We Now Know Where One Imperiled St. Croix Valley Bird Spends the Winter

Scientists uncover new information about the migration of the musical wood thrush.




2 minute read

“This is the only bird whose note affects me like music. It lifts and exhilarates me. It is inspiring. It changes all hours to an eternal morning.” – Henry David Thoreau about the wood thrush, 1853

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Most people probably haven’t seen a wood thrush, but might know its sweet song. The birds live deep in forests, and while they won’t likely show up at backyard bird feeders, their calls are some of the prettiest in the wild kingdom.

The song has become harder to hear in recent decades, silenced by the loss of its forest habitat. Deforestation opens the door for increased harm from a parasitic bird species. Cowbirds – which lay their eggs in other birds’ nests and trick the mothers into raising cowbird chicks – live in open country, and the more open country there is, the harder it is for thrushes to hide.

“As forests are cut into smaller fragments, it apparently becomes easier for cowbirds to penetrate these small woodlots and find more of the thrush nests,” the Audubon Society says.

There are now about half as many wood thrushes as there were 50 years ago.

With the bird in peril, an ambitious research project was launched in the St. Croix River Valley last summer to help understand its habitat needs. Scientists from the Smithsonian Institute captured wood thrushes at Carpenter Nature Center in Hastings and at Warner Nature Center in Marine on St. Croix. They then placed bird-sized backpacks on them, which contained GPS recorders that could document the birds’ migration.

Now, the first data has been retrieved. Because the backpacks are too small to carry transmitters, the only way to find out where the bird has been is to recapture it and download the coordinates. The researchers recently did just that, and found out that the bird had traveled to wooded highlands in southern Mexico and back since last summer, a round-trip journey of more than 3,600 miles.

This type of relationship between the St. Croix Valley and Central America is the basis for the Sister Park agreement signed two years ago between National Parks in the Upper Midwest and on Costa Rica’s Osa Peninsula, which is another hot wintering ground for the migratory bird species that nest along the St. Croix River.

Even though wood thrushes don’t breed in Central America, and thus don’t have to fear cowbirds interfering with their nests, the impacts of habitat loss can be felt even greater. While there are 1.3 million square miles in their summer habitat in the United States and Canada, all those birds need to find somewhere to spend the winter in just 185,000 square miles down south. With the birds facing shrinking homes on both ends of their migration, this new research could help guide efforts to preserve their population both here and on the narrow isthmus between Mexico and South  America.

And if that happens, everyone’s ears ought to be happier.