Endangered

(fws.gov)

U.S. Endangered Species Act; Migratory Bird Act

(animaldiversity.org)

This bird is threatened by habitat loss and degradation as a result of urban and agricultural development, as well as non-native plant species. Brown-headed cowbirds are also an issue, as lay their eggs in Least Bell’s vireo nests which is known as brood parasitism (fws.gov).

They reside in southern Baja California, Mexico during winter, then go up to southern California in spring. They are also located inland in 3 deserts: the Mojave, Colorado, and Vizcaíno (fws.gov).

Least Bell's Vireo Survey Guidelines (fws.gov)

Least Bell's Vireo Minimum Qualifications (fws.gov)

These birds are primarily found in willow-dominated riparian woodlands during breeding time, but they also nest in neighboring mulefat scrub, chaparral, and oak woodlands. In desert habitats, Least Bell’s vireos can also be found in mesquite thickets. Their diets primarily consist of insects and spiders found on nearby plants’ leaves and branches. During nesting, pairs of Least Bell’s vireos will obtain plant fibers and spiderweb silk to make an open-cup style nest in the fork of a branch. These nests are usually only a few feet off the ground due to the short plants in their habitats (fws.gov).

There is a lack of research in how brood parasitism on Least Bell’s vireo can be reduced based on location in a habitat and how movement may help recover their numbers.

Habitat Conservation Plan for the Least Bell's Vireo (federalregister.gov)

Draft Recovery Plan for the Least Bell's Vireo (fws.gov)

Least Bell’s vireos have a white belly with a gray backside. They have a faint white eye ring and thin white wing bars. The wings are short and rounded, and the bill is short and straight. For body length, they are usually 4.5-5 inches (fws.gov). Least Bell’s vireo can look similar to many other vireos or warblers, one in particular being the warbling vireo. The warbling vireo also migrates to southern California and can be seen in the same areas. Warbling vireos are more yellowish-gray and lack the apparent wing bars on a Least Bell’s vireo. They also have a white stripe over the eye, while the Least Bell’s vireo has a white eye ring (allaboutbirds.org).

Least Bell's Vireo Story (Vireo bellii pusillus) | U.S. Fish & Wildlife Service (fws.gov)

Least Bell's Vireo (Vireo bellii pusillus) | U.S. Fish & Wildlife Service (fws.gov)

Bell's Vireo | Animal Diversity Web (animaldiversity.org)

Bell's Vireo Similar Species Comparison | Cornell Lab (allaboutbirds.org)