A Cave Called Home

by | Mar 20, 2018 | Fauna

Mexican free-tailed bats fly over the entrance to a retired guano mine at Bracken Cave. PHOTO Adam Barbe

Mexican free-tailed bats fly over the entrance to a retired guano mine at Bracken Cave. PHOTO Adam Barbe


It’s like the sunset — it seems to take all day, and then it happens in a blink. At first you only hear the rustle of oak leaves, the approaching scream of a hawk looking for a meal, the anxious chatter of the viewing party. Eyes and camera lenses focus on a hole in the ground 100 feet wide and pitch black. This is Bracken Cave in Comal County, Texas, a short drive northeast of San Antonio. It is home to the world’s largest colony of bats. Spectators gather here for a tour offered by a pair of charismatic hosts with Bat Conservation International (BCI); the couple helps manage the site, a 1,500-acre preserve surrounding the cave. The sun begins to set, and suddenly things start to change.

If you look closely, you can make out the preparatory swirl of a few dozen bats in the dark cavern. Visitors gasp as coachwhip snakes four feet long or more dart out from the rocks below and lie in wait beside the cave entrance. When the bats are finally ready, they burst from their home. As they pour from the cave and climb into the blue sky, you can start to see the scale of the colony — estimates number them at more than 15 million. They come out at dusk each night in a spectacular parade that can last more than four hours. These are Mexican free-tailed bats (Tadarida brasiliensis), and though they are small enough to fit in your palm, en masse their display is monumental. As they fly overhead, their wings sound like rain on an umbrella.

They fly in search of food. Mexican free-tailed bats feed on the wing, catching insects out of the air. They mostly seek moths; the corn earworm moth (Helicoverpa zea) is a favorite and one of the most significant crop pests in North America. Adult corn earworm moths lay their eggs on corn silks or leaf hairs, and in Texas — where growers plant almost 3 million acres of corn each year — they are right at home. (Native host plants include fall panicgrass, Panicum dichotomiflorum, and members of the genera Ipomoea, Portulaca and Lupinus.)

But bats have been in Texas much longer than our modern farms. They are here because of the geology of the Edwards Plateau, where caves and sinkholes have been carved out of soluble bedrock by rainwater, providing an excellent habitat. There are more than 10,000 such shelters in the karst regions of Texas; together they hold 100 million Mexican free-tailed bats. And these huge populations of bats consume an astonishing number of insects to support themselves — up to 18,000 metric tons each year.

Departing bats must be wary of lurking predators such as snakes and hawks. PHOTO Adam Barbe

Departing bats must be wary of lurking predators such as snakes and hawks. PHOTO Adam Barbe


When standing at the entrance of Bracken Cave, it is unmistakably clear where all that food goes. The smell of guano rises up from the floor of the cavern and hangs heavy in the Texas heat. Here, beneath the world’s largest bat colony, guano is piled sixty feet deep or more. This creates ammonia gas that is so highly concentrated inside the cave it is lethal to humans. The bats withstand the fumes by lowering their metabolism and increasing their stores of carbon dioxide, which neutralizes the harmful ammonia. Guano stores in Bracken Cave were mined extensively in the 1800s; in fact, guano (an excellent natural fertilizer that can also be used to make gunpowder) was once Texas’ most significant mineral export.

The bats at Bracken Cave have a long history as a precious economic resource for Texans. That history continues today as Mexican free-tailed bats provide pest control and prevent crop losses in Central Texas worth more than $700,000 every year — just one of many reasons BCI and The Nature Conservancy are helping to protect the colony at Bracken Cave. The Wildflower Center and Lake|Flato Architects are contributing by developing a plan for new facilities at the cave, including a visitors pavilion and observation path; these will provide improved access and new educational opportunities while minimizing disturbance to the landscape. Increased visitation to Bracken Cave will help fund the important work of BCI, helping to ensure the health of the colony and the karst landscape for years to come.

Of course, there is also the grander value, that which can’t be priced or purchased. The sunset in Texas Hill Country is regularly a spectacle, but when set against the nightly flight of 15 million Mexican free-tailed bats, it is downright spellbinding. As the stream of bats flowing from the cave begins to slow, their individual bodies become clearer — and smaller. Each bat weighs less than an ounce. When they leave for the evening, they fly low enough to the ground to be snatched from the air by snakes or get hopelessly tangled into nearby cacti and thorny shrubs. Despite their numbers, life outside the cave is dangerous for the bats. But they fly each night, and what they offer us — pest control, mineral resources, a magnificent show — can only be repaid by protecting their home in Central Texas as a natural treasure that, like the sunset, both moves us and stops us in our tracks.

Keaton Daniel is a former Wildflower Center intern. He currently studies writing and human ecology at College of the Atlantic.