Bell's Vireo is a small, insectivorous, neotropical migrant that breeds in the central and southwestern United States and northern Mexico and winters primarily in central and southern Mexico and Baja California. Discovered by John James Audubon and named for his friend and companion, John Bell, on Audubon's 1843 expedition to the Missouri River, the “greenlet” (as Bell's Vireo is also known) inhabits dense vegetation characteristic of early successional stages, or any successional stage exhibiting dense understory vegetation—riparian areas, brushy fields, young second-growth forest or woodland, and mesquite brushlands.
Although drab in appearance with no plumage differences between the sexes, Bell's Vireo's song is unmistakable if unmusical: a scratchy 2-phrase “cheedle cheedle chee? Cheedle cheedle chew!” Within a breeding season, males are territorial and most pairs are monogamous, although some males and females may switch mates between nest attempts. Males and females share nest duties, and readily renest following unsuccessful attempts until they fledge young or reach the end of the breeding season.
Like other vireos, Bell's Vireos are heavily parasitized by the Brown-headed Cowbird (Molothrus ater), and parasitism has exacerbated declines resulting from habitat loss and degradation throughout the species' range. Since the Least Bell's Vireo was listed as Endangered in California in 1986, an upsurge of research has expanded our knowledge of the breeding ecology and demography of Bell's Vireos, particularly with regard to parasite-host interactions. Considered a flagship species representative of many low shrub-nesting species, this vireo is increasingly the subject of research and conservation promoting protection of avian biodiversity.