Homes for Native Songbirds

Endangered vireos build first nests in restored habitat

California’s dwindling population of least Bell’s vireo – a bird many Californians haven’t even heard of – might have vanished forever without the dedication of people like Tracie Nelson.

When this tiny gray songbird was placed on the endangered species list in 1986, it barely made the news, even though its future seemed bleak. At the time, there were just 300 known breeding pairs left in the state of California. California and northern Baja California are the least Bell’s vireo’s only known breeding habitats.

Least Bell’s vireo. Photo by Peterson B Moose, U.S. Fish and Wildlife Service.

The trill of a vireo is distinct and recognizable, and these birds were once so abundant that the oldest living generation of farmers in the Central Valley can still remember their song.

Today habitat restoration projects like River Partners’ Hollenbeck Canyon Wildlife Area site in San Diego County have given least Bell’s vireos a fighting chance. Biologist and Site Manager Tracie Nelson has championed the project since its rough start during the harshest years of the recent California drought. Today there is proof of success: Two least Bell’s vireo nests in shrubs planted by River Partners.

Nelson says at least one individual least Bell’s vireo was sighted at Hollenbeck last year, and others have also been seen at nearby Rancho Jamul. The nests are an encouraging sign that the birds are moving in and making Hollenbeck their home.

“Because the habitat has been expanded, they’re moving up the stream as the streamside habitat is maturing,” says Nelson. “That habitat wasn’t suitable until the restoration occurred.”

Restoration Project at Dulzura Creek in the Hollenbeck Canyon Wildlife Area.

Ecological Reserve and Wildlife Area

On May 19th, 2020 Kevin Clark from the San Diego Natural History Museum was conducting a bird survey when he discovered the two vireo pairs nesting in the restoration site. One pair was still actively building their nest in a planted elderberry. The second pair had completed their nest in a planted laurel sumac and were incubating three of their own eggs and one cowbird egg.

Cowbirds are obligate brood parasites that lay their eggs in the nests of other birds. Clark removed the cowbird egg to give the vireos a better chance at successfully fledging all of their own young.

Unfortunately when Clark returned on June 1, both nests had fallen victim to another threat:  depredation by other native birds. This is a common fate for songbird nests, and many species display an impressive resilience by immediately building another nest. Researchers have documented Bell’s vireos attempting up to seven nests in a single season. We hope and expect the pairs at Hollenbeck will do the same.

Least Bell’s vireo nest containing three vireo eggs and a cowbird egg. Photo by Kevin Clark.

Shrubs like elderberry and laurel sumac make ideal nesting sites, but they also attract insects that make good vireo food. River Partners chose these species and the other components of the plant palette for their broad mixture of characteristics.

“I saw one of the vireos sing from perched at the top of a planted sycamore,” says River Partners Ecologist Emma Havstad, “then collect nesting material from a planted mule fat, and carry it into the planted elderberry where the nest is being built.” These behaviors are strong indications that the new habitat is high quality, and likely to attract a range of important species.

Least Bell’s vireo nest at the Hollenbeck Wildlife Area. Photo by Kevin Clark.

River Partners restored the Rancho Jamul and Hollenbeck site with the help of the Wildlife Conservation Board (WCB) and California Department of Fish and Wildlife. Planted in 2013, this was one of River Partners’ more challenging sites. The early planting got off to a slow start as it struggled through several drought years. “Our soils are dry there are not a lot of organics,” says Nelson. “It does take a little more to get things going.”

As the end of the project’s timeline approached, it became clear that more funds were needed to realize River Partners’ vision for restoration of the area. River Partners got help from WCB and executive director John Donnelly, who believed in Rancho Jamul’s potential and provided us with much-needed financial backing and support. WCB project manager Don Crocker and CDFW also gave critical support and guidance that helped made the Rancho Jamul success story possible.

Thanks to habitat restoration efforts like the one at the Hollenbeck Canyon Wildlife Area, the least Bell’s vireo population has moved towards a slow but steady recovery. The birds are also present on River Partners’ Otay River Delta restoration site.

During his bird survey, Clark also spotted two yellow-breasted chats and a yellow warbler at Hollenbeck Canyon. The presence of other songbirds is another hopeful sign that maturing vegetation on our restored riparian landscape is providing new homes for fragile wildlife.