Overview
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.
Vireos (Vireonidae)
ORDER
The vireos are one of the one hundred eighteen families of birds in the order PASSERIFORMES (pronounced pas-ser-i-FOR-meez); a large taxonomic order that also includes the bulbuls, dippers, and gnatcatchers.
FAMILY TAXONOMY
A family mostly restricted to the New World, the Vireonidae (pronounced vir-eh-ON-uh-dee) is composed of fifty-nine species of vireos in six genera.
NORTH AMERICA
Thirty-seven species of vireos in four genera occur in North America. In addition to various vireo species, the Neotropical peppershrikes and greenlets are also members of this family.
KNOWN FOR
Like most members of the Vireonidae, the Red-eyed Vireo is known for its habit of constantly singing throughout the day during the breeding season.
PHYSICAL
Vireos are small birds with rather stocky heads, medium length tails, and fairly long wings. They have medium length legs and feet, and rather strong medium length bills with a small hook at the tip.
COLORATION
Members of the vireo family show green, brown, or gray upperparts, and white or yellow underparts. All show facial markings such as spectacles, eyebrows, and eyelines on gray or bluish heads, and some species have light-colored wingbars.
GEOGRAPHIC HABITAT
Various members of the Vireonidae occupy most types of forest and scrub habitats in North America including pinyon-juniper forest (the Gray and Plumbeous Vireos), desert riparian zones (the Warbling Vireo), and mangroves of southern Florida (Black-whiskered Vireo). Members of the family either occur high up in the trees, or low down in thick scrubby vegetation.
MIGRATION
Most vireos are long distance migrants to Central and South America.
HABITS
Vireos do not form flocks, nor do they nest in colonies, although they readily join mixed species flocks during migration and on their wintering grounds. Vireos typically forage by slowly moving through the vegetation to glean larvae and other invertebrates from the leaves and branches, often smacking a prey item on a branch to kill it. Birds that sing slow phrases generally forage high up in the trees, while those that sing rapid chattering songs skulk in the low, thick vegetation.
CONSERVATION
Although no vireo species in North America is endangered, a distinct subspecies, the “Western Warbling Vireo” has declined due to destruction of the southwestern riparian habitats in which it lives.
INTERESTING FACTS
The Cassin’s, Plumbeous, and Blue-headed Vireos were at one time thought to be distinct subspecies of the same species, the Solitary Vireo. They were split after studies demonstrated that their plumages, songs, and habitat use differed enough to rank them as separate species.