Bat guano was once a big business—guano was Texas’ largest mineral export before oil! In fact, Austin is a seasonal home to North America’s largest urban population of Mexican free-tailed bats, which live beneath the Congress Avenue Bridge. Approximately 1.5 million bats reside there.
- https://www.nature.org/en-us/about-us/where-we-work/united-states/arizona/stories-in-arizona/top-10-bat-facts/