The (Last but Not) Least Bell's Vireo

By: Joelle Saute

Photo by Brian E. Small

Have You Seen This Bird?

If this bird looks at all familiar, chances are you have probably seen it. The Least Bell's Vireo is a migratory songbird that resides in California and Baja California. It is a subspecies of the Bell's Vireo.

Although they might not have many distinctively attractive aspects, this little bird is still recognizable. The head is ashy grey with pure white under wing coverts. The breast can be slightly brown or white with a hint of yellow on the sides of the body under the wings.  

If you have spotted this insectivore, it was most likely while you were in a riparian habitat. This is the only habitat in which they breed. Riparian habitats are located along bodies of water, especially where the soil is damp and sandy. There are many niches within a riparian ecosystem, but these birds tend to stay in the lower canopies, especially in willows. They also need dense shrubs in order to protect their nests. 

Interestingly enough, this type of ecosystem actually supports more bird species than any other type of habitat in California. Sadly, only about 10% of these areas that were present in the 1850's remain in California. 

Riparian Habitat
Photo from Sharing the Horizons 

What is the Least Bell's Vireo Like?

For such a small bird, they can really travel. The distance from their wintering to breeding ground is about 2,000 miles. They reach the breeding ground in Southern California around mid-March.

They can also live up to 7 years. Unfortunately, a large percentage of the young die before they become 1. 

Since they are indeed songbirds, this is the main way in which males establish their .5-.75 acre territory. Each male has anywhere from 5 to 15 unique songs.

Check out what their song sounds like!

 The females are in charge of choosing the nesting site. Once one is chosen they build their nest out of spider webs, bark, and various other materials. The nest is shaped like an open cup.

Nest with a Vireo inside.
Photo from Arkive

They lay the eggs in the nest a few days after completion of construction. The 3-4 eggs incubate for 14 days. The parents will care for the hatchlings for about 2 weeks after they have fledged. Each breeding pair will fledge as many as 5 nests per breeding season. This season ends around September.

                                            What's Changed?

Present and Historical range
 of the Least Bell's Vireo
Map from US Fish and Wildlife
 Recovery Plan

The Least Bell's Vireo has been found in less and less areas throughout the state. The largest population that still remains can be found in San Diego County. This is a large shift from its historical range along most of the southern coast and all throughout central California. The vireo does still winter in Baja California, but the range they occupy is significantly smaller.

Their populations began to noticeably decline after 1940. By 1986 there were only about 300 pairs found in California. 

They're In Luck!

After being federally listed as endangered in the Endangered Species Act (ESA) on May 2, 1986, a recovery plan was formulated. The plan was published on May 6, 1998, but action was taken to protect critical habitats in 1994 . This plan proved to be a success because by 1996 there were 1,346 pairs of Least Bell's Vireos in California. They have even expanded their regions in some areas they had left. In order for these birds to be delisted, they need to have stable populations of several hundred breeding pairs in the Salinas River, San Joaquin Valley, and Sacramento Valley for at least 5 years. Conversely, the IUCN Red List only has them listed as Near  Threatened. 

Why Were They Listed?

Brown-headed Cowbird
Photo from Audubon

As with a vast majority of species on the ESA list, the main cause of the Least Bell's Vireo population decline was a result of habitat destruction. Riparian habitats were removed to provide land for agriculture, livestock grazing, commercial development, and many other human centered purposes. Sadly, this was not the only reason for their decline. In fact, another bird, the Brown-headed Cowbird, caused catastrophic destruction for the Least Bell's Vireo. Cowbirds are known as brood parasites. These birds lay their eggs in the nests of other birds and have the adults of those nests raise their young. The cowbird eggs hatch before the Least Bell's Vireo eggs and thus gain a majority of the food from the adults, leaving the other hatchlings to starve and die.  Cowbirds have even been expanding their ranges, increasing their harm towards the endangered species. To add to the intensity of the issue, the Least Bell's Vireos are also currently living in smaller populations so this increases their risk of extirpation. 

Example of Brood Parasitism from the
 Brown- headed Cowbird (2 large brown spotted eggs) in a Least Bell's Vireo nest.
Photo from Arkive

What's Being Done?
The main course of action currently being done is in regards to the riparian ecosystem. Since this is one of the most critically endangered ecosystems in the US, efforts are being placed to conserve what remains as well as improve the conditions.

Riparian Habitat Restoration
Photo from Enviornmental
 Collaborative

There are also efforts in regards to removing cowbirds from the vicinity of Least Bell's Vireo habitats. Some of the less intense measures being taken include developing management plans for the remaining populations of the vireos, controlling non-native plant species that pose some type of risk to the nesting areas, reintroducing vireos to locations they used to occupy, and emphasizing public education of the issue at hand. The full recovery plan provides more details on these specific efforts. 

And You Can Help!
By donating to either the Audubon Society or the Center for Biological Diversity you can aid in the efforts to preserve this delicate bird. 

And if money is not an option, you can always choose to educate others on the issue. 
As the Audubon Society says, 
"Spread the word. It's the least you can do."

*This species still has a long way to go!*

For Additional Information please visit these helpful sites:
California Partners in Flight Riparian Bird Conservation Plan
Interesting Article about their breeding in the Central Valley
Want to get involved with bird watching?


References

"Bell's Vireo." , Life History, All About Birds. N.p., n.d. Web. 16 Nov. 2015.

Least Bell's Vireo. Center For Biological Diversity, n.d. Web. 16 Nov. 2015.

Patten, Michael A. "Least Bell's Vireo." US Bureau of Land Management (n.d.): n. pag. Print.

"Species Profile for Least Bell's Vireo (Vireo Bellii Pusillus)." Species Profile for Least Bell's Vireo (Vireo Bellii Pusillus). N.p., n.d. Web. 16 Nov. 2015.