Austin , Texas is the summer home to North America 's largest urban bat colony ( under the Congress Avenue bridge ) , an estimated 1,500,000 Mexican free-tailed bats , who eat an estimated 10 to 30 tons of insects each night , and attract 100,000 tourists each year . The Congress Avenue Bridge in Austin , Texas is the summer home to North America 's largest urban bat colony , an estimated 1,500,000 Mexican free-tailed bats , which eat an estimated 10,000 to 30,000 pounds of insects each night . An estimated 100,000 tourists per year visit the bridge at twilight to watch the bats leave the roost .