On summer evenings in downtown Austin, crowds gather along the shore of Lady Bird Lake and on the Congress Avenue Bridge itself. They are not waiting for a concert or fireworks. They are waiting for bats. Around sunset, a dark ribbon begins to pour out from under the bridge, and for close to an hour it keeps coming: as many as a million and a half Mexican free-tailed bats, heading out to feed.

The bats did not always live there. When the bridge was rebuilt in 1980, engineers left narrow gaps, about an inch wide, between the concrete beams underneath. The gaps turned out to be almost perfect for bats, which need tight, dark, warm spaces where they can roost safely during the day. Within a few years, the bridge held one of the largest city bat colonies in North America.

At first, many people in Austin were alarmed. Newspaper stories warned that bats could spread disease, and some residents asked the city to get rid of the colony. Bat scientists and conservation groups worked to change that view. They explained that bats rarely bother people who leave them alone, and that no one should ever pick up a bat found on the ground. They also pointed out what the bats do each night.

A single free-tailed bat can eat a huge number of insects in one night, including moths whose caterpillars damage crops such as corn and cotton. Together, the colony eats several tons of insects every night. Farmers in Central Texas benefit, even if they never see a bat.

Today the bats are one of Austin's best-known attractions. Visitors from around the world come to watch the evening flight, and signs near the bridge explain the bats' habits. A colony that people once wanted to remove has become something the city is proud of.