Located less than 20 miles from downtown San Antonio, the cave is the summer home for the bats. This time of year is particularly important to them.
RELATED: Bat Rocks Out in Speaker-Like Roost
Beginning in March and April, expectant female bats travel to the cave after spending the winter in Mexico. Males roost in smaller groups elsewhere while the females congregate.
Right about now, the end of June, each healthy female bat gives birth to a single pup. As a result, the cave's already large population of bats nearly doubles. According to Bat Conservation International, the pups cluster tightly on the cave walls at up to 500 baby bats per square foot.
The mothers at this time are ravenous, and must eat more than their body weight in insects each night since they are eating for two. Their fave foods include moths, beetles, dragonflies, flies, wasps and ants, so for homeowners hoping to control insects like flies and ants, bats are a huge help.
To human eyes, the baby bats look basically the same, but a mother bat can find her own pup among the millions -- primarily based on scent and sound -- when she returns after hunting. The atmosphere in the cave is intense, with all of the bat pups vocalizing at around the same time, hoping to grab their mother's attention.
RELATED: Why You Shouldn't Be Scared of Vampire Bats
The video shows the massive number of bat mothers leaving the cave, an exit that begins about three hours before sunset. The females must nurse their pups at least twice daily, so their evening hunts are limited by that need.
The pups learn to fly when they are 4–5 weeks old. Bat Conservation International reports: "If all goes well on its first flight, a young bat drops into complete darkness, flies at a speed of at least 20 feet per second, and turns an almost complete somersault with millimeter precision to land on the cave wall just seconds after taking off."
Collisions with other young bats, or even the cave walls, can lead to sudden death, which is welcomed by the many meat-eating dermestid beetles that line the cave's floor, hoping for a meal to drop in on them.
In late July, the young bats who survive this arduous upbringing join their mothers during the evening hunts for insects. As the video shows, the spectacle of bats leaving Bracken Cave gives locals and tourists a close-up view for one of nature's most spectacular sights.
WATCH VIDEO: Kayaking Through The Largest Urban Bat Colony In The World