Illustration by David Beadle
An unmarked vireo, the warbling is most often located by its song, which it delivers for hours. The birds tend to work mid and top parts of broad, leafy trees. Polytypic. Length 5.5" (14 cm).
Identification Among the dullest of vireos, lacking wing bars and spectacles; gray with brownish or greenish tones to the upper parts. The face pattern is ill-defined, with a dusky postocular stripe and pale lores; white eyebrow lacks a dark upper border. The underparts are typically whitish. Fall: in fresh plumage, greener above with yellow wash on flanks and undertail coverts.
Geographic Variation Two western subspecies, especially swainsoni, are smaller than the nominate eastern sub-species, have a slighter bill, and tend to be more olive above with a grayer crown. Eastern and western birds might represent 2 separate species. Two additional subspecies in Mexico.
Similar Species Fall birds greenish above, often with extensive yellow below, and can be confused with the Philadelphia. Yellow on the warbling restricted to flanks; it lacks dark lores of the Philadelphia and is more likely to be heard calling, particularly in flight.
Voice Call: a nasal eahh mobbing call; typical vireo. Note commonly uttered in flight. Song: eastern gilvus song is delivered in long, melodious, warbling phrases. Song of western swainsoni similar but less musical, higher tones.
Status and Distribution Common. Breeding: deciduous woodlands, primarily riparian areas. Migration: western birds have prolonged spring migration (early March–late May). Eastern subspecies a circum-Gulf migrant, rare on eastern Gulf Coast; arrives mid-April in Texas, by early May to Great Lakes. Peak fall migration in northern United States late August–mid-September. Mostly gone from United States by mid-October; stragglers at southern areas to November, rarely December. Winter: mostly Mexico and Central America. Casual to southern California, southern Arizona, and southern Louisiana. Vagrant: western Alaska.
Population Cowbird parasitism implicated in decline of some western populations. Ontario population has recently increased.
—From the National Geographic book Complete Birds of North America, 2006
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