Mammals
Mexican Free-tailed Bat
Tadarida brasiliensis · also called Brazilian free-tailed bat, guano bat
At dusk on a summer evening in central Texas, the sky over a cave mouth turns into a moving river of bats — millions of them, for hours, dense enough to register on weather radar. The Mexican free-tailed bat gathers in the largest numbers of any mammal on Earth, and once airborne it is the fastest animal in level flight.
Overview
Tadarida brasiliensis is a small, narrow-winged bat, about 9 cm long and 10 to 15 g, grey-brown, with the last third of its tail projecting past the membrane — the feature that names the free-tailed bat family. It ranges from the southern US to Argentina, roosting by the million in caves, mines, and bridges and foraging over open country for high-flying moths and other insects.
Numbers without equal
Bracken Cave, near San Antonio, holds an estimated 15 to 20 million of these bats in summer — the densest concentration of any mammal known. Several other southwestern caves hold millions more. In Austin, about 1.5 million roost under the Congress Avenue Bridge, the largest urban bat colony in North America and a nightly tourist draw.
The speed record
A 2016 study that tracked tagged bats leaving a Texas cave from an aircraft measured ground speeds over 160 km/h in straight, powered flight — faster than any bird has been shown to fly on the level. A diving peregrine falcon is quicker, but that is a dive, not level flight. For horizontal speed under its own power, this bat holds the record.
Status
The Mexican free-tailed bat is Least Concern and hugely abundant, but that abundance is concentrated into a few giant roosts. Damage to one cave can hit millions of bats at once, and the species is also killed at wind turbines and carries heavy pesticide loads, so conservation centres on protecting the key roost sites.
Mexican Free-tailed Bat: common questions
How fast can a Mexican free-tailed bat fly?
Very fast. A 2016 study fitted tiny transmitters to bats leaving a cave in Texas and tracked them from an aircraft, recording ground speeds of more than 160 km/h in level, powered flight, with the fastest individual near 45 metres per second. That is quicker than the fastest documented level flight of any bird, including the swifts often cited as the fastest fliers. Some of the top speeds were helped by tailwinds, and the peregrine falcon is still far faster in a dive, which is not level flight, but for straight-line horizontal speed under its own power the free-tailed bat is the current record holder among animals.
Where is the largest colony of Mexican free-tailed bats?
Bracken Cave, on protected land north-east of San Antonio, Texas, holds an estimated 15 to 20 million free-tailed bats during the summer breeding season. It is regarded as the largest known concentration of mammals anywhere on Earth. The bats are almost all females and their young, gathered in a maternity colony, and their nightly emergence to feed is one of the great wildlife spectacles of North America. Several other caves in Texas and the wider Southwest hold colonies of one to several million, and the total regional population runs into the tens of millions.
Why do so many people come to watch them?
Because a mass emergence is dramatic and predictable. At dusk in summer, millions of bats spiral out of a cave or, in Austin, out from under the Congress Avenue Bridge, forming a dark ribbon that streams across the sky for a long time. Crowds gather on the bridge and the riverbank in Austin every evening in season. Beyond the spectacle, the bats are economically important, because they eat enormous quantities of moths whose caterpillars attack cotton, corn, and other crops, and studies have valued this natural pest control in the hundreds of millions of dollars a year for agriculture in the south-central United States.
Are Mexican free-tailed bats endangered?
The species is listed as Least Concern and is one of the most numerous mammals in the Americas. But its abundance is misleading, because a large fraction of the entire population squeezes into a handful of enormous roost caves. Damage or disturbance to one of those caves, a collapse, vandalism, disturbance during the breeding season, or closure of a mine, can affect millions of animals at once. Free-tailed bats are also killed in large numbers at wind turbines, accumulate agricultural pesticides through their insect diet, and, in northern parts of the range, face the fungal disease white-nose syndrome. So conservation focuses on protecting the key roosts.