The Bell's Vireo ( Vireo bellii ) is a small North American songbird.

The Least Bell's Vireo ( Vireo bellii pusillus ), is an endangered subspecies in Southern California.

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Species that were once common but now are gone or endangered include the Southwestern Willow Flycatcher, Western Yellow-billed Cuckoo, Least Bell's Vireo, and Warbling Vireo.

A study of nests of Bell's Vireo highlighted a potential limitation of these control programs, demonstrating that removal of cowbirds from a site may create an unintended consequence of increasing cowbird productivity on that site, because with fewer cowbirds, fewer parasitized nests are deserted, resulting in greater nest success for cowbirds.

However, the road east of there faced delays from environmentalists over the endangered Least Bell's Vireo songbird, which faced habitat destruction, as well as those concerned with the destruction of homes and businesses for the right of way for the freeway.

A Bell's Vireo, of which the least Bell's Vireo is a subspecies.

However, in July, the U. S. Fish and Wildlife Service expressed concerns regarding the San Diego River crossing, since construction might result in the destruction of the endangered least Bell's Vireo habitat.

# REDIRECT Bell's Vireo

Consideration of Bell's Vireo has been a factor in several land development projects, to protect Least Bell's Vireo habitat.

The decline of the Least Bell's Vireo is mostly due to a loss of riparian habitat and brood parasitism by the Brown-headed Cowbird.

The Bell's Vireo makes a well-camouflaged nest but when found the bird will stand its ground against intruders.

Cowbirds use Bell's Vireo nests as their own nurseries.

Bell's Vireo often uses Western poison oak shrubs or vines as nesting locations.

Cowbirds sometimes lay eggs in Bell's Vireo nests.