Bird of the Month: Wilson's Warbler
By Andy McCormick
PC: Mick Thompson (Wilson’s Warbler)
Scientific Name: Cardellina pusilla
Length 4.75 in
Wingspan 7 in
Weight 0.27 oz
AOU Band code WIWA
Wilson’s Warbler is often found along wooded streams, in low shrubs, willows and alders (Kaufman). It sings loudly and regularly and can lead a birder trying to see it on a frustrating chase through dense underbrush, but persistence will pay off with a view of this lovely bird. Its song is more of a descending chatter chi chi chi chi chi chet chet, than a song.
The Wilson’s is bright yellow on the forehead, face, throat, chest, belly and undertail coverts. However, its tail is dark on both the upper and lower sides. Its back and wings are a yellowish olive separating it from other yellow warblers. A definitive field mark is the black cap on the male and grayer cap of the female, which also has more muted yellow plumage. Its beady black eye is prominent.
Wilson’s Warbler is an early migrant in the west with northward movement beginning in March and peaking in Western Washington by mid-May (Ammon and Gilbert). Fall migration begins in August and most of these warblers winter south of the United States usually in forest edge habitat, riparian areas and thorn brush areas, habitat that is similar to its northern breeding areas (Ammon and Gilbert). Pacific Coast Wilson’s Warblers typically migrate around the western coast of Central America.
As a result of recent genetic research Wilson’s Warbler has been placed in the genus Cardellina, diminutive for the Latin carduelis, goldfinch, or a kind of finch. The species name pusilla is Latin for very small or tiny (Holloway). It is formerly in the genus Wilsonia, named after the famed ornithologist Alexander Wilson, who first named this bird the Green Black-capt Flycatcher (Mearns and Mearns). It moves sprightly; flipping its tail as it forages in low shrubs showing great agility as it forages for insects of all kinds.
Wilson’s Warblers are m ore numerous in the West, and some of the highest breeding densities of the Wilson’s occur in Western Washington. This warbler most often nests on the ground or up to three feet above ground in forest edge habitat. It does not like to be under forest canopy. The female builds an open-cup nest of dried leaves, grasses and moss. Along the Pacific coast the subspecies C. p. chryseola usually deposits two to four eggs in the nest. At higher elevations of the Rocky Mountains the subspecies C. p. pileolata deposits more eggs, sometimes as many as six or seven. Incubation takes less than two weeks. The young are fed by both parents and leave the nest within two more weeks (Kaufman). Some pairs will have a second brood.
The population of Wilson’s Warblers remains strong although there have been declines throughout Western North America. “Degradation and loss of primary breeding habitat, western riparian woodlands, are likely among the leading causes of declines… Protecting and restoring native riparian habitat within the breeding range will likely play a key role in long-term conservation of this species” (Ammon and Gilbert).