The least Bell's vireo (Vireo bellii pusillus) is federally listed as endangered. This small songbird -- it weighs less than an empty aluminum soda can -- breeds from April to July around San Diego County, but in the winter, it migrates southward to the Cape Region of Baja California.

At least that's what biologists think. But USGS Western Ecological Research Center ornithologist Barbara Kus wants to make sure. 

Kus and her research group monitors the population trends and demography of the least Bell's vireo, surveying riparian habitat around the San Luis Rey River, Marine Corps Base Camp Pendleton and the San Diego River.

But to really understand how to help the recovery of this endangered species, biologist need to know all the factors that affect vireos throughout the entire year. While their southern California surveys are extensive, discovering where vireos are actually spending winters will shed light on the factors that affect their health and survival before they return to the U.S. to breed.

"To put all this together, we need to know their whole annual cycle," says Kus, who is based out of the WERC San Diego Field Station. "We've banded several thousand individual birds since 1987, but we've only found three in our winter surveys in Baja California. We know the birds survive because more than half of them come back to San Diego, so where are they actually spending their wintering days in Mexico?"

Kus and her team will soon be visiting new sites in Baja California, surveying riparian habitats and dry arroyos for these songbirds to look for birds that were marked in San Diego. Additionally, the team will capture and band birds in Mexico, in hopes that they will be sighted somewhere in the U.S. during breeding season.

It's a game of "connect the dots" across international boundaries that Kus is happy to play.

-- Ben Young Landis

Photo of a least Bell's vireo in Mexico courtesy of Steve Mlodinow.