To learn more about the study, watch the video: Wood Thrush Tagging, YouTube
Anna Buckardt Thomas was crouched behind a tree about 20 paces off trail at Brown’s Woods county park, watching a mist net while playing a recording of a male wood thrush on a loop hoping to elicit a response.
The researcher with the Iowa Department of Natural Resources is working as part of a western hemisphere study on animal migration from Canada to Central and South America, and she needs to catch and apply small radio tags to 14 more wood thrushes this summer to meet her quota.
“Wood thrushes are fairly common, but the population is declining across its range by as much as 60 percent since 1970. We think that’s primarily due to the loss of habitat but we need to know more about what happens during migration,” she said.
Wood thrushes are small birds that live in mature forests with shrubby understory and feed on insects in the leaf litter. Iowa is on the western edge of the bird’s range, with a small population living in limited pockets of habitat.
She will need to net 8-9 different sites in order to tag the 27 birds for the Iowa portion of the study. Buckardt Thomas and her fellow researchers have identified areas where wood thrushes have been confirmed then slip in and set up a portable mist net roughly 10 feet high by 20 feet long in a forest clearing. They hide, then play a recording of a thrush through a speaker to rile up the local birds.
Males are territorial and will challenge the intruder. If everything works out it will fly in and get tangled in the net, where the researchers work quickly to safely remove the bird, collect some biological data and apply the radio tag. The bird must meet a minimum weight or it will be released without a tag. All of this work is completed by trained researchers under authorization from the US Geological Survey’s Bird Banding Lab.
The first bird was caught on May 28 at Ledges State Park; the final bird was caught on June 24 at Browns Woods. The birds have been mostly males as the females have been on the nest and are less responsive to audio lures. Nationally, about 500 wood thrushes will be tagged this year.
These birds will be tracked through an international network of radio antennas that will detect the radio tags on the birds, when the birds pass nearby on their migration.
The Motus Wildlife Tracking System, motus being the Latin word for movement, began in 2013 by Birds Canada, near Toronto. Motus is a system of coordinated automated radio telemetry stations used to track long-distance movements of small animals.
Automated antenna arrays connected to radio receivers are being installed throughout the Western hemisphere and birds, bats and insects are netted and trapped then outfitted with small tags that emit a radio signal every few seconds. When the tagged animal passes near a receiver station, it records the tag and identifies the animal to which it was attached. So multiple detections over time can build a map of migration for a tagged individual. The network of Motus stations and the wildlife that it has recorded is available online at https://motus.org/
Iowa is an important flyway for migrating birds with nearly 1 billion birds flying through the state each fall. The state has 25 Motus receiver stations detecting birds and providing more information on migration patterns. That information can inform conservation decisions.
Iowa began installing stations in August 2021 as part of a Fish and Wildlife Service Grant that funded equipment for 40 stations in the Midwest and a dozen in Central and South America.
Anna Buckardt Thomas is the state avian ecologist with the Iowa Department of Natural Resources.