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The Wood Thrush (Hylocichla mustelina) is an iconic songbird of shaded woodlands that breeds throughout most of eastern North America. For many nature-lovers, its beautiful, flute-like ‘ee-o-lay’ song is the sound of the summer. Sadly, Wood Thrushes are in decline, with 60% of their population lost in the last 50 years. There are several reasons for this decline but the most obvious is habitat loss. Wood Thrushes need large patches of mature woodland for breeding. Unfortunately, our eastern forests are being shrunk, fragmented, or even lost altogether due to development. Other factors may reduce their survival during migration or on their non-breeding grounds in Central America. These pressures may be particularly intense in some parts of their range.
To shed light on this, Dr Sarah Kendrick of US Fish and Wildlife proposed a bold and unprecedented collaborative project. It involved researchers placing radio-transmitters on Wood Thrushes throughout their range so they could be tracked using the Motus system.
The Motus system is a network of towers that can detect any animal with a radio-transmitter within 15 km. These towers operate 24/7 so they can still detect animals passing by at night when they are otherwise invisible to researchers. The network is especially dense in the eastern United States. It includes data-rich towers at our DuPont Environmental Education Center in Wilmington and Bucktoe Creek Preserve near Kennett Square. Remarkably, the data is open access and clicking on any tower on the Motus homepage shows you the list of animals it has detected. Just click here to see the data from the DEEC tower and here for the data from Bucktoe Creek. Coverage is expanding throughout Central America so any tagged Wood Thrushes have a good chance of being detected during migration and the non-breeding season.
The massive scale of this project allows us to answer some key questions. Do Wood Thrushes from very different parts of their breeding range migrate using different routes or at different times? Also, do birds from different states inhabit different areas during their non-breeding season? Are there differences in the migration patterns of males and females, or between experienced adult birds and juveniles migrating for their first time?
The Delaware Nature Society was invited to be a part of the project because of our work on bird conservation. We were able to participate thanks to initial funding from several donors and our partners at Mt Cuba Center as well as a grant from Delaware Audubon. Chief among these donors were the Flying Penguins! This is a team of three enthusiastic young birders, Ellie, Christian and Hudson. These talented teens raised money for bird conservation by competing in New Jersey Audubon’s famed World Series of Birding. The Flying Penguins were happy to see the money they raised go toward conserving one of their favorite birds!
Our field work started with a bang in the summer of 2024. Adult and juvenile Wood Thrushes were captured by setting up soft mist-nets in woods where they were known to breed. These nets were closely monitored, and any birds captured were carefully taken out and given a numbered metal leg band, then sexed, aged, and measured. Finally, they were fitted with a light weight (1g) battery-powered radio-transmitter. This sits snugly against their lower back and is held in place with soft elastic cord looped above their thighs so they can walk and fly freely. The birds were gently released and flew away as normal.
We placed radio-transmitters on 26 adult and juvenile Wood Thrushes at several sites in the Piedmont. These were Ashland Nature Center, Coverdale Farm Preserve, Flint Woods Preserve and Mt Cuba Center in northern Delaware, and Bucktoe Creek Preserve in Pennsylvania. Our colleagues in other states have been equally successful. In just this first summer, over 500 Wood Thrushes were tagged in 25 states and one Canadian province! This means a staggering number of Wood Thrushes will soon be lighting up the Motus network as they migrate south.
The batteries last up to 450 days so we should be able to follow tagged birds during the fall and winter of 2024. We should also be able to track them as they head north next spring. It’s not impossible that we could still be tracking this summer’s birds during the fall and winter of 2025! These year-round data are critical for identifying areas where Wood Thrushes concentrate during migration and the non-breeding season so we can prioritize these forests for preservation. Once our tagged birds start heading south, we will be providing updates on their progress!
This is the start of a broader Wood Thrush initiative the Delaware Nature Society is undertaking. We will be integrating tracking data into our field studies to help educate landowners, land managers, and the general public about the best ways to conserve this and other woodland birds. Everyone should be able to experience the sublime song of this beautiful bird emanating from a healthy forest.