In Austin, the Congress Avenue Bridge crosses Lady Bird Lake. Under the bridge lives a huge colony of bats. A colony is a big group of animals that live together. In summer, more than one million bats may live there.

These bats are called Mexican free-tailed bats. During the day, they sleep in narrow cracks under the bridge. When the sun goes down, they fly out to hunt. So many bats come out at once that they look like a dark cloud moving across the sky.

The bats eat insects, such as moths. In one night, the colony can eat thousands of pounds of bugs. That helps farmers, because many of those insects eat crops.

On summer evenings, crowds of people gather by the lake to watch the bats fly out. In the fall, the bats fly south to Mexico for the winter.