The Fascinating Lives of Mexican Free-tailed Bats

© J. Scott Altenbach, University of New Mexico

Guest author:  Amy Sugeno

Here in the Texas Hill Country, we are lucky to enjoy many natural wonders: nesting Golden-cheeked warblers, migrating monarchs, and Texas alligator lizards, to name just a few.  But did you know central Texas is also home to the largest concentrations of Mexican free-tailed bats in the United States? There are at least 11 major seasonal roosting sites in central Texas, each of which are home to at least 500,000 Mexican free-tailed bats. In fact, an estimated 100 million Mexican free-tailed bats call central Texas their summer home! A few of these sites are available for public viewing, and visitors come from all over the world to witness their spectacular evening emergences.

Although a small percentage of Mexican free-tailed bats overwinters in Texas, most begin arriving to Texas in the spring (around mid-March) from their wintering grounds in Mexico, Central America, and possibly even as far south as South America. Females seek out one of only a handful of maternity sites to raise their young, called pups, which are born in early to mid-June. In these maternity sites, such as the ones at Congress Avenue Bridge and Eckert James River Bat Cave Preserve, the temperature and humidity must remain relatively stable in order for the pups to survive. Inside of a maternity cave, temperatures can reach upwards to 100 degrees or higher, especially where the highest densities of pups congregate, sometimes up to 500 pups per square foot.

© Thomas Kunz, Boston University

After a gestation period of 4-6 weeks, the pups begin to fly, usually around the end of July and beginning of August. It is at this time when the most spectacular emergences occur. With nearly twice as many bats needing to emerge each night to feed, they must emerge earlier in the evening, which makes them easy to see. And if they are easier for humans to see, they are also easier for birds of prey to see! Red-tailed and Swainson’s hawks and peregrine falcons are fairly common aerial predators, and watching them swoop and dive into the stream of emerging bats is a stunning sight. After the sun goes down, Great-horned owls pick up where the hawks and falcons left off. On the ground, several different snake species, raccoons, ringtails, and foxes all visit during emergence time to try and catch a low-flying or fallen bat or a bat whose wing has become impaled on the branch of a shrub.

The life history of Mexican free-tailed bats is full of fascinating facts: they can forage up to 1 mile high; they can fly up to 60 mph; mothers can find their own pup from the hundreds of thousands or millions of other pups in the roost; they can eat their own weight each night (they mostly eat moths); and they can survive in maternity roosts with ammonia levels high enough to kill a human!

And this is only the beginning. If you’d like to learn more, the best (and most fun!) way is to visit one of the fantastic viewing sites in our area. The two closest ones which are open to the public and offer educational presentations are Old Tunnel State Park and Eckert James River Bat Cave Preserve. If you want to explore further afield, Devil’s Sinkhole State Natural Area, Frio Bat Cave, and Stuart Bat Cave in Kickapoo Caverns State Park are great places to go.

Happy batting!

© Thomas Kunz, Boston University

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