On summer evenings, crowds of people gather near the Congress Avenue Bridge in downtown Austin. They are not waiting for a parade or a concert. They are waiting for bats.

Up to one and a half million Mexican free-tailed bats live in narrow cracks under the bridge during the warm months. It is the largest bat colony living in a city anywhere in North America. In the winter, most of the bats fly south to Mexico.

At sunset, the bats begin to pour out from under the bridge. They fly out in long, dark ribbons that twist across the sky, and the stream of bats can keep going for many minutes.

The bats are leaving to hunt. Mexican free-tailed bats eat insects, including moths that damage farm crops. In one night, the colony can eat thousands of pounds of insects.

Not everyone was happy when the bats first moved in. After the bridge was rebuilt, the bats found the new cracks underneath and began living there. Many people in Austin were afraid and wanted the bats gone. Bat scientists explained that the bats would not hurt people who left them alone, and that they helped the city by eating pests. Over time, people’s feelings changed. Today, the bats are one of the most famous sights in Austin.