The bats of Bracken Cave comprise the largest known bat maternity colony in the world, producing millions of young each year. In March and April, expectant females return to Bracken after overwintering in Mexico. Most males roost elsewhere in smaller groups. In the latter part of June, females give birth to a single pup, nearly doubling the cave's population. As the bats give birth, cave walls become densely packed with naked pups, clustering tightly at up to 500 per square foot.
Babies roost separately from their mothers.
Before leaving her infant in the nursery with the other pups, each mother spends up to an hour getting acquainted with her baby's scent and vocalizations. She nurses her baby at least twice a day. After four to five weeks, youngsters begin learning to fly under the most difficult of conditions. 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. It must also avoid several collisions a second with thousands of other young fliers testing their skills, while at the same time relying on a sonar system that is itself being tested for the first time.