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Wilson's WarblerWhat to look for to identify Wilson's Warbler
Contributed by Kenn Kaufman
Published: April 22, 2011  Click to enlarge
This is one of the smallest warblers, slim and small-headed, with a relatively long tail. What it lacks in size it makes up for with activity, flitting with restless energy, flipping its tail about. Experienced birders may learn to recognize it, even from a glimpse, by its overall yellow tone and typical actions.
A key field mark for Wilson’s Warbler is its lack of field marks. Many warblers have contrasting wing bars, and many have white spots in the tail, but Wilson’s has neither. Wilson’s also lacks any streaks on the body or a distinct pattern on the face. On some birds, especially in eastern populations, the ear-coverts (cheeks) are dark enough olive to set off a narrow, partial eye-ring of yellow, encircling the back of the eye. On others, the ear-coverts are so pale that no such eye-ring can be discerned.
Few other warblers are so unmarked. Some Orange-crowned Warblers along the Pacific coast are plain and yellow overall, but they have a dark eye-line that Wilson’s lacks. Dull female Hooded Warblers can be superficially similar, but they flash white outer tail feathers. Some female Yellow Warblers can be patterned much like dull female Wilson’s, but they have a different shape, heavy-bodied and short-tailed, with a larger bill.
Beyond the species ID, Wilson’s Warbler has regional variations worth noticing. The subspecies nesting in eastern Canada, from Newfoundland to the edge of the Rockies, Wilsonia pusilla pusilla, is dullest, tending toward dull greenish yellow. The one nesting on the Pacific slope from southwestern British Columbia to southern California, W. p. chryseola, is brightest, a rich golden or orange-yellow. The subspecies nesting from Alaska south through the Rockies, W. p. pileolata, is intermediate between the other two. All three races are so variable that it’s rarely safe to identify them in migration, away from their nesting grounds. The variation gives us another reason to look closely at “Mr. Wilson” when we get the chance.
Incidentally, “Mrs. Wilson” varies not only in color but in the extent of the black cap. Adult females of the Pacific race, chryseola, usually show a mostly black cap. First-year females of the eastern pusilla usually show little or no black there. Between these extremes there is almost continuous variation: Western females average more black than eastern ones, and adults average more than first-year birds. The upshot is that, as with other birds, it’s useful to be able to recognize Wilson’s Warbler by more than just one “diagnostic” mark.
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What to look for
Overall shape.
Small and slim, with relatively short, thin bill, long tail.
Wing and tail pattern. Unlike many warblers, has no contrasting pattern in wings and no trace of paler spots in tail.
General color. Suffused with yellow all over: brightest golden yellow on those of the Pacific coast, dullest greenish yellow on eastern birds.
Crown pattern. Neat black cap on males, reduced or absent on many females.
Face pattern. Mostly plain yellow, making isolated black eye conspicuous. On some, olive ear-coverts set off a narrow, yellow, partial eye-ring.
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Parallel warblers
Wilson’s Warbler and Orange-crowned Warbler are neither similar nor closely related, but there are interesting parallels in their distribution. Both are more common in the west than in the east. They nest across the boreal zones from eastern Canada to Alaska, but in the western U.S. their breeding ranges extend far southward in the mountains and along the coast. In both species, the birds nesting along the Pacific coast are the most brightly colored, those nesting in eastern Canada are dullest, and those of the Rockies are more or less intermediate in color.
Wilson’s birds
Alexander Wilson (1766-1813) is often regarded as the first true ornithologist in North America. His American Ornithology, published in nine volumes from 1808 to 1814 (the last was completed posthumously by his friends), established the baseline that John James Audubon relied upon in producing his better-known Birds of America two decades later. But Wilson didn’t live long enough to see his own work recognized. Several birds named after him were described to science after his death: Wilson’s Plover in 1814, Wilson’s Phalarope in 1819, Wilson’s Storm-Petrel in 1820. The bird that we now call Wilson’s Warbler was described to science by Wilson himself, in 1811, but he called it Muscicapa pusilla, the “Green Black-capt Flycatcher.” The genus Wilsonia, in which the warbler is now classified as Wilsonia pusilla, was not named until 1838.
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Kenn Kaufman is the author of the Kaufman Guide to Birds of North America, Kaufman Field Guide to Advanced Birding, and other books about birds.
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