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As the sun dips in the Texas sky, the soft whooshing from Bracken Cave grows louder and louder. Finally, countless numbers of Mexican free-tailed bats appear, swirling in a vortex as they climb out of the cave and pour into the evening sky in search of insects. Located on San Antonio’s suburban fringe, Bracken is the summer home of the world’s largest bat colony. All told, 20 million free-tails migrate here every March to raise their young before returning to Mexico in November.

The cave is a rare bright spot for bats these days. Wind farms threaten migratory tree-dwelling species nationwide, and deadly white nose syndrome (WNS), named for a fungus that dusts the animals’ faces, has killed at least 5.7 million hibernating bats in 19 states*. Fortunately, biologists have found no sign of the disease in Texas yet (see “Going to Bat for Bats,” January-February 2011). Even if WNS were to make its way to Texas, it likely wouldn’t devastate the Mexican free-tailed bat population, because the species doesn’t hibernate. (Hibernation is when the disease kills, by causing species like the little brown myotis to repeatedly wake up and use up fat reserves before winter is over.)

“We’re cautiously optimistic that it wouldn’t be detrimental to the free-tails,” says Mylea Bayless, a conservation biologist with Bat Conservation International, the nonprofit that owns the cave and the surrounding 697 acres. “Though it’s possible that if white-nose reaches them, they’ll take it everywhere they migrate.” BCI has worked with Texas agencies to develop a WNS response plan.

The key agricultural role bats play is gaining appreciation. Each night free-tails emerge hungry for insects, including pests like the cotton bollworm. In south-central Texas alone the night flyers save cotton farmers $741,000 in pesticides, according to a 2006 study, and in April researchers reported in Science that bats save farmers about $22.9 billion in pest control nationwide. 

Besides conserving free-tails, BCI manages the Bracken Cave preserve to benefit other wildlife, including the 43 bird species found on the property. To protect the federally endangered golden-cheeked warbler, for instance, the group has maintained stands of cedar trees, the bird’s preferred habitat. As a result, since 2005 the number of breeding pairs on the site has increased from six to nine. “It’s very impressive that they’ve got nine breeding pairs,” says Bob Benson, executive director of Լ����Ƶ Texas. “I think they’ve gone about restoring the land in a very sound way, managing it for all the flora and fauna that they can.”

*This story was updated on January 17, 2012 to reflect the announcement by U.S. Fish and Wildlife Service biologists and partners that an estimated 5.7 million to 6.7 million bats have died from white-nose syndrome; the previous estimate was more than one million.