Wilsons warbler

Appearance

The Wilson's warbler is a small passerine, ranging from 10 to 12 cm in length, with a wingspan of 14–17 cm and a mass of 5–10 g. 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 males of the eastern, nominate race. Individuals from Alaska and the west-central portion of the species' range average slightly larger than those found in eastern and Pacific coastal populations.

Naming

The Wilson's warbler was first described in 1811 by the ornithologist Alexander Wilson, who placed it in the genus ''Muscicapa''. The species was moved to ''Wilsonia'' by the naturalist and ornithologist Charles Lucien Bonaparte in 1838. Zoologist Thomas Nuttall moved it to ''Sylvania'' in 1840, and by 1845, many authors included it in ''Myiodioctes.'' In 1899, the American Ornithological Union returned the species to ''Wilsonia.'' The species is currently assigned to the genus ''Cardellina''. The specific epithet ''pusilla'' means "small."

Habitat

The breeding habitat is fairly open woodland with undergrowth or shrubs and thickets in moist areas with streams, ponds, bogs, and wet clearings.

Reproduction

Nesting generally begins in early March in west coast populations, and extends into August in the northern range. The female does the majority of the nest building.

The cup nest is typically constructed of vegetation and lined with grasses and hair. It is often sunken into moss or sedges at the base of shrubs. The clutch varies from 2 to 7 eggs, which are creamy or off-white with fine reddish spots. The young are altricial. The montane populations generally have a higher clutch size and nest success rate than those on the coast. The eggs hatch at 11–15 days and the young fledge at 8–13 days; adults care for them for several weeks.

Some montane populations are polygamous.

Food

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. Feeding birds often twitch their tails or flick their wings nervously. The observed feeding rate of the male Wilson's warbler was not significantly different between males with or without mates. It also eats a few berries.