Tens of millions of Mexican free-tailed bats migrate to the United States every spring. But where exactly they come from, no one knows.
08.06.25
By Rachel Harper
Every summer, millions of Mexican free-tailed bats swirl out of Bracken Cave in Texas in one of the most spectacular wildlife events in the world. But where did they come from? Where are they going? And how do we even begin to answer these questions for a tiny, nocturnal animal flying thousands of miles across international borders?
That’s the mystery bat researcher Kristin Dyer, PhD Student at the Becker Lab at the University of Oklahoma, is working to solve. Her mission: unravel the routes, timing, and health challenges of migratory bats as they travel between Mexico and the southern United States—and understand how these migrations are evolving in the face of climate change and parasitic threats.
Seems simple, right? While some animal migrations have been documented for centuries, tiny, nocturnal winged mammals that like to hide in caves and crevices are tougher to track than most. That’s where technology comes in using two bat-tagging methods: PIT tags and Motus radio tags.
Two Ways to Track a Bat
Think of PIT tags and Motus tags as complementary ways to solve the same mystery—with each offering a different piece of the puzzle. PIT Tags are Radio Frequency Identification, or RFID chips (your dog or cat might have one if they have a microchip). Their frequency is only picked up if scanned by a close antenna. Motus Radio Tags have a longer range and their frequency is picked up by the Motus network throughout the Americas.
| Motus Tag | PIT Tag |
|---|---|
| Motus Tag | PIT tag |
| Lightweight radio transmitters sewn onto bats with dissolvable sutures | Implanted just under a bat’s skin, about the size of a grain of rice |
| Temporary: transmit data for about 9 months | Permanent: last the bat’s entire life |
| Detected by a continent-wide network of towers—including a new one at Bracken Cave | Only detected when the bat flies close to a specialized antenna |
| Ideal for tracking long-distance movements, especially between countries | Ideal for known roost locations |
| More expensive (~$300 each), but offer broader reach | Affordable and best for specific sites over longer periods of time |
Motus is a collaborative research network led by Birds Canada and partners with scientists and organizations across the globe. Motus stations are specialized receivers used in radio telemetry to track the movements of flying animals, such as birds, bats, and insects. They allow researchers to study and identify important migration routes, stopover areas, and wintering locations – and provide valuable insights on animal behavior. Using these tools, Dyer can study not just where bats go, but how their movement is affected by threats like parasites and shifting weather patterns.
Enter Bracken Cave Preserve
Dyer has some pretty big fish to fry (or more accurately: lots of bats to track) so BCI plays a small but important role in her research. Firstly, BCI helped to get a Motus tower installed on Bracken Cave Preserve (which will also support data for other migratory species!) And while Dyer is just one person, Bracken Cave Preserve Coordinator Krystie Miner and volunteers help to tag and analyze bats on off months when Dyer is off at other sites. Added perk: BCI is proud to give Dyer a place to crash at the Bracken field house because that’s the kind thing to do for bat biologists forced to stay up into the late hours of the night!
Why Study Migrating Bats?
Mexican free-tailed bats (Tadarida brasiliensis) are not just migratory—they’re migratory while pregnant. That means every spring, females are flying up to 1,100 miles north with pups developing inside them. It’s an incredible physical feat—and one that leaves them vulnerable. As bats travel and cluster in maternity colonies, parasites like bat flies and fleas jump from host to host. Some carry Bartonella, a bacteria that can infect other animals, including humans. Infected bats might struggle to migrate efficiently—or at all. Bats rely on seasonal cues like temperature and rainfall to time their migration. But 20 years of radar data shows that fall migration back to Mexico is now happening nearly a month earlier than it used to at Selman Cave in Oklahoma. That shift could throw off the timing of reproduction, parasite cycles, and migration routes—further complicating an already delicate process.
Cracking the Code of Migration
By combining PIT tag detection with continent-spanning Motus radio data, researchers like Dyer are piecing together one of the last great mysteries of mammal migration. With climate change and parasites threatening bat populations, understanding their movement is more important than ever. These bats aren’t just fascinating—they’re vital. They eat crop pests, reducing a need for pesticides, and form one of the planet’s most awe-inspiring wildlife spectacles at Bracken Cave. With the help of tiny tech and big collaborations, we’re finally learning where they go—and what they need to survive.
Bonus Content
Learn more about Kristin Dyer’s work
The Bracken Team
Fran Hutchins – Director, Bracken Cave Preserve
Fran has been with Bat Conservation International since 2006, directing educational bat flight programs and the restoration work on BCI’s Bracken Cave Preserve and working with Central Texas landowners protecting other bat roosts. His work at the preserve protects the largest colony of bats in the world.
He is often asked to speak at various events, sharing his passion for informing the public about bats. This extended’s to schools, zoos, Scouts and organizations from around the world. He has been featured on Texas Country Reporter, Travel Channel, several documentaries as well as Texas Highway Magazine.
In 2013 he was recognized by the US Forest Service for Wings Across the Americas / BatsLIVE education program.
Fran is also a Caver and Texas Master Naturalist and Eagle Scout.
Krystie Miner – Project Coordinator
Krystie began working with BCI in January 2023 as the Project Coordinator for Bracken Cave Preserve. She coordinates volunteers to lead tours of Bracken Cave and to help with numerous other research, conservation, education, and citizen science projects. She also aids in other BCI projects as needed, including White-Nose Syndrome surveys and swabbing agave plants for eDNA.
Krystie has a background in wildlife research and conservation and has been working with bats since 2014. She has a B.S. in Ecology from Boise State University and an M.S. in Conservation Biology from Texas State University. She resides in San Antonio, Texas, where she is within a half hour drive to Bracken Cave and spends her spare time exploring the hill country with her dog.
About the Author
Rachel Harper – Digital Marketing Manager
Rachel joined Bat Conservation International in 2021, fostering BCI’s digital presence and community to support ending bat extinctions worldwide.
Rachel has a Bachelor’s in Marketing and over ten years experience in the digital conservation field, ranging from directing the Communications department at a local nature center to crafting social media strategies for San Diego Zoo Wildlife Alliance, and volunteering as Marketing Director for a nonprofit in Costa Rica.
Rachel grew up in Reno, Nevada and spent much of her childhood hiking and camping in Lake Tahoe, which inspired her passion for conserving wildlife and learning about nature. She now enjoys hiking, snorkeling, and experiencing wildlife in San Diego, California.
