Media & Education
BATS Magazine

Volume 39, Issue 1, 2020

From Guano Harvesting to Bat Warfare

Bracken Cave’s wild history of wartime experiments, fertilizer harvesting and, ultimately, preservation

By Linda Lombardi

Bracken Cave in Texas is home to more than 15 million Mexican free-tailed bats from March to November each year, when pups are born and raised. It is the largest bat maternity colony on Earth. But this cave near San Antonio isn’t just the largest of its kind. It’s also a cave with a wild and unusual history.

For many generations, its value was in what the bats produced. Guano (bat poop, to the rest of us) was mined for use as fertilizer going back at least as far as the 1800s, when colonists first settled the area. It was Texas’ most valuable mineral product before oil was discovered, and Bracken Cave had plenty of it. Fran Hutchins, director of Bracken Cave Preserve, says recent measurements estimate the current depth of guano to be between 75 and 100 feet in the 117-foot-tall cave.

Guano was also later used in the production of gunpowder during the Civil War and World War I. The cave’s bats were enlisted in the service of war during World War II, when military researchers tried to develop a way to use them to deliver tiny firebombs. The bat bombs were never put to use, but they did give the researchers a taste of their own medicine.

“When they experimented on the base, some of the bats got out and flew over to some of the buildings and set them on fire,” Hutchins says.

Since the 19th century, the cave was owned by the Marback family. They mined it, while being good stewards, until BCI founder Merlin Tuttle learned about the cave in 1992. BCI acquired partial ownership of the five acres containing the cave before later owning the land outright. With the cave located less than 20 miles from downtown San Antonio, it was clear that five acres wasn’t enough to protect it. In the early 2000s, using a combination of private and public funds, BCI and partners worked to acquire another 692 acres around the cave.

Sounds like a happy ending, right? But in a last-minute plot twist, a developer bought the land on the south and west boundaries of the preserve and planned to build around 3,500 homes.

With the closest house only about a half-mile from the cave and the entrance right next to the preserve’s gate, millions of bats would be flying over those homes for several hours each night in season. “What’s wrong with that?” asks the bat lover. Unfortunately, there was a huge potential for human-wildlife conflict.

“It’s pretty cool to sit in your backyard and have a glass of wine and watch the bats fly over—until your dog or your 5-year-old brings one into the house,” Hutchins says. “That was just too many homes, too close to the cave.”

Fortunately, bat lovers won the fight. With the support of local government, citizens, and the Nature Conservancy, the developer was convinced to sell the land in 2014, and it’s now protected. The cave is surrounded by a large nature preserve that protects the bats, other wildlife, and the city’s aquifer. “We had a ton of community support, and that’s what helped us win,” Hutchins says.


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