U.S. Fish & Wildlife Service

Sacramento Fish & Wildlife OfficeServing the people, conserving the fish, wildlife, and plants of California

A Unit Of The Pacific Southwest Region

Kids' Species Information

Least Bell's Vireo

Least Bell's Vireo


Endangered. This means the species is in danger of dying out. We are working to prevent that. Read a .

The Least Bell's Vireo is 1 of 4 subspecies of the Bell's Vireo species. The other 3 are not endangered.


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. The pattern is also common in fish.

It's hard to see little birds like vireos that spend most of their time in trees and bushes. So we often use bird songs and calls to identify them.


Insects and spiders.


Dense shrubs and small trees along rivers and streams.


First eggs are laid in April. The average number of eggs is four. See photo of baby vireo (pictured left) that has been removed from the nest for banding.

Young birds leave the nest 10-12 days after hatching. This is called fledging.

The parents immediately make a new nest and raise another set of babies. Each set is called a brood. Each couple may have up to four broods per year.


Least Bell's Vireos winter in Southern Mexico. They head south by September and come back in March.


Other birds such as scrub jays and hawks. Coyotes, opossums ("possums"), raccoons, rats, house cats and other small mammals. Snakes.

Least Bell's Vireos build their nests about 1 meter above the ground. (About 3¼ feet) This makes them vulnerable. Predators like coyotes couldn't reach high nests.


The Least Bell's Vireo once was common in the Central Valley. The removal of 90% of riparian habitat forced most of them out. Before habitat was restored at San Joaquin River National Wildlife Refuge, the last confirmed breeding was in 1919.

By the 1940's, birders could no longer hear Least Bell's Vireos in the Valley at all. Searches in the 1970's and 1980's also came up empty-handed. The species continued to hold on in Southern California and Mexico.

In 2005, a pair nested at the San Joaquin River refuge. They came back in 2006.


Least Bell's Vireos are musical, chatty birds. Some males have up to 15 different songs. Hear a song by hitting the play button. (635 KB WAV file)


Some birds lay their eggs in other birds' nests. Cowbirds lay eggs in Least Bell's Vireo nests. The vireo parents feed the baby cowbirds. This reduces the amount of food for the baby vireos.

Other threats: Loss of habitat. Killing of vireos by cats.


There are many things you can do to protect birds. Here is some information on migratory bird conservation. It is about migratory song birds. But much of it applies to all birds.

Keep your cat inside. Cats kill millions of birds per year. Even well-fed cats kill birds. It is just their nature to hunt. Living indoors is also much safer for the cats themselves.

When you go to the beach, pay attention to signs warning you that birds are nesting. Many shore birds nest right on the beach. They are easily disturbed. Don't let your dog chase or bark at them.

Whenever you go to natural areas, observe any signs telling you how to protect wildlife and plants.

See What You Can Do to Help Wildlife and Plants (201 KB PDF) for more ideas.


You may have trouble seeing a least bell's vireo. But wherever you live, you can watch birds. See the American Birding Association's Young Birders' Home Page

Photo credits: Po-Hon Liu

Words to Learn

Scientists who study birds are called ornithologists.

Ornithologists call the whole Bell's Vireo species Vireo bellii. Scientific names are in Latin or Greek.

The Least Bell's Vireo is a subspecies of Bell's Vireo. Ornithologists add pusillus, which means very small in Latin, to the scientific name. This makes the species' full scientific name Vireo bellii pusillus.

Least is not a put down. It just means that this is the smallest subspecies of Bell's Vireo.

Vireos are in the Vireonidae family.

Birds that lay eggs in another bird's nest are called brood parasites.

The colors and patterns of a bird's feathers are called plumage.

The common names of bird species are capitalized. So we write Least Bell's Vireo. But if you are writing about vireos in general, you should use lower case. Learn more from this Wikipedia article.

See a basic bird diagram. (483 KB PDF)

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Last updated: November 29, 2017