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Researchers from the USGS Western Ecological Research Center have reported a rare case of two songbird species cross-breeding in San Diego County -- an odd couple which raised four chicks to maturity.

Biologist Melissa Blundell and lead scientist Barbara Kus observed the successful breeding of a male Least Bell's Vireo (Vireo bellii pusillus) with a female White-eyed Vireo (Vireo griseus) in May 2010 along the San Luis Rey River, near Oceanside, Calif.

Here's some context: while Least Bell's Vireos regularly breed in Southern California and down to Baja California, the White-eyed Vireo is a native of our eastern shores, from Massachusetts down to Texas. And between 1969 and 2009, only eight White-eyed Vireos have been sighted in San Diego County.

Least Bell's Vireo with White-eyed vireo
A male Least Bell’s Vireo (left) building a nest with a female White-eyed Vireo (right) at the San Luis Rey River, Oceanside, San Diego County, Calif. Photo courtesy Lisa D. Allen/USGS.

Based at the WERC San Diego Field Station, Kus and her research group monitors the population trends and genetic health of the Least Bell's Vireo, which is federally listed as endangered. Vireos are small birds -- measuring less than 5 inches in length and weigh less than an empty aluminum soda can.

"This is the first documented case of a White-eyed Vireo breeding in California and appears to be the first successful nesting of a mixed pair of any two vireo species observed in the field," write Kus and Blundell in their report to the Wilson Journal of Ornithology.

Genetic evidence shows that the Least Bell's Vireo and White-eyed Vireo are more closely related to each other than the 40 some species of vireos known. Additionally, the two species have similar breeding behaviors, like shared nest building by males and females.

So was this a case of the endangered Least Bell's Vireo not being able to find a mate?

"It's likely it was the other way around," says Kus. "The White-eyed female was way out of her range." Researchers think that the wayward bride was in a situation where there were no potential mates of her own species -- leading her to accept courtship from a local species.

DNA samples collected from all four hybrid chicks are being analyzed to learn more about their genetics. All four chicks were observed to survive past fledging, and since Kus and her colleagues conduct intensive surveys and monitoring of Least Bell's Vireos throughout Southern California annually, they should be able to detect these hybirds if they survive and return to breed in Oceanside in future years.

-- Ben Young Landis

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