Cassin's Vireo

Cassin's Vireo is a western counterpart of the Blue-headed Vireo, sharing the same basic pattern of white spectacles, olive back, and wing bars, but with muted, less contrasty coloring—a grayer-green head that blends more gradually into the back and throat rather than standing out sharply. The underparts are whitish with a pale yellowish wash on the flanks.

It was split from the former "Solitary Vireo" complex along with Blue-headed and Plumbeous Vireos in 1997, based on differences in plumage, voice, and range; Cassin's occupies the Pacific states and parts of the interior West.

Key field marks

• Gray-green head blending into the olive back (less contrast than Blue-headed Vireo)
• Bold white spectacles
• Whitish underparts with pale yellow flanks
• Two white wing bars

Similar species

• Blue-headed Vireo: bluer, more contrasty head sharply set off from a white throat; mainly eastern range.
• Plumbeous Vireo: entirely gray, lacking olive or yellow tones; interior Southwest range.
• Hutton's Vireo: smaller, plainer, resident in similar western woodlands but lacks the crisp spectacled pattern's contrast and has a thinner bill.

Range and habitat

Cassin's Vireo breeds in coniferous and mixed woodlands of the Pacific states and parts of the interior western US and southwestern Canada, from lowland Douglas-fir forest to montane pine-oak woodland.

Migration

Most populations migrate to winter in Mexico, though a few individuals linger along the Pacific coast in mild winters.

Behavior

Forages deliberately in the mid- to upper canopy of conifers and oaks, gleaning insects from needles and leaves.

Voice

A slow, burry, warbled series of short phrases similar to Blue-headed Vireo's song but often slightly harsher or more hurried.

Feeding

Insects and caterpillars during the breeding season, supplemented with berries in fall and winter.

Nesting and breeding

Builds a hanging cup nest suspended from a forked branch in a conifer or oak. The female incubates 3-5 eggs for about 2 weeks.