those that live in coastal California build their nests as high as 5 feet above the ground in shrubs.

Nest Description

Females use large leaves or sedges to form the base of the nest. Once the base is in place, they add moss, strips of bark, fine plant material, grass, and hair to make a cup-shaped nest in a little depression on the ground. Nests in shrubs are generally wedged between small twigs to keep them in place. It takes the female about 5 days to complete a nest without any help from the male. Ground nests are 3–4 inches wide with an interior cup of about 1–2 inches wide; nests in shrubs are slightly larger.

Nesting Facts
Clutch Size:2-7 eggs
Number of Broods:1-2 broods
Egg Length:0.6-0.7 in (1.5-1.8 cm)
Egg Width:0.5-0.5 in (1.2-1.3 cm)
Incubation Period:10-13 days
Nestling Period:9-11 days
Egg Description:White to creamy white with fine reddish brown speckling or spotting.
Condition at Hatching:Eyes closed and mostly naked with one or two tufts of down on the head or back.
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Behavior

Behavior Foliage GleanerWilson’s Warblers are always on the move, either hovering to grab an insect or flitting to and fro in the understory, often twitching their tail. Unlike many other warblers, they spend much of their time foraging in low to middle levels of the forest. Males choose and defend territories that include nesting and foraging habitat. When another Wilson’s Warbler enters their territory they drop their wings and cock their tails upwards while hopping between branches and giving harsh calls. They typically mate with one partner during the breeding season, but like many other warbler species, some sneak off to mate with an additional mate, a phenomenon known as extra-pair copulation. Pairs typically break up after the summer and they find new mates in the following years. Only females incubate and brood eggs and nestlings, but males help with feeding nestlings and fledglings. If an adult detects a nest predator such as a Steller’s Jay near the nest, the parent may perform a broken-wing display to distract the predator. Other nest predators include mice, squirrels, chipmunks, domestic cats, and snakes. Many males and females defend territories on the nonbreeding grounds in Mexico and Central America, others join foraging flocks with other species. Back to top

Conservation

Conservation Common Bird in Steep DeclineWilson’s Warbler populations declined by nearly 2% per year between 1966 and 2015, resulting in a cumulative decline of 61% according to the North American Breeding Bird Survey. Populations in the West are declining at a greater rate than populations in the East. Despite these declines, they are not listed on the 2016 State of North America’s Birds Watch List, however the 2014 State of the Birds Report listed Wilson’s Warbler as a Common Bird in Steep Decline. Partners in Flight estimates a global breeding population of 60 million with 61% spending some part of the year in the U.S., 39% breeding in Canada, and 72% wintering in Mexico. The species rates 10 out of 20 on the Continental Concern Score. Primary threats include habitat loss on the breeding grounds, along migratory pathways, and on the wintering grounds. In some areas, Brown-headed Cowbirds pose a threat by laying their eggs in the nests of unsuspecting Wilson’s Warblers, a phenomenon known as brood parasitism. The warblers then raise the cowbird chicks at the expense of their own. Back to top

Backyard Tips

Wilson’s Warblers do not visit feeders, but you can provide habitat for them in your yard by landscaping with native trees and shrubs. Creating a bird-friendly backyard for Wilson’s Warblers even if they are not breeding in your area may help them out during migration. Head on over to Habitat Network to learn about which native species are good matches for your yard and more.

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Credits

Ammon, Elisabeth M. and William M. Gilbert. 1999. Wilson's Warbler (Cardellina pusilla), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Karlson, Kevin and D Rosselet. 2015. Birding by Impression. Living Bird no. 25:34-42.

Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2015.2. Patuxent Wildlife Research Center, Bird Banding Laboratory 2015.

Mearns, B. C. and R. F. Mearns. 1992a. Audubon to Xántus: The lives of those commemorated in North American bird names. New York: Academic Press.

North American Bird Conservation Initiative. 2014. The State of the Birds 2014 Report. US Department of Interior, Washington, DC, USA.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Ruegg, K. C., E. C. Anderson, K. L. Paxton, V. Apkenas, S. Lao, R. B. Siegel, D. F. Desante, F. Moore and T. B. Smith. 2014. Mapping migration in a songbird using high-resolution genetic markers. Molecular Ecology no. 23 (23):5726-5739.

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, USA.

Stephenson, T. and S. Whittle (2013). The Warbler Guide. Princeton University Press, New Jersey, USA. 

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