Swainson' s Thrushes, from a local population near Bolinas, CA, spend their winters together in Mexico, according to a new tracking study released by Point Blue Conservation Science, (Point Blue, formerly PRBO). This result is important because it shows how the conservation of habitat for these local populations in California is tightly linked with climate and habitat changes in Mexico, where these birds spend their winters, 1,600 miles away.
The Swainson's Thrush is one of the most melodic of all the songbirds, and can be heard singing now by hikers, walkers, and cyclists enjoying trails near streams throughout the bay area.
Using tiny tags to track a bird's location, biologists from Point Blue have pinpointed the wintering locations of Swainson's Thrushes, tracking them from study areas in Marin County to their wintering homes in central Mexico. Published this week in the journal The Auk, the study illustrates the linkages between nesting and breeding locations of migratory birds. Swainson's Thrushes have lost 95% of available nesting habitat along streams and rivers throughout California. They have also almost completely disappeared from the Sierra, and projections on Point Blue's "Where Will the Birds be" interactive website (choose bird distribution, then riparian, then Swainson's Thrush) illustrates that the Swainson 's Thrush will occur in fewer places in California, given climate change.
During a time when many birds are threatened by climate and land-use change, being able to protect migratory birds on both their breeding grounds and wintering grounds is more important than ever. Science findings like these help guide people who are protecting habitat for these birds to conserve the appropriate locations for the species' entire lifecycle. Point Blue scientists, working out of the Palomarin Field Station, attached small tags that record day length to breeding Swainson's Thrushes in Point Reyes National Seashore and in Marin County Open Space District's Bolinas Lagoon Preserve before they headed south for the winter. When the birds returned the following two springs, 12 tagged birds were safely recaptured, the tags were removed, and the data downloaded to a computer.
"Until now, all we knew was that these birds likely wintered in Mexico or Central America. We're very excited to finally pinpoint where Swainson's Thrushes spend the winter," explained Ren้e Cormier, an avian ecologist at Point Blue and lead-author of the study.
Swainson's Thrushes winter across a large area that includes parts of Mexico, Central America, and South America. However, the results of this study show that rather than dispersing across this wide area, Swainson's Thrushes from a local population winter together in a relatively small area. This result is important because it implies that local conservation of this species in California may be tightly linked with weather patterns and habitat changes in a relatively small portion of the area where they winter.
"We have a good understanding of Swainson's Thrush breeding ecology and annual survival and more recent studies have begun to shed light on their distribution and survival during the winter period in Mexico and Central America." explains Dr. T. Luke George of the Colorado State University. "This study is extremely important because it gives us a much better understanding of the links between the breeding and wintering populations and provides important information for conservation throughout the entire life-cycle."
Operating since 1966, Point Blue's Palomarin Field Station has been one of the premier west-coast locations for long-term studies on birds and their habitats. These data sets that extend over several decades allow scientists to assess human-caused and natural changes over time to guide conservation action in a time of rapid environmental change.
"Today we are facing unprecedented changes in land-use and our climate" explained Ren้e Cormier. "The information in this study will help us understand where our migratory birds may be vulnerable to these changes, and what we can do to help protect them and the ecosystems on which they -- and we -- depend." Co-authors, along with Ren้e Cormier and Diana Humple, were Nat Seavy and Tom Gardali also of Point Blue. The manuscript was published in the journal The Auk.
- Ren้e L. Cormier, Diana L. Humple, Thomas Gardali, Nathaniel E. Seavy. Light-level geolocators reveal strong migratory connectivity and within-winter movements for a coastal California Swainson's Thrush (Catharus ustulatus) population. The Auk, 2013; 130 (2): 283 DOI: 10.1525/auk.2013.12228
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