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 does migrate for the winter months from southern Arizona-(the Sonoran Desert), to southern Mexico.

The vireo is 11-14 cm (4-6 inches) in length, with a gray head, back, and flanks, and whitish underparts. It has a solid white eye spectacle and white wing bars.

The song, given persistently, consists of short, rough whistled phrases of several notes, spaced about 2 seconds apart. The phrases often alternate ending on a high note and a low note, giving an impression of question and answer.

It prefers open woodlands of the western mountains and foothills. It is usually found in the mid to lower portions of the forest canopy where it slowly and deliberately forages for insects among the foliage.

Cassin's Vireo builds a cup nest out of bark strips and down in the fork of a twig. It lays 2 to 5 white eggs with some brown spots.

This species was formerly considered to belong to the same species as Plumbeous Vireo and Blue-headed Vireo. At that time, this complex of species was referred to as the "Solitary Vireo".

This vireo is named after ornithologist John Cassin.

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