Overlooked Winter Bird Communities Are Unexpectedly Diverse

Every fall, many of the songbirds that breed in the U.S. clear out for the winter, heading for tropical habitats in Central and South America, but the forests and wetlands they’re vacating won’t be empty during the winter months. Instead, a shift change occurs, as birds that breed in the boreal forest of northern Canada move in to take their place. One third of these “Neotemperate” migrant species are in decline, but the importance of high-quality winter habitat to this vulnerable group of songbirds has long been overlooked. For a new study forthcoming in The Condor: Ornithological Applications, Kristen Dybala, Melanie Truan, and Andrew Engilis, Jr. of UC Davis’s Museum of Wildlife and Fish Biology sought to change that by comparing the summer and winter bird communities of two watersheds in California’s Central Valley. Contrary to expectations, they found that just as many species used the riparian habitats in winter as in summer, and the bird community in winter was actually more diverse.

Temperate riparian areas like the ones in this study are often managed specifically to benefit breeding birds, but the habitat where a songbird spends the winter is also key to its survival, affecting its physical condition, the timing of its migration, and its reproductive success. Dybala, Truan, and Engilis surveyed birds in the lower Cosumnes River and lower Putah Creek watersheds between 2004 and 2012, counting birds along transects in January and February and again during the breeding season in April through June. Their results reveal an unexpectedly diverse winter bird community including a large proportion of declining boreal-breeding species. Managing and restoring riparian habitat to meet birds’ winter needs, in California and elsewhere, is an overlooked conservation opportunity that could make a meaningful difference for vulnerable wildlife.

Initially, Engilis and his colleagues had trouble even getting funding to study temperate winter bird communities, but their perseverance paid off. “We are sure that if similar analyses were done in other regions of the U.S., they would find similar results: habitat conservation and restoration doesn’t just benefit breeding birds, but supports continental populations of boreal-breeding songbirds that require winter habitat for the half of their life spent off the breeding grounds,” says Engilis. “The importance of conservation measures for wintering songbirds seems intuitive but is glossed over or even ignored in regional planning documents, and we hope that this paper provides a framework to rethink the broader implications of habitat restoration, particularly in riparian systems.”

“This work highlights a long overlooked component of the non-breeding ecology and conservation of North America’s migratory landbirds,” agrees Jeff Wells, Chief Scientist of the Boreal Songbird Initiative. “Boreal breeding birds, in particular, become the common winter birds of ecological communities across the U.S., yet most conservation work is focused only on the breeding birds of those regions. This research demonstrates that birds from the boreal forest region are vital parts of wintering bird communities in the U.S. and should no longer be neglected in conservation planning and research.”

Every winter, Neotropical migrants like the Black-headed Grosbeak (left) are replaced by Neotemperate migrants like the Fox Sparrow (right) in the riparian habitat of California's Central Valley. Image credit: A. Engilis, Jr.

Every winter, Neotropical migrants like the Black-headed Grosbeak (left) are replaced by Neotemperate migrants like the Fox Sparrow (right) in the riparian habitat of California’s Central Valley. Image credit: A. Engilis, Jr.

Summer vs. winter: Examining the temporal distribution of avian biodiversity to inform conservation is available at http://www.aoucospubs.org/doi/full/10.1650/CONDOR-15-41.1. Contact: Kristen E. Dybala, kdybala@pointblue.org.

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society.

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