Mexican free-tailed bats (Tadarida brasiliensis) like the ones found under the Pantano Wash bridge, are found in the western United States, south through Mexico, Central America and into northern South America.

They are medium-size bats, weighing between 0.4 and 0.5 ounces, with a wingspan between 12 and 14 inches. Their fur is reddish to dark brown or gray in color. They have broad, black, forward-pointing ears and wrinkled lips. Their tails extend more than a third beyond the tail membranes; most other bats have tails that are completely enclosed within the tail membranes. Their wings are long and narrow.

Mexican free-tailed bats prefer to roost in caves but will also inhabit attics, bridges and abandoned buildings. They choose roosts near water because it attracts the insects they eat. Females produce a single baby each summer, and all of the babies roost in a "nursery" of sorts, in the highest and warmest reaches of the cave or other roost site. Free-tailed bats can live up to 18 years.