Wilson's Warbler, Timber, Washington Co., Oregon on 17 May 2009 by Greg Gillson.
The Wilson's Warbler is a rather common migrant throughout the Pacific NW. It also breeds in dense riparian edges. These birds are especially common near the ground in dense wet Coast Range understory edges, and regenerating red alder clear-cuts, of the Pacific NW.
Birds arrive in spring in early May and remain into mid September.
It is a bright yellow bird, greener above, especially on the tail. Males have black caps, females often show a trace. The beady black eye really stands out in the middle of the yellow face. Birds west of the Cascades are more golden yellow (as photographed above).
The song is a rapid trill that drops in pitch at the end: chi-chi-chi-chi-chi-chi-chet-chet. The call note is similar to a Winter Wren call, a sharp chip.
The naming of this bird has an interesting history. Peter Simon Pallas, in Russia, described a new species of bird brought back by Joseph Billings from Kodiak Island, Alaska in 1790, then a Russian territory. Pallas named it "pileolata," meaning a skullcap. From this root we have pileolated or pileated, meaning a cap or crest, and referring specifically to the pileum, the top of a bird's head from bill to nape. The description was published in September 1811, after Pallas' death. The bird became known as the Pileolated Warbler.
However, earlier in February of that same year, 1811, Alexander Wilson published a record of a "new" species found in New Jersey. He named it the "Green Black-capped Flycatcher." Even though the record from Alaska was 17 years earlier, Wilson published a few months earlier than Pallas. Thus we have today Wilson's Warbler and not Pileolated Warbler. [Read The Pileolated Warbler in the Spring 1992 American Birds article by John Ferrand, Jr.]
The name has been changed several times, apparently in about this order: Green Black-capped Flycatcher, Wilson's Black-capped Flycatching-Warbler, Wilson's Black-capped Warbler, Wilson's Warbler.
The yellower birds of the West have long gone by the name Pileolated Warbler, but subspecies names were dropped in the 1950's. If you were to look in the 1940 book Birds of Oregon by Gabrielson and Jewett, or the 1953 book Birds of Washington State by Jewett, Taylor, Shaw, and Alrich, you would find that the bird I photographed above is called the Golden Pileolated Warbler.
After doing this research I'll never look at a Wilson's Warbler the same way again....