# UKBirds

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## Wilson's Warbler

### Cardellina pusilla

The Wilson's Warbler (Wilsonia pusilla) is a small New World warbler. It is primarily greenish above and yellow below, with rounded wings and a long, slim tail. The male has a black crown patch which is greatly reduced or missing entirely in the female. It breeds across Canada and south through the western United States, and winters from Mexico south through much of Central America. It is a very rare vagrant to western Europe.
The Wilson's Warbler is a small passerine, ranging from 10 to 12 cm (3.9 to 4.7 in) in length, with a wingspan of 14-17 cm (5.5-6.7 in) and a mass of 5-10 g (0.18-0.35 oz). It has a plain green-brown back and yellow underparts. The male has a small black cap. Males of the western race W. p. chryseola are greener above and brighter than the eastern race. The Western-central and Alaskan Warblers are typically slightly larger in size than those found on the Eastern and Pacific coast populations. Some of the common names include Chipe coronoa negra, Reinita Gorrinegra, Reinita de Wilson, Chipe Careto, Reinita de Capucha, Chipe Coroninegro in Spanish speaking populations and Paruline - calotte noire in French.Its song is a series of descending notes typically in a loud, rapid series of chatter-like tones. The male makes a hurried, staccato sound resembling chatter that will drop in pitch near the end of the call called a flat chuff.
The breeding habitat is fairly open woodland with undergrowth or shrubs and thickets in moist areas with streams, ponds, bogs, and wet forest clearings. Wilson's warbler migrations for breeding to Northern Canada and Northwestern US in the summer with a winter migration to Southern Mexico. Winters in some overgrown clearings and coffee plantations as well as forest edges, deciduous forests, tropical evergreens, pine-oak forests, mangroves, thorn-scrubs, riparian gallery forests, brushy fields, and mixed forests across the Northern Hemisphere of Canada, the United States, and Mexico. Prefers secondary growth, riparian habitats, lakes, montane and boreal forests with overgrown clear-cuts.
The Wilson warbler can inhabit tall shrubby vegetation characteristic of treed swamps. Lack of vegetation succession in coastal forests can limit the species diversity of riparian birds in harvested landscapes that have experienced destroyed natural habitat ranges after fires. The riparian species are then limited to the non-forested portions of the habitat surviving fire.The Wilson's Warbler is an insectivore, feeding primarily on insects gleaned from leaves and twigs, or caught by flycatching. Some of these insects include beetles, bees, or caterpillars. The Wilson's warbler is an active forager moving rapidly through shrubs, on the ground, and sometimes in taller trees during the winter. They often twitch their tails or flick their wings in a nervous manner while foraging. The observed feeding rates of the male Wilson's warbler was not significantly different between males with or without mates. It also eats a few berries.The majority of the breeding range occurs between the Northern United States boreal forest zones. Nesting generally begins in early March for the West coast populations into August for the Northern ranges. The female does the majority of the nest building.
It builds a cup nest, typically constructed of vegetation and lined with grasses and hair from other animals resembling a bowl like shape. It is often sunken into moss or sedges located at the base of shrubs or under the lower branches of shrubs. The clutch varies between 2-7 eggs consisting of a creamy or off-white color with fine reddish spots throughout. The young are altricial. The montane populations generally have a higher clutch size and nest success rate than those on the coast. The eggs will hatch at 11-15 days and the young fledge at 8-13 days while one or the other parent will take care of them for several weeks.
The Wilson's warbler can be territorial as well as seasonal monogamous although some Montane populations are polygamous (one male breeds with multiple females). It has been predicted the Wilson warbler's paternal provisioning is typically essential to the female's general fitness when the territorial males have removed from their habitats.
The Wilson's warbler nests often experience parasitism from the Brown-headed cowbird in which the Brown-headed cowbird (Molothrus ater) will abandon its own eggs in the nests of the Wilson's warbler to raise.
Ground-nesting species, like the Wilson's warbler, have been studied to exhibit greater risk of detection by predatory species while at older ages they had a higher frequency and less rapid modulation warning calls than similar tree-nesting species.