Vireo bellii pusillus

Vireo bellii pusillus

Least Bell's Vireo
FWS Focus

Overview

Least Bell's vireos are small birds. They are only 11.5-12.5 centimeters long. (About 4.5 to 5.0 inches) They have short rounded wings and short, straight bills. There is a faint white eye ring. Feathers are mostly gray above and pale below. This is a common protective marking in birds. Seen from below, the bird blends into the clouds. From above, it blends into the landcover.
Characteristics
Overview

The least Bell’s vireo is a small migratory bird. Historically, the least Bell’s vireo also ranged throughout the California Central Valley and the central California Coast ranges. Listed as endangered in 1986 because of historical and ongoing habitat loss, the least Bell’s vireo also suffered reduced nesting productivity because of brood parasitism by brown-headed cowbirds (Molothrus ater). Cowbirds are a North American species that historically did not occur in California, but 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 young of host species. 

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 10 times that number now. Despite the numerical increase, large areas of its historical range remain unoccupied.

Scientific Name

Vireo bellii pusillus
Common Name
Least Bell's Vireo
FWS Category
Birds
Kingdom

Location in Taxonomic Tree

Species

Identification Numbers

TSN:

Characteristics

Characteristic category

Physical Characteristics

Characteristics
Physical Characteristics

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.  

Sound

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

Size & Shape

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

MeasurementsLength: 4.5 to 5 inches

Color & Pattern

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

Characteristic category

Behavior

Characteristics
Behavior

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. 

Characteristic category

Habitat

Characteristics
Habitat

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.

Desert

Arid land with usually sparse vegetation.

Forest

A dense growth of trees and underbrush covering a large tract.

Characteristic category

Food

Characteristics
Food

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

Characteristic category

Lifecycle

Characteristics
Reproduction

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. 

Geography

Characteristics
Range

Today, the least Bell’s vireo 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.

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