By Austin Young
The Call to Action
Wood Thrush (Hylocichla mustelina) is one among many North American bird species expressing substantial population declines in recent decades. We are in urgent need of ways to understand their migratory pathway, survival across years, and the relative importance of the habitats that these birds use throughout the calendar year, i.e. during the breeding season, migration period, and overwintering months.
Check out the following link to watch a really cool interactive map of GPS-tagged Wood Thrushes migrating thousands of miles between breeding and overwintering locations.
In fact, simply knowing where populations go and how the birds use habitats across a broad landscape remains one of the primary constraints to successful bird conservation. Without knowledge of where individual birds go and how individuals use habitats in different places throughout the year, we cannot address the issues that are driving the declines of many species.
From late May through June 2024, we deployed 10 radio transmitters on Wood Thrushes breeding in the Tennessee River Gorge. This effort is part of an international, collaborative Wood Thrush tracking project that seeks to increase our knowledge of this secretive songbird. Tracking these birds could guide the conservation and management of this still common, but declining species as well as guide our management of the forests that these birds are using, both locally in the river gorge and beyond.
Capturing Wood Thrushes
It’s June and the early summer air is warm and humid as we hike up the side of the Tennessee River Gorge in the predawn hours. As we ascend the canyon with our gear, we hear several Wood Thrushes singing from the forest canopy a few hundred yards ahead. By the way, every spring and summer male Wood Thrushes, and hundreds of other North American passerines, sing loudly and proudly from a comfortable perch in order to attract prospective females that are in search of a mate and to defend their breeding territories from other males. As a result of this behavior, this is also the time of the year that secretive woodland species, like the Wood Thrush, are most readily found and captured.
Fast forward a half hour, we have now identified the specific territory of one of the singing thrushes. Using headlamps, we set up a mist net and a pair of Bluetooth speakers. We play a few Wood Thrush songs from the speakers and within minutes, a male Wood Thrush has come down from his canopy-level perch to check out the sounds from the speakers. In doing so, he has flown into the net!
As an important aside, we do not recommend using playback often for birding or photography. Here is a helpful article outlining the responsible use of audio playback in the field.
With the bird in hand, we unload our backpacks and set up a temporary bird banding station on the forest floor. One of the bird banders on the team attaches a lightweight, aluminum leg band to the bird that comprises a unique alpha-numeric code that functions as a “name” for that individual Wood Thrush. Next, we record a series of measurements, including the age and sex of the bird. In this case, we have a second-year male Wood Thrush. Meaning that this is a male in it’s second-calendar year. This individual was born in the summer of 2023, and during the subsequent autumn he migrated south to Central America or southeastern Mexico, and has now migrated back north to breed the spring of 2024.
Attaching the Transmitter
The bird is now ready to be fitted with a radio transmitter tag. This tag is less than 3% of the total body weight, and itself weighs 1.2 grams, on average. The tag is attached to the bird using a well-known method called the leg-loop harness. The leg-loop harness is similar to a rock-climbing harness worn around a climber’s thighs and waist. The tag itself sits on the rump of the bird in fanny-pack fashion. Once the tag is appropriately attached to the bird, we admire the bird one last time before it silently flies back into the dense, dark forest from where we captured it.
A tagged Wood Thrush (left) perched in thick woodlands of a predawn forest. The radio transmitter tag is the white blob on the bird’s back. The antenna noticeably extends down the back along the center of the tail.
The radio transmitter (right) comprises a battery-powered unit that emits radio waves with a long antenna flowing out the back. The two loops on either side of the transmitter go around the legs of the bird. This method ultimately positions the transmitter on the lower back of the Wood Thrush.
Tracking the Wood Thrush
Finally, we pack up the gear and make our way back to the office. The bird we tagged earlier this morning can be seen on the Motus project website along with the other Wood Thrushes tagged in Tennessee. Now we sit and wait! The tag emits a radio frequency that can be detected by Motus towers around the globe. This includes our tower, as pictured here. By using our local tower, we can assess when the tagged birds leave their breeding sites and how many come back to the same area next year! In general, however, when a tagged bird is detected by a tower, that is a data point. Collectively, the data points tell a story that explains to us the extensive movement and survival information about Wood Thrushes in addition to some details regarding the forests they use. It is also important to recognize that if a bird is not detected by an existing Motus tower, this is also a data point because it tells us where Wood Thrushes are not.
Click here for a map of all Motus towers to see the extensive network of towers across the globe!
Importantly, you can track our locally tagged Wood Thrushes, and others from around the state in Tennessee, by visiting this link to the Motus project.
A trained and licensed bird bander holding a tagged Wood Thrush.
What We Hope to Learn
We hope to discover where our local Wood Thrushes migrate to in the winter and the routes they take. We have a general idea of where they go during the migration periods and winter months, but in reality we do not know specifically where most Wood Thrushes go and what habitat they use during that time frame. We also don’t know how long they stay in those parts of the world and how many of them survive into the next year.
By collecting movement data from these birds, we are informing global conservation of the forests they use and our local forests on Tennessee River Gorge Trust lands. Here at TRGT, we seek to conserve a healthy forest for generations. Learning more about the iconic, but declining, Wood Thrush may provide insights we need to manage the forest to be healthy and sustainable for the current generations of our community and all who use it.
Photo Credit: Kevin Livingood