Point Blue Ecologists Use Novel Tracking Technology to Unravel Mysterious Migratory Patterns of Swainson’s Thrushes

A decoy used to lure Swainson’s thrushes into the mist net just visible in the background. Ecologists used commercially-available purple martin decoys, which co-investigator Will Richardson from Tahoe Institute for Natural Science then painted to attract and fool the thrushes.

Point Blue Conservation Science / Diana Humple

May 2021 - Where do different populations of a migratory songbird go when they migrate? This mystery was first put forth by Audubon scientists over a century ago. The answer might hold the key to protecting declining populations of a once-common species, the Swainson’s thrush. The thrushes are common along the central coast of California. They're frequently seen in stream-side areas of Point Reyes National Seashore and Golden Gate National Recreation Area. But just 250 kilometers away, in seemingly suitable habitat in the Cascade-Sierra mountain ranges, they are bafflingly rare. Point Blue Conservation Science ecologists at the Palomarin Field Station, located in Point Reyes, wanted to find out why. So, in 2014 they began a research collaboration with the Tahoe Institute for Natural Science.

Recent advances in tracking technology have finally enabled this research to happen. The team captured the 30-gram birds in mist nets and fitted them with tracking tags small enough for them to carry. Ecologists then released the birds and returned to the release site a year later, after the birds' marathon journeys to the tropics and back. Their aim was to recapture them, remove the tags, and acquire the data stored on the tags in order to determine where the birds had been!

Study co-author Ryan Burnett holds the first Swainson's thrush captured in the study. After capture, the birds were fitted with tiny radio tracking tags and then released. A year later, the ecologists returned to the release site to re-capture the birds and recover the tags.

Point Blue Conservation Science / Diana Humple

This process was a shared adventure full of disappointment and elation. It tested the team’s collective experience in field ornithology and bird behavior, as well as their patience and persistence. “We had to think like a thrush, and sometimes act like one. We spent many hours lying on our backs, crouched in a bush, sitting in standing water, letting mosquitoes bite rather than risk swatting them and spooking the bird we were seeking,” says Point Blue Avian ecologist Diana Humple.

What the ecologists discovered in the 12-months of data contained in those tiny bird backpacks surprised them. Swainson’s thrushes from all three breeding regions studied—the San Francisco Bay Area coast, Lassen, and the Tahoe area—migrated to distinctly different wintering regions in Latin America. San Francisco Bay Area birds migrate predominantly to wintering grounds in Western Mexico. Meanwhile, Lassen birds head to Central America, and nearby Tahoe birds all the way to South America! In other words, Swainson’s thrushes breeding in the Cascade-Sierra migrate longer distances. They are also exposed to a greater degree of relative forest loss in both their breeding and wintering regions. As a result, the authors suggest that these birds are currently more vulnerable to environmental change than coastal (SF Bay Area) breeding birds.

Left: A Swainson’s thrush wearing a light-level geolocator tag that has just been affixed prior to the bird’s release and its year-round migration. Right: Two recovered tags awaiting analysis to determine where the two birds carrying them went during their last year’s out-and-back-again migration.

Point Blue Conservation Science / Diana Humple

Estimated wintering destinations, relative forest loss (from 2000–2017; Hansen et al. 2013, Science), and migration distances for three breeding populations of Swainson’s Thrush in California (see Figure 1 in paper for more details).

Point Blue Conservation Science

“As with many migratory species, enhancing populations of Cascade-Sierra Swainson’s Thrushes will require actions across more than one region and country. In this time of great environmental change, preserving the genetic diversity that exists within different populations of species will be critical to allowing them to adapt to a novel world, and knowledge of the vulnerability of populations will be critical to directing limited resources to conserve them,” Says Humple.

Further reading:

Adapted from a blog post for ‘Nature Research’ by Point Blue’s Palomarin Field Station Program Lead, Diana Humple, and Point Blue’s Sierra Nevada Group Director, Ryan Burnett, entitled Unraveling a Migration and Conservation Mystery.

Read a summary of the publication in this Publication Brief.

Explore the full publication:

Humple, D. L., Cormier, R. L., Richardson, T. W., Burnett, R. D., Seavy, N. E., Dybala, K. E., Gardali, T. (2020). Migration tracking reveals geographic variation in the vulnerability of a Nearctic-Neotropical migrant bird. Scientific Reports 10:5483.

Visit Point Blue's website to learn more about Point Blue’s Palomarin Field Station located in the southern end of Point Reyes National Seashore, more long-term bird monitoring studies, what they've learned about the migratory destinations of other local species, and opportunities for visiting once they reopen to the public.

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