From McCarthy to Bolivia: Chronicle of a Swainson’s Thrush

Swainson’s thrush tagged with a transmitter.

Photo courtesy of National Park Service. 

National Park Service Biologist Carol McIntyre takes a “life cycle stewardship approach” to migratory birds that breed in Alaska’s national parks. What that means is her interest is not just in what the birds are doing when they come here to breed, but also “across the entire year and across the entire life of the bird.” This can help scientists to identify what challenges the birds face, and what habitat is critical for them, during all the stages of their travels. McIntyre talked about her research on Swainson’s thrush migration during a WISE Science Lecture Series talk about bird research in Wrangell St Elias National Park earlier this year. 

The Swainson’s thrush is a relatively inconspicuous robin-sized bird with a breathtaking song. During all times of year, it sticks strictly to forest habitat, often at the edge of grasslands or other open areas. Although the general Swainson’s thrush winter range has been known, it was not known specifically where within that range local Swainson's thrushes go. McIntyre has been researching this question since 2015, starting with a study on the birds in Denali National Park. 

Swainson’s thrushes migrate alone during the night, which makes visual observation difficult. A series of tagging studies was needed to learn more about their movements. Adult males are easier to catch and place tags on because their territorial nature makes them curious about other males’ songs. This makes it possible to lure them in using recorded birdsong.

Rest stop for a Swainson’s thrush en route to South America.

Image courtesy of National Park Service

In 2018, McIntyre’s team outfitted adult male Swainson’s thrushes near McCarthy with transmitters that gave precise information about their migratory routes and stopovers. What she found is that many of them travel 12,000 km to the Andes to spend the winter, and quite a few go to Bolivia. This is in the southern part of their winter range, which means that the birds from the Wrangells come from farther north than many other Swainson’s thrushes and then fly farther south than many others do. This phenomenon is called “leap frog migration.” 

Many of the birds who summer near each other in McCarthy also winter quite close together in Bolivia. This is called “strong migratory connectivity.” This made the birds more relatable to me: How many of us have traveled to a warm climate, and found friends from Alaska eating at the same restaurant? 

The transmitter study gave information not only about the birds’ winter endpoint, but how they got there and where they went along the way. One bird that McIntyre highlighted in her talk spent a month flying hard through the center of Canada until it took a break in Indiana. The bird spent about a week resting in a small patch of trees, surrounded by a mix of woods, farmland, and housing. Then it flew without much rest across the Gulf of Mexico and into southern Mexico. Is that the Swainson’s thrush equivalent of the Seattle Airport? 

I wonder how many people living their lives in that Indiana neighborhood know how precious that little spot of forest is. 

After another short rest in southern Mexico, the thrush arrived at “McCarthy South” in Bolivia (or is McCarthy “Bolivia North?”). McIntyre pointed out that the thrushes spend as much time in the Andes as they do in the Wrangells. She also noted that the forested habitat the birds winter in exists as a strip, with grasslands to the west and increasing agricultural development to the east. This could have significance for conservation of this bird in the future. 

The bird McIntyre followed in her talk did not retrace its steps to find its way back to the Wrangells. The return journey took it to the northern edge of the Amazon in Columbia, the forests of Costa Rica, and the side of a volcano in El Salvador. The bird then took a huge flight and rested in Oklahoma before traveling fairly directly through Canada and back to McCarthy.

It’s tempting to say, “home to McCarthy,” but is it home? As McIntyre pointed out, a McCarthy Swainson’s thrush only spends about 27% of its time there. If we want them to keep coming back, it’s important to know what happens to them for the other 73% of the time. 

A link to the recording of Carol McIntyre and Jeremy Mizel’s bird research talk is currently available at WISE-edu. McIntyre also recommends the Arctic Animal Movement Archive on the website movebank.org.

By Allison Sayer

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