Eckert James River Bat Cave steward Vicki Ritter assists a baby Mexican free-tailed bat on Aug. 5, 2018. Credit: Monika Maeckle / For the San Antonio Report

A drone hovering above the nightly emergence at the Eckert James River Bat Cave near Mason last weekend caused the Nature Conservancy of Texas to close the cave to the public two months early, the conservation organization announced Thursday.

The maternal bat colony, home to 1.6 million Mexican free-tailed bats and their offspring each summer, typically welcomes visitors from March through October. The Eckert James River Bat Cave, which is owned by The Nature Conservancy (TNC) also was closed during July following an incident in which pesticide sprayed on a neighboring ranch drifted onto the property, driving away visitors and potentially harming the bats.

“Recent events at the preserve have given us reason to pause, step back, and reconsider the operations that support the cave’s nightly public viewings,” TNC announced on its website. The eight-acre preserve sits southwest of the Hill Country town of Mason near State Highway 29 in Mason County.

Last weekend, a mysterious drone appeared over the cave during the bats’ nightly emergence. It was not known who was flying the drone or from what location it was being piloted. TNC officials voiced concerns that the drone might pose a danger to the bats and interfere with the whirlwind “bat-nado” that occurs when they emerge en masse from the cave each night to feed.

The drone flight over a small crowd of spectators at the bat cave prompted legal and safety questions.

“If the drone was flying over nonparticipating subjects, it was an illegal flight, no questions asked,” said Wes Pruitt, CEO of White Cloud Drones, a San Antonio-based drone operating company licensed by the Federal Aviation Administration. “The rules state unequivocally you cannot fly over nonparticipating subjects,” meaning “people who didn’t know about it or agree to have the drone fly over them.”

Unlicensed or inexperienced drone pilots flying too close to the bats could cause harm and force them to adjust their behaviors, said Fran Hutchins, director of Bracken Cave, the world’s largest colony of bats. Owned by Bat Conservation International (BCI), Bracken Cave is just outside San Antonio.

“Any wildlife – birds or bats – you have the impact issue, the bat hitting the drone or the drone hitting the bats,” he said. “It’s human harassment. It creates stress – not a good thing, especially at a maternal colony.”

Hutchins added that when he and BCI personnel have overseen flights with professional drone pilots at Bracken Cave, they maintain a distance of 100 feet from the bats.

Merlin Tuttle, founder of BCI and Merlin Tuttle’s Bat Conservation, said he didn’t have huge concerns about the impact of responsible drone flights on bats, “as long as you’re not doing it in their faces,” he said.

Tuttle said that while drones should remain at least 20 feet from the bats, when flown responsibly they have “no greater impact on the bats than hawks.”

The drone episode followed an incident at the cave on June 29 when aerosol pesticides drifted over to the bat cave from a neighboring exotic game ranch while visitors gathered for the nightly bat emergence. Coughing and gasping, visitors were exposed to the pesticide as they ran for their cars.

Following the pesticide spraying, TNC closed the cave for two weeks during peak visitor season. On July 11, a notice appeared in the Mason County News stating that a neighboring landowner had treated his property with a fog pesticide identified as Permethrin.

The bat cave reopened July 18, and a Texas Department of Agriculture investigation is expected to be complete by Sept. 1.

Other pristine bat cave preserves exercise tight controls over visitor access. At the Bracken Cave, which is home to more than five million bats, visitors make appointments to come to the cave and are accompanied by a steward while there.

“Access is by reservation,” said Hutchins. “The primary way is as a BCI member. We limit the number of people and regulate group size and frequency, mainly because we’re so close to San Antonio.”

TNC spokeswoman Vanessa Martin said the conservancy will work with BCI, the Texas Parks and Wildlife Department, the U.S. Fish and Wildlife Service, and other agencies to reevaluate its operating procedures and visitor access policies over the coming months.

“Not much has been updated about the visitor experience at the bat cave since it was opened in 1990,” she said. 

San Antonio Report co-founder Monika Maeckle writes about pollinators, native plants, and the ecosystems that sustain them at the Texas Butterfly Ranch website. She is also the founder and director of...