General

Cassin's Vireo: Small vireo with olive-gray upperparts, white underparts, and pale yellow flanks. Head is gray with white eye-ring that extends to brow. Wings are dark with two white bars. Female is slightly duller. Juvenile has less gray on crown and sides of head.

Range and Habitat

Cassin's Vireo: Breeds from British Columbia and southwestern Alberta south to central Idaho and along the west coast to southern California. Most leave the U.S. in fall but a small number spend winter in southeastern Arizona; prefers dry, open forests in mountains and foothills.

Breeding and Nesting

Cassin's Vireo: Three to five creamy white eggs with black and brown speckles are laid in a nest made of twigs, fine grass, and stems, lined with finer grass and hair, and built 4 to 30 feet above the ground in a tree or bush. Incubation ranges from 11 to 12 days and is carried out by both parents.

Foraging and Feeding

Cassin's Vireo: Eats insects and fruits. Forages high in trees by gleaning food from bark, branches, or foliage, sometimes hovering briefly to pick food off vegetation or catch insects in flight.

Vocalization

Cassin's Vireo: Song is a mixed jumble of hesitant phrases, punctuated by short deliberate pauses, often repeated.

Similar Species

Cassin's Vireo: Blue-headed Vireo has blue-gray crown contrasting more sharply with throat and back. Plumbeous Vireo lacks green hues except on rump, yellow edges on flight feathers, and yellow on flanks. Hutton's Vireo is smaller with incomplete eye-ring, dingier underparts, and less contrasting plumage. Bell's Vireo lacks full eye-ring and has weaker wing-bars.