Wilson’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
Wilson’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
Female 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.
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.
|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.|
Wilson’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
Wilson’s Warbler populations declined by nearly 2% per year resulting in a cumulative decline of about 60% between 1966 and 2019, according to the North American Breeding Bird Survey. It is listed as a Common Bird in Steep Decline by Partners in Flight who estimate the global breeding population at 81 million and rate Wilson’s Warbler 10 out of 20 on the Continental Concern Score. Primary threats include habitat loss on the breeding grounds, along migratory pathways, and on 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 comparatively huge cowbird chicks at the expense of their own.Back to top
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