Fairly dull vireo, intermediate between Blue-headed and Plumbeous. Grayish-olive overall, but with yellowish flanks and greenish back. Note white spectacles contrasting with gray head and two whitish wingbars. Breeds in mixed coniferous forests, especially in mountainous areas; winters in Mexico. Often separable from Blue-headed and Plumbeous Vireos by range, but can overlap during migration and winter. Cassin's is less contrasty than Blue-headed, especially between the gray face and whitish throat, and the greenish back blends into the nape and crown. Some birds are best left unidentified.