The Tennessee River Gorge is one of many areas in the eastern U.S. used as summer breeding grounds for the Louisiana waterthrush (also known as LOWA) after migrating north from their wintering grounds in Central America.
As summer ends, the waterthrush heads back south, only to return the following spring, often to the same section of stream. The LOWA, feeding on aquatic macroinvertebrates found in clean gravel-bottom streams flowing through deciduous forests, serves as an excellent indicator of stream health.
In 2016, the Tennessee River Gorge Trust began a pilot project to attach light-level geolocator devices on the backs of male LOWAs to observe the physical effects of carrying the device and discover a functional way to track the species. The devices map the birds’ migration routes and wintering grounds by measuring ambient light levels in reference to time. This can later be used to calculate the subject’s latitude and longitude each day.
Identifying both stopover and wintering locations is essential for establishing conservation priorities and understanding all factors potentially playing a role in fluctuating population trends. The physical effects of carrying the devices were studied because previous attempts by others to attach geolocators to LOWAs yielded few results because of problems involving the harnesses and attachment techniques.
In 2016, the trust’s avian technicians captured 33 LOWAs in the gorge, attaching geolocators to 16 and using the other 17 as a control group, giving them only a color leg band.
Unlike radio collars or GPS-based locators, which generate real-time data on larger animals, the geolocators’ data is only obtained when the devices are removed from the birds upon their return in spring. The technicians attempted to recapture the same LOWAs when they returned to the gorge the following year. To do so, the technicians combed the gorge with binoculars and set fine mist nets across the stream once a marked LOWA was sighted. A robotic decoy and speaker system were used to entice the territorial males to fly into the net. The technicians eventually recaptured five of the 16 geolocator-marked birds and seven of the 17 control birds, which was higher than with previous geolocator studies by others. Importantly, the birds returned in good health and showed no harm from carrying the geolocators.
Data extracted from the geolocators, with the help of Dr. Henry Steby and Gunnar Kramer at the University of Toledo, uncovered fascinating results.
The birds had wintered in a range spanning from southern Mexico through eastern Honduras, with both coastal and trans-Gulf migration routes. Surprisingly, the birds migrated at incredible speeds, averaging seven days for the roughly 1,500-mile trek, with two completing the flight in four days!
The project was one of the first successful attempts to track the migration of LOWA, and the trust plans to continue the research, hoping these techniques will open the door to gathering more complete life cycle data on the Louisiana waterthrush. This information will help the trust and other conservationists better understand this important indicator species and how to use it to make management decisions.
Funds for the project were provided by a grant from the Benwood Foundation and contributions from the Tennessee Ornithological Society and private donors. A component the grant offers is the opportunity to connect members of Chattanooga’s Latino community with the gorge by demonstrating the fascinating connection the birds’ migration makes between the gorge and Central America.
This piece was adapted from an article by Eliot Berz.