SAN ANTONIO — As the sun sets every summer night at Bracken Cave in the Hill Country, a vortex of Mexican free-tailed bats rushes forth. For as long as four hours a night, a stream of bats leaves the cave to dart south, over the tree line, to hunt. The scene at the cave is repeated in reverse at dawn, when the bats return.

Making up the largest colony of flying mammals in the world, the bats can number as many as 20 million. But a plan to build a subdivision near the cave has conservationists worried about the colony’s fate.

“There’s nothing else like this in the world,” Fran Hutchins, Bat Conservation International’s coordinator for the cave, said of the colony. “And we don’t really know what will happen if they build here.”

The cave’s size draws the bats to migrate from smaller caves to congregate and give birth. From March to October, every bat eats its weight in insects each night.