Cassin's Vireo: Small vireo, olive-gray upperparts, white underparts, pale yellow flanks. Head is gray with white eye-ring that extends to brow. Wings are dark with two white bars. Until the 1990s was classified as the Solitary Vireo, along with the Blue-headed and Plumbeous Vireos.
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.
It is a fearless defender of its nest. Both the male and female will vigorously scold a predator and dive at it. The female often will not leave her nest and sometimes can be picked up off of it by a human observer.
The Cassin's Vireo was formerly lumped as a "Solitary Vireo," along with the Plumbeous and Blue-headed vireos, it is now considered a separate species.
Two subspecies are recognized. One is widespread in western North America from Canada to the northern part of Baja California. The other is found only on the very southern tip of Baja California more than 500 miles away.
A group of vireos are collectively known as a "call" of vireos.
The Cassin's Vireo is native to the United States, Mexico, Guatemala and Canada. This terrestrial bird has a fairly large range of nearly 1.5 million square kilometers within its natural range. The population of Cassin's Vireo is more than 4 million individual birds. In 2000, Cassin's Vireo was rated as Lower Risk; however, this was downgraded to a Lest Concern rating due to its large population and lack of evidence that the population has decreased over the past generations.