Every evening from spring to fall, crowds gather near the Congress Avenue Bridge in downtown Austin. They are not waiting for a parade or a concert. They are waiting for bats.

Under the bridge lives a colony of Mexican free-tailed bats. By late summer it can hold as many as 1.5 million bats, which makes it the largest urban bat colony in North America. Most of the bats are females. They arrive in the spring, and in early summer most of them give birth to a single baby, called a pup. In the fall, the colony flies south to spend the winter in Mexico.

The bats did not always live there. When the bridge was rebuilt, narrow gaps were left in the concrete underneath it. These dark, warm spaces turned out to be a perfect place for bats to roost during the day before they fly out to hunt at night. Within a few years, hundreds of thousands of bats had moved in.

At first, many people in Austin were afraid. Some believed the bats were dangerous and asked the city to get rid of them. Scientists and bat experts worked to change people's minds. They explained that bats rarely bother people, as long as people never touch or pick up a bat. They also pointed out how useful the bats are.

Each night, the bats fly out to hunt insects, including moths that damage farm crops. A colony this size can eat thousands of pounds of insects in a single night.

Today, the bats are one of Austin's best-known sights. On summer evenings, visitors line the bridge and the riverbank to watch dark clouds of bats pour out from under the bridge at sunset and stream away across the sky.