Mexican free-tailed bats are rather small bats with a wingspan of 290-325 mm. They are named free-tails because the lower half of their tail is free of the membrane (see photo). The ears almost meet at the midline of the face, but are not joined. They are usually a dark to light brown color.

They are found at lower elevations throughout California, across southern Nevada and southern Utah, Arizona, New Mexico, Texas, and parts of Oklahoma. They are also found throughout the southeastern United States from Mississippi down through Florida and over to South Carolina.

The habitat of the Mexican free-tailed bats differs in various parts of the United States. In the West and Southeast they live primarily in buildings: in the south they are mostly cave dwelling. Free-tails are one of the most social of all bats, always found in a colony, sometimes colonies of one thousand or more live in buildings. Several million bats live in Carlsbad Caverns, and larger nursery colonies live in Texas, Arizona, and Mexico.

The great numbers of these bats are an exciting sight to see emerge during sunset. The free-tails usually emerge about 15 minutes after sunset. The bats leave the cave and spread out in several directions, resembling a river of bats. Bats leave the cave at speeds of up to 35 miles per hour, then increase in velocity to 40 to 60 miles per hour.

Mexican free-tails feed primarily on small moths, and all food is captured on the wing. Most insects are 5-9 mm. in length. Most other bats catch food with their tail membrane, but free-tails have a very short membrane. In flight, the bats point their toes backward, extending the membrane almost to the tip of the tail. The long narrow wings and straight, rapid flight of the free-tails make them easily recognizable in flight. Unlike most other species of bats, the free-tails do not use night roosts often. They have been spotted occasionally with other species of bats.

The large colonies of the Mexican free-tails are nursery colonies where the bats congregate to bear and raise their pups. Some males remain present. Most of the other males tend to gather in caves near the maternity colonies.

A single pup is born each year from June to mid-July. Birth dates within a colony are very close together. In one Texas colony two-thirds of the births occurred during a 5 day period, and 90% within a 15 day period. The young take their first flight at about five weeks of age, this is later than most bats, which can fly within three weeks after birth. It is hypothesized that free-tails develop slower than other bats.

During the day the young ones cling together in large masses at roosting sites separate from their mothers. The mothers leave the young behind when they emerge to feed, only carrying the young when they move them from one place to another, especially when the colony has been disturbed. Mothers will return to the pup several times a day and during the night to nurse them.

Mexican free-tails are found in huge populations throughout their range. In Bracken Cave Texas, the population was estimated at 20 million bats. In Oklahoma the populations were estimated at 6,960,000 among various caves. At Carlsbad Cavern the population has been estimated from as high as 8,700,000 in June, to a low of 250,00 at the present time. In Florida, where the free-tails do not enter caves, large colonies are found in buildings. Colonies of 50,000 individuals are common and some are much larger.