Cassin's vireo (Vireo cassinii) is a small North American songbird, ranging from southern British Columbia in Canada through the western coastal states of the United States. This bird migrates, spending the winter from southern Arizona (the Sonoran Desert) to southern Mexico. The vireo is 4.3–5.5 inches in length, with a gray head, back, and flanks, and whitish underparts. It has solid white "spectacles" and white wing bars. The Cassin's Vireo has been spotted in the COSA from April to May and September