Guide to Boreal Birds
The bird formerly known as the Solitary Vireo was recently split into three separate species by American ornithologists: Cassin's, Plumbeous, and Blue-headed Vireos. Cassin's Vireo occupies the westernmost part of the former Solitary Vireo's range.
5-6" (13-15 cm). Very similar to both Blue-headed and Plumbeous Vireos, with plumage roughly intermediate between the two. Duller and grayer overall than Blue-headed but shares that species' yellow sides and flanks, olive back, white wing-bars, and bold white "spectacles."
Song a series of phrases; intermediate between clear notes of Blue-headed Vireo and rough, husky notes of Plumbeous Vireo. Call a husky chatter.
3-5 white eggs, lightly spotted with brown, in a pendant cup of bark strips and down, placed in a forked twig of a small forest tree.
Coniferous and mixed forests.
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 the fall but a small number overwinter in southeastern Arizona.