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Vireos on the Verge

Why is San Diego's rarest chaparral bird thriving just south of the border?

For the last 75 years, the grey vireosmall, unassuming species of songbird—has declined steeply north of the U.S.-Mexico border in San Diego and Imperial Counties. Today, it remains the rarest bird that breeds in San Diego County’s chaparral habitatThis is due, in partto brown-headed cowbirds, which lay their eggs in vireo nests (called nest parasitism)and scrub-jays, which prey on grey vireo eggs and chicks 

In 2022, The Nat’s Birds and Mammals Department completed four, week-long surveys in northern Baja California. They found that these small birds are persisting in good numbers in both the Juárez and San Pedro Mártir mountain ranges. Grey vireo numbers in Mexico are so good, in fact, that they exceed the current population numbers for the entire state of California.   

Now more questions arise: Why is the gray vireo doing so much better in Mexico than just above the border in the United States? What can we learn from its ecology in Mexico that would aid its conservation in the United States? How many other California species are thriving better south of the border at the southern limits of their range 

Together with our colleagues in Mexico, we’re hoping to answer these questions for the gray vireo in the coming yearsDespite their sharp decline in California, grey vireos are still flying under the radar of most conservation efforts and research, but The Nat has been keeping an eye on them for over two decades. Uncovering Mexico’s population numbers has proven to be an important and enlightening step in our effort to clarify the status of this bird, for which the future is increasingly uncertain.

This research is made possible in part by the Jiji Foundation.

Posted by Cypress Hansen, Science Communications Manager.

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