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...