Between 10 and 20 million Mexican free-tailed bats emerge from Bracken Cave in Texas every day during the spring and summer. It is the largest gathering of mammals of any kind in the world. Here is the sound of huge clouds of bats returning to the cave after a long night of foraging for insects. As they dive, they make a buzzing/whooshing sound with their wings. Bat Conservation International allowed me to visit the site last weekend to make recordings. I can't say enough about the good work they do to protect bats around the world. Thank you BCI! The recording was made with the collaboration and support of the Acoustic Atlas at Montana State University.