Photo: Mexican Free-tailed bats
Mexican Free-tailed bats emerge from under the Congress Avenue bridge in Austin, Texas

Photograph by Merlin D. Tuttle, Bat Conservation International

The sun is setting on a summer night. At the entrance to a deep, dark cave, a few bats fly out and begin swirling high up into the air. More and more of the small, furry creatures appear.

Within a few minutes, a whole river of bats is pouring out of the cave, and they keep coming, millions and millions of them.

Sound like a scary horror movie? Believe it or not, this scene takes place every night, all summer long, at Bracken Cave in Texas, where 20 million bats zoom outside to feed on insects.  “It is magical,” says Jim Kennedy of Bat Conservation International in Austin Texas. “It is one of the most majestic animal movements on the planet.”

A group of bats is called a colony. Every March or April, Mexican Free-tailed bats (Tadarida brasiliensis) migrate up to 1,000 miles (1609 kilometers) from their winter home in Mexico to this cave, where they raise their young. Soon after they arrive, each mother gives birth to one pup. As many as 500 baby bats live crammed into one square foot (30.48 cubic centimeters) of space, and all those bodies create nice warm temperatures to keep them cozy and comfortable.

If you’ve ever lost sight of your parents on a crowded playground, you may be wondering how mother bats recognize their offspring. They use their sense of smell to tell which pup is theirs when it is time for the babies to nurse.

The cave, a quiet place in winter, teems with life all summer—and not just with bat residents. Bat poop, called guano, soon covers the floor, providing food for tiny organisms like bacteria and fungi. And then there are the bugs, says Kennedy: “You walk in with your headlight on your helmet on, and the floor of the cave is seething with bugs. Gnats are flying around, and beetles are crawling up your leg!”

When the adult bats leave the cave at night to feed, they are helping humans. They eat tons of insects in a single night, including many pests that eat farmer’s crops. And they get rid of mosquitoes, too.

When twenty million bats fly out of one cave opening, they make a column so thick that they show up on radar at the nearby airport. It can take three hours for all of them to emerge. “You can feel the breeze from their wings, and see the vegetation rustle,” says Kennedy. “The flap of all those wings makes a very relaxing soft sound. I’ve always thought it would make a great tape to help you go to sleep.”

So much for bats being scary!

Fast Facts:

  • Mexican Free-tailed bats can fly up to 60 miles (97 kilometers) an hour with the help of tail winds. 
  • They go as high as 10,000 feet (3048 meters), up where planes fly. 
  • No other kind of bat in the world forms such large colonies. 
  • These bats are most common in central Texas and Mexico, but also live in much of western North America and areas of Central and South America. 
  • They don’t just live in caves, but find homes in protected places like mines, tunnels, hollow trees, and under bridges. 
  • Bats made a contribution to the Civil War: In 1863, a gun powder factory was built in Texas that got a key ingredient, saltpeter, from bat guano.