Tadarida brasiliensis (Geof. St. Hilaire)
Mexican Free-tailed Bat, Le Conte’s Free-tailed Bat, Free-tailed Bat, Guano Bat.
Poorly known. Possibly found statewide, but most remaining populations are in southern half. Occurs only in human-made structures. HIGH CONSERVATION CONCERN.
A medium-sized (adult forearm = 36-46 mm [1.4-1.8 in.], wingspan = 290-325 mm [11-13 in.]; weight = 9-15 g [0.3-0.5 oz.]) bat, and the only molossid species occurring in Alabama. Easily distinguished by wrinkled lips, long narrow wings, and by distal half of tail, which protrudes beyond tail membrane. Pelage velvety, dark brown to dark gray (Barbour and Davis 1969, Lowery 1974, Schmidly 1991). Ears large and rounded; do not join in middle of forehead. Stiff hairs protruding from toes as long, or longer than, foot (Barbour and Davis 1969). A distinctive, musky odor produced by gular gland, enables humans to detect colonies from several hundred feet away. Two subspecies recognized in the United States (Barbour and Davis 1969, Lowery 1974).
In Alabama and the southeastern United States, a bat primarily of the Coastal Plain and Piedmont. T. b. cynocephala, the eastern subspecies, occurs from extreme southeastern Virginia through eastern Texas and Arkansas. The highly migratory, western subspecies, T. b. mexicana, occurs throughout the Southwest from central Texas and Oklahoma to California and Oregon, and northward to Kansas, Colorado, and Utah (Barbour and Davis 1969). Has been documented throughout the lower two-thirds of Alabama; a record from Jackson County is considered outside the normal distribution (Holliman 1963).
Uses a variety of natural and artificial roost sites. In the eastern United States, never uses caves and is nearly, if not totally, dependent on human-made structures for summer and winter roosts (Jennings 1958, Kiser 2000, Schmidly 1991). Frequently found in attics and walls of masonry and wooden structures, including schools, courthouses, churches, and homes. Expansion joints of bridges and sports stadiums also are favored summer roosts (Kiser 2000). Prior to European settlement of the United States, the eastern subspecies probably used large hollow trees almost exclusively (Jennings 1958). Has been reported in hollow trees in Louisiana and black mangrove trees in Florida (Jennings 1958, Lowery 1974). Throughout the west, caves and cliffs are the primary summer roost sites, although it also uses buildings and other structures for rearing young (Barbour and Davis 1969, Lowery 1974, Schmidly 1991). Nearly all T. b. mexicana migrate to caves in Mexico for winter, although a small percentage (often first-year males) remain in caves and bridges in the United States (Schmidly 1991). Will use bat houses for rearing young, as night roosts, and as migratory stopover and wintering sites.
Diet consists of primarily moths.
LIFE HISTORY AND ECOLOGY:
A bat of the tropics and subtropics. A poor hibernator, the western subspecies stays active in winter by migrating to Mexico (Schmidly 1991). In the southeastern United States,
enter periods of torpor when temperatures drop below 20oC (68oF). A year-round resident of Alabama. Mate in early spring before leaving winter roosts and shortly after arriving at maternity roosts. Females produce one offspring per year after an 11-12 week gestation period, with birth occurring in late May or early June (Barbour and Davis 1969, Schmidly 1991). Twins have been reported from one colony in Tuscaloosa, Alabama. Buildings used as maternity sites may contain a few dozen bats in winter, but often are deserted after autumn (Barbour and Davis 1969). Winter roosts can be difficult to locate. Migrate only short distances between summer and winter roosts, from 305 meters (1,000 feet) up to 135 kilometers (84 miles) (Kiser 2000). Maternity colonies in Alabama typically consist of several hundred individuals, often sharing roost sites with big brown bats and evening bats. In the 1990s, four colonies in Alabama were observed containing more than 1,000 individuals, and one of these may have contained 5,000 to 10,000 (Kiser 2000). In contrast, colonies in the West can number from the thousands to millions. Just 13 Texas caves shelter an estimated 100 million individuals. Bracken Cave, near San Antonio, shelters 20 million free-tailed bats in summer (Barbour and Davis 1969), the largest concentration of mammals on the planet. That colony is estimated to consume 200 tons of insects nightly. A fast, high-altitude feeder that feeds primarily on small moths, but also on beetles, flying ants, leafhoppers, and true bugs (Schmidly 1991). One of the most frequent users of bat houses. A specially designed 5.5 x 5.5-meter (18- x 18-foot) artificial roost constructed in 1991 at the University of Florida in Gainesville currently is occupied by 100,000 free-tailed bats (Kiser 2000). This colony is believed to be the largest extant concentration of T. b. cynocephala remaining.
BASIS FOR STATUS CLASSIFICATION:
Until 1996, T. b. cynocephala was listed as a Species of Special Concern by the Alabama Department of Conservation and Natural Resources, Division of Wildlife and Freshwater Fisheries (DWFF). Few active roost sites in Alabama were known, although the species was described by Holliman (1963) as “moderately common.” The discovery of new roosts in the early- and mid-1990s in 15 counties throughout the central and southeastern portion of the state indicated the species was perhaps more common than previously thought, and its protected status was removed. However, of 26 roost sites surveyed in Alabama from 1990 to 1994, only two, both in bridges, are thought to remain. Throughout the southeastern states, free-tailed bats are locally common, but secure or protected roost sites are few. Widespread exclusion from buildings and deliberate destruction of colonies have contributed to population declines in recent decades. Modern building designs, and renovation and demolishing of old buildings have left fewer roost sites available. Pesticide exposure also may have played a role in species decline.
W. Mark Kiser