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Bird Bio: Cassin’s Vireo

Bird Bio: Cassin’s VireoCAVI
By: Amanda Cornell, KBO Field Intern

The Cassin’s Vireo (Vireo cassinii) is one of three vireo species that were once lumped together as the “Solitary Vireo.” It is a small, greenish-gray bird that can be distinguished from other vireos by its white eye rings and lores, which look like a pair of “spectacles” across the top of the beak. The Cassin’s Vireo diet consists largely of insects gleaned from forest foliage. These birds normally breed in conifer/mixed conifer hardwood forests from British Columbia to California and Nevada; they spend their winters between southern Arizona and Guatemala.

The Cassin’s Vireo, known as the “question and answer bird”, has a distinctive, two-part, four-note song, which makes it easy to identify and track in the field. He is the only bird who sounds like he is asking himself a question, then answering himself, too. Male Cassin’s Vireos are also known for singing from their nests, a habit which is appreciated by KBO field interns who are trying to document nesting habits for a study about how prescribed burning within riparian areas affects bird nesting success.

Although populations of Cassin’s Vireo are widespread, they are unevenly distributed throughout the forest; the birds tend to prefer warm, dry forests and usually stay away from cool and open areas. Perhaps the biggest threat to Cassin’s Vireo is their nests being heavily parasitized by Brown-Headed Cowbirds. This parasitism reduces the number of eggs a female vireo lays, decreases the hatching rate of vireo eggs, and increases the rate of starvation of vireo nestlings through competition with the larger cowbird nestlings. However, despite the pressures of nest parasitism, Cassin’s Vireo populations have remained stable, and some have actually increased in recent years.

This article appears in KBO’s Summer 2007 newsletter. 


Birds of Oregon edited by D.B. Marshall, M.G. Hunter, & A.L. Contreras; The Sibley Guide to Bird Life and Behavior by D.A. Sibley.