From Birds to Bats

Every summer evening at BCI’s Bracken Cave Preserve near San Antonio, the sky churns with life. Millions of Mexican free-tailed bats (Tadarida brasiliensis) stream out of the cave in a rushing spiral at dusk to feed.

But come fall, Bracken—which is the largest known bat colony on Earth and the largest known gathering of mammals on Earth (with 20 million animals during peak season)—empties out. While scientists know that many of the bats, like birds, fly south for the winter, their precise migratory routes and winter roosting sites aren’t well known. A newly installed tower perched on the Bracken Cave Preserve landscape might contribute to solving the mystery.

Launched in 2014 by Birds Canada, the Motus Wildlife Tracking System comprises a network of radio-receiving towers across North America designed to track birds tagged with tiny radio transmitters. As tracking technology becomes smaller and lighter, researchers are taking advantage of the Motus system for smaller flying creatures, including bats.

When a tagged bat passes within approximately 12 miles of any Motus tower, that receiver logs the signal, allowing researchers to trace where and when the animal was detected. Over time, those individual pings connect to form a migration map.

Dyer’s work helped establish a foundation for integrating Motus towers into larger sites where bats are found, including Bracken Cave. “We’re doing this because we don’t know a lot about the corridors, routes, or timing of the Mexican free-tailed bat migration,” says Bracken Cave Preserve Director Fran Hutchins. “We know they start showing up in mid-February, they’re here all summer, and they start leaving in September. But we don’t know where all the roosts are in Mexico. Finding the different roosts in Mexico will help us protect them in the future.”

Bracken Cave installed its Motus tower in spring 2024, and even in the early stages of the project, researchers are already seeing movement they had never been able to document before.

In October 2025, Hutchins and his team tagged 18 Mexican free-tailed bats at the preserve. Only a few days later, one of the bats pinged in Big Bend, Texas. “It may spend the winter there, or it may go farther south into Mexico,” Hutchins says. This data could help scientists understand Mexican free-tailed bats that migrate shorter distances or even stay within Texas year-round.

As the Motus network expands and more towers are added across the Southwest U.S. and Mexico, researchers expect clearer, more detailed maps to emerge. For now, each new ping—each tiny signal from a bat passing a tower—adds another piece to a migration puzzle that, until recently, was almost entirely out of reach.