The Bracken Bat Column

Bracken Cave, in a hillside in southern Comal County, holds the largest concentration of mammals anywhere on earth. Up to twenty million Mexican free-tailed bats use it as a maternity roost through the summer, almost all of them females and their single pups. On a July evening the emergence begins around dusk and continues for hours. The column rises out of the cave mouth in a slow counterclockwise spiral and then streams away to the southeast to feed, dense enough that National Weather Service radar picks it up as a spreading echo blooming out of empty country. Bats have used the cave for thousands of years, long enough that the guano deposit beneath the roost is metres deep and was mined commercially in the nineteenth century. They are hunting insects at altitude, sometimes as high as ten thousand feet. The Bracken colony alone is estimated to take more than a hundred tons of moths in a night, largely corn earworm and other crop pests riding the wind north, and the value of that to Texas agriculture has been calculated many times and always comes out large. The cave itself is not open to the public. Bat Conservation International owns the 1,458-acre preserve around it and co-manages several thousand more acres with The Nature Conservancy and the City of San Antonio, and visits are by reservation on a limited schedule. The column itself is another matter. On a summer evening it climbs high enough to be picked out from roads miles away across this corner of the county, heading southeast in a dark, unsteady ribbon that nothing else in the sky resembles.