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U.S. Geological Survey

Publication Date

August 2003


Biological characteristics: Species: Tadarida brasiliensis, the smallest free-tailed bat, is 3 1/2 - 4 3/8 inches in length, with a forearm length of 1 3/8 - 1 3/4 inches, and a mass of 11-14 grams. They are snub nosed and thickset, dark brown or gray on top, with hairs that are whitish at the base. Ears of this species are separated at the base, and the tail is naked, with at least half extending beyond the interfemoral membrane. Wings are narrow, and flight is straight. Spoon-shaped bristles on the hind toe are used to groom themselves, and thumb and toe claws have double talons. Status in Estuaries: In the east and on the west coast this bat hibernates but does not migrate; bats in Texas and the southwest usually migrate to Mexico in October and return in March. Older males may remain on the wintering grounds year-round ( Thies et al., 1996 ). This species lives in large colonies in caves and buildings. Young hang together in a nursery, and mothers return and feed any of them. The lifespan is usually 13-18 years for males and 12 for females, who produce 1 offspring per year (McCracken, 1986). Abundance and Range: Breeding occurs in southern U.S., from Oregon, Nebraska, Louisiana, and South Carolina south to Mexico. These bats winter on their breeding grounds or migrate to Mexico. This is one of the most abundant North American mammals, and the most common bat in the southwest U.S., with an estimated population of 120-150 million in the U.S. (McCracken, 1986). However, the population in Carlsbad Caverns, New Mexico, has declined from 8-9 million to several hundred thousand since the 1930's. Site Fidelity: High cave site fidelity due to limited number of maternity caves. Return to cave nightly unless migrating, and return to same cave after seasonal migration if available. Sometimes must relocate due to human disturbance and habitat loss (McCracken, 1986). Ease of Census: Patterns are distinguishable in departure and arrival to and from cave, but thousands may be moving together. Though difficult to get an accurate count, probably should be classified as "simple". Easier to estimate during roosting. Adults roost in densities of 1800 per square meter, and newborns 5000 per square meter (McCracken, 1986). Feeding Habits: May travel up to 150 miles to feed, though some (including those at Carlsbad Caverns) stay within 50 miles of their cave. Feeding occurs in flight, from sunset to sunrise, at approximately 10-15 mph and includes insects such as moths, ants, beetles, and leafhoppers. Attached pdf contains ecotoxicological information and references. Open Access See Extended description for more information.