Tadarida brasiliensis is the most gregarious of all mammals. The immense colonies (up to 20 million animals) are one of the wonders of nature. Emergence from large colonies begins about 15 minutes after sunset. How long emergence lasts depends on the size of the colony. The bats may fly considerable distances to their feeding areas because areas near the roost cannot support enough insects to feed them. Rapid, long distance flights are made possible by long, narrow wings. These wings are adapted for speed, but they do not provide enough lift for bats to fly up from the ground without climbing high enough to drop and gain speed. Once at the feeding areas, the bats drink, feed, and rest before returning to their roost. The return of the bats is almost as exciting to watch as the emergence, and it lasts as long or longer. At all but the largest colonies, the bats return by sunrise. Northern populations of the Brazilian free-tailed bat migrate southward to warmer climates for winter; the bats in Kansas migrate to southern Texas or northern Mexico beginning in September. They begin to return to Kansas in late April. Maximum reported lifespan in the wild is 8 years. Maximum potential lifespan is about 15 years.
Moths are the primary food of this bat, although other kinds of insects also are eaten. The quantity of moths eaten in one night by a large colony of these bats is enormous, thus punctuating the ecological importance of this species and its benefit to agricultural interests.
The Brazilian free-tailed bat is regarded as a "big city bat" because of the large colonies in which it lives. And, as in many large cities, there are perils to avoid. For example, solid waste pollution resulting from guano produced by millions of bats literally can fill up a cave. If a pup loses its grip and falls onto the surface of the guano, it literally is eaten alive by the dermestid beetle larvae that live in the guano. Air pollution resulting from the urine produced by all the bats is another problem. The ammonia literally can bleach the bats blonde. Crime in the streets is represented by all the predators that arrive each evening at the cave entrance just before the exodus. These may include snakes, coyotes, raccoons, opossums, and armadillos, while circling overhead are owls. When the column of bats emerges, they are flying in such close formation that they beat one another with their wings. Those that fall to the ground are immediately grabbed by a predator while the owls above repeatedly fly through the column picking off bats. Other sources of mortality include pesticide poisoning, rabies, accidents, and destruction of bats in their roosts by humans.