Completed in 1998, the Chiroptorium is designed to house 1 million Mexican Free-tailed Bats. (For more details see the article How to Build a Bat Cave (PDF) from Bat Conservation International that chronicles the construction of the Chiroptorium.)
Five years passed with no significant population and staff hypothesized the problem being 3 plate glass windows inside the cave for the purpose of human-viewings of the bats at roosting height. Indications were that the bats were either not seeing or echolocating the smooth surface of the glass and were bumping into the windows. The windows were covered in the winter of 2003 and several thousand bats arrived that summer.
Then on August 5, 2003, J. David went up to the cave before sun-down to view his “several thousand” and what he saw changed the whole picture: several hundred thousand were emerging from the cave, coming out in a column over the road and flying off into the night sky.
Strong emergences regularly lasted 30 minutes each night. Scientists estimated at that time that we had at least 200,000 bats! Bats returned the following summer, estimated over 5,000. But the best news came on June 22, 2004. Dr. Gary McCracken, a leading bat biologist who is studying the eating habits of Mexican Free-tailed Bats, came to the ranch before heading to his Uvalde research site. After viewing the five minute emergence, Dr. McCracken and several staff members went into the cave to see how much guano was on the ground.
What they found was several meter squares of pups on the walls. Dr. McCracken estimated that about 5,000 pups roost in one-meter-square and we had several meters on the wall. Adult bats can roost 200 per square foot, and pups can roost up to 500 per square foot. Every year since, we have supported a growing maternal colony. Last official scientific documentation done by Boston University documented 160,000 in 2011. Comparable observations of length and strength of emergences lead us to believe we had nearly double that in 2015.