(by Pam Hunt)

Of the 27 Wood Thrushes tagged in NH this past summer, the farthest traveled is a bird that covered roughly 3000 miles from Mt. Wantastiquet in extreme southwestern NH to Costa Rica, arriving at the latter on November 2. But this story isn’t about Wood Thrush #592, he is instead the hook for a brief discussion of what the Motus Wildlife Tracking System can tell us about the non-breeding ecology of thrushes. Sticking with Wood Thrush to start, the same project NH Audubon is involved in during the breeding season has started up in winter, where researchers hope to deploy over 100 tags on this species from Mexico to Costa Rica. In the latter country, they’ve already tagged 23, one of which was back for at least its second winter. This bird was originally captured in November 2023 and spent the entire winter at a site along the Caribbean slope of Costa Rica. It departed north on April 13, 2024, and passed over Louisiana, Georgia, and Pennsylvania in less than a week in early May. No one knows where it ended up and nested, but it was back over western Pennsylvania on September 29, continued south through the Appalachians, and showed up in Belize on October 11. By October 23 it was back in Costa Rica, presumably in the same winter territory, where it was being detected daily through December 13. So yes, in case you weren’t aware, many of “our” migratory birds defend territories during the non-breeding season. Males and females are separate, and because there isn’t a need to feed hungry nestlings the territories are much smaller. Wood Thrushes spend up to six months of each year on their winter territories, compared to four to five here on the breeding grounds. Throw in the time spent migrating, and they ultimately spend less than half their time here in New Hampshire. It’s clearly an important period since this is when breeding occurs, but also highlights the need for studies across the entire annual cycle: threats that occur from September to May are at least as important as those that operate locally, and ignoring them does a disservice to conservation efforts for migratory species.

Two other species of thrushes were detected by Motus towers in Costa Rica in the last month. One was a Veery tagged during migration in New Jersey on September 11. By October 7 it had made its way south along the coast to Georgia and next appeared in Costa Rica on November 28. Veeries winter in South America, however, so this bird didn’t stick around. It passed by four towers in a row that same night: covering 90 miles in 2.5 hours. Lastly, there’s Swainson’s Thrush, a common breeder in northern New Hampshire as well as across Canada and south in the mountains of the western U.S. Over 2300 Swainson’s Thrushes have been fitted with Motus tags across this broad range, making it the second-most-common species tracked using this technology (after Semipalmated Sandpipers). The resulting collection of tracks provides us with a phenomenal picture of this species’ migration. While populations of Swainson’s Thrushes from the Pacific Northwest and California spend the winter from Mexico to Costa Rica, those from most of the range continue to the Andes of South America. If you look at migration tracks on a map, you can see that birds generally avoid the deserts of the Great Basin and Southwest. This is known as a “migratory divide” – a geographic feature that splits a population into two or more groups that show different migratory behavior. In a fun coincidence, a Swainson’s Thrush tagged in British Columbia passed over the very same towers at the previously mentioned Veery, on the same evening, and traveled at the same speed (35-40 mph). For all we know they were in the same flock, and it’s sobering to realize that a Veery from the Northeast and Swainson’s Thrush from the Northwest shared some airspace over Costa Rica on their way to South America.

Article cover photo by Pam Hunt of a Wood Thrush signing.