Tiny San Diego Songbirds Take Winter Holidays in Baja

Release Date:

When it comes to vacation planning, the least Bell’s vireo (Vireo bellii pusillus) is pretty savvy. USGS Western Ecological Research Center ornithologist Barbara Kus has been surveying this endangered species, trying to determine where the tiny songbird migrates to during the winter season. 

The findings will help U.S. and Mexico governments determine habitat management plans in areas that host vireos and other migratory birds during their various seasonal stops.

Sightings in San Diego this summer have offered some interesting insights.

“We've spotted two birds that we banded on the wintering grounds in southern Baja back in February 2012,” reports Kus, whose lab is located in San Diego.

“One bird is a female, who was banded as a nestling at Camp Pendleton a few years ago. We found her near Miraflores in February, and she is back at Camp Pendleton this year,” says Kus. “The other bird is a male that we banded at San José del Cabo on the wintering grounds. He was recently spotted on the Tijuana River in San Diego County.”

Idyllic, low-key getaways in Baja California Sur, bathed in seabreeze and sunshine? Sign us up, too!

“This is the first time a least Bell's vireo from a known wintering location was detected on the breeding grounds, giving us some information on exactly where the San Diego birds spend the other half of their year,” says Kus, who coordinates her research with the U.S. Fish and Wildlife Service, California Department of Fish and Game, the U.S. Navy and Marine Corps, the U.S. Army Corps of Engineers and others.

USGS researcher holds an endangered least Bell's vireo

A least Bell's vireo in Baja California Sur, with some fresh leg bling. Image Credit: Barbara Kus/USGS.

Kus and her team have banded several thousand individual vireos in San Diego County since 1987, but until this year, they had only found three in their winter surveys in Baja California. They knew the birds survived the migrations, because more than half of them are seen in San Diego in the summer.

It was a matter of time and looking in the right places.

WERC Ecologist Calls in Birds with Recorded Calls

USGS Western Ecological Research Center scientist Suellen Lynn sets up a mist net during the Mexico vireo survey. Image Credit: Barbara Kus/USGS.

“We look at thousands of vireo legs every breeding season, checking for bands,” says Kus. “We’ve finally closed the breeding-and-wintering season loop, at least for these two!”

WERC scientist Suellen Lynn assisted Kus on the 2012 Baja expedition. The Camp Pendleton female was sighted by seasonal biologist Ryan Evans -- a special feat since female least Bell's vireos don't sing and are harder to spot than males. The Tijuana River male was spotted by Arlene Arnold, natural resources specialist with the U.S. Navy.

Least Bell’s vireos -- which weigh less than an empty aluminum soda can -- will breed in San Diego County from April through July, before heading about 1,000 miles southward to kick back in the riverine and dry arroyo habitats.

Learn about the Kus lab’s vireo research here: https://www.usgs.gov/centers/werc/science/population-structure-and-demography-least-bell-s-vireo-and-southwestern-willow

Profile for Dr. Barbara Kus: https://www.usgs.gov/staff-profiles/barbara-kus

Blogpost edited July 17, 2012, with research team mentions.

-- Ben Young Landis

Barb Kus Profile WERC

USGS Western Ecological Research Center scientist Barbara Kus in the field with her quarry: a least Bell's vireo. Image Credit: Suellen Lynn/USGS.