The least Bell’s vireo is a small migratory bird. The westernmost of four subspecies of Bell’s vireo, the least Bell’s vireo winters at the southern end of the Baja California Peninsula, Mexico. In mid-March to early April, it migrates north to nest. Today, its breeding range is primarily coastal southern California and northern Baja California, Mexico. It also occurs away from the coastal region at isolated oases in the Mojave, Colorado, and Vizcaíno deserts. Historically, the least Bell’s vireo also ranged throughout the California Central Valley and the central California Coast Ranges. Its breeding habitat is primarily willow-dominated riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian woodlands, although it also forages and sometimes nests in neighboring mulefat scrub, oak woodlands, and chaparral. In the deserts, it is also found in mesquite thickets. The least Bell’s vireo was listed as endangered in 1986 because of historical and ongoing habitat loss and reduced nesting productivity resulting from brood parasitism by brown-headed cowbirds (Molothrus ater). Cowbirds are a North American species that historically did not occur in California. They expanded into California in the early 1900s. Cowbirds are obligate brood parasites; they lay their eggs in the nests of other bird species to be raised by those “host” species, typically at the expense of the host species’ own young. Thanks to conservation efforts, the least Bell’s vireo United States population has increased from 291 territories at the time of listing to more than 10times that number now. Despite the numerical increase, large areas of its historical range remain unoccupied.  

The least Bell's vireo is gray songbird with a distinctive question-and-answer song: Cheetle-cheetle-cheetle-chee? Cheetle-cheetle-cheetle-chew.

Least Bell's vireo have short, rounded wings and a short, straight bill.

Measurements
Length: 4.5 to 5 inches

Least Bell's vireo have a faint white spectacle-like eye-ring, and thin white wingbars. 

The westernmost of four subspecies of Bell’s vireo, the least Bell’s vireo winters at the southern end of the Baja California Peninsula, Mexico. In mid-March to early April, it migrates north to nest. 

Its breeding habitat is primarily willow-dominated riparian riparian
Definition of riparian habitat or riparian areas.

Learn more about riparian woodlands, although it also forages and sometimes nests in neighboring mulefat scrub, oak woodlands, and chaparral. In the deserts, it is also found in mesquite thickets.

Least Bell’s vireos primarily eat insects and spiders gleaned from leaves and branches.

Pairs use plant fibers and spiderweb silk to build an open-cup nest in the fork of a branch. The nest is typically only a few feet from the ground.