Description: The Mexican free-tailed bat (Tadarida brasiliensis) is a medium sized bat. They have broad, black, forward pointing ears. Like other members of the genus Tadarida, they possess characteristic wrinkled lips. Their tails extend more than one third beyond the tail membranes, which gives this species its “free-tailed” moniker (most other bats have tails that are completely enclosed within the tail membranes). Their wings are short (that is, from the leading edge to the trailing edge) and broad (that is, from side to side).
Color: Mexican free-tailed bats are red-brown to dark brown or dark gray in color on the dorsum and light-colored below – typically pale yellow to dirty white. Their broad, forward pointing ears are naked and dark brown to black in color. The interfemoral membrane is similarly dark in color.
Range: The Mexican free-tailed bat occurs throughout the southern half of the United States with a more northerly distribution in the west and more southerly distribution in the east. There are a few limited population centers scattered in more northerly locations in the Midwest. The species can also be found throughout Mexico excluding the Yucatan peninsula.
Roosting: Mexican free-tails are predominantly cave-roosting bats. Certainly their largest colonies are located in caves. Occurring in smaller numbers, Mexican free-tailed bats may also roost in attics, under bridges, or in abandoned buildings. Most Mexican free-tailed bat colonies, large or small, are located close to water – proximity probably being an essential component of the life history strategy for an insectivorous species that associates in such large numbers.
Characteristics: Mexican free-tailed bats are extremely fast fliers, certainly among the fastest-flying bat species. Their long, narrow wings are appropriate for high speed flight. Their short, silky fur and ear orientation may contribute to speed and lift respectively. Roosting in numbers that can exceed several millions, they form the largest colonies of any mammal (including humans). Their large roost numbers also play a factor in their role as “guano bats”, extraction of which was once a major source of “natural” fertilizer. In the early 20th century, bat guano played an important role in California agribusiness.
Feeding: Mexican free-tails emerge around, including just before sundown and feed exclusively on flying insects – primarily moths, flying ants, and beetles – and exploit insect blooms, insect-rich agricultural fields, and river-associated high density insect populations.
Migration: Although there are some populations that remain within the continental U. S. over winter, this species is highly migratory and most of its populations move southward into Mexico and Central America during northern hemisphere winter months. It appears that this species is active throughout the year. Populations may be sedentary in temperate climates but most migrate to warmer winter climates in order to remain active.
Breeding: Adult Mexican free-tailed bats mate prior to the northward migration. Upon arrival on the breeding grounds, adults typically separate with females forming often immense nursery colonies while the males form bachelor colonies, which can range from but a few individuals to 10s of thousands in number. Females typically produce one pup each year and unlike many bat species, pups are born during a very narrow time frame, most often in the month of June, albeit with some variation in climate-based onset date.