Nature Notes; Mexican Free-Tailed Bats

Hundreds of thousands of Mexican free-tailed bats migrate to West Texas each year. On cliffs, tunnels, bridges and caves, these summer residents birth and raise young.

And each night, they take flight – with an appetite. The bats are critical in controlling insect populations in the region.

Mexican free-tailed bats, also known as Brazilian free-tailed bats, are among the most abundant mammal species in the world. They are found from Chile to California. West Texas is home to two dozens bat species. But Mexican free-tailed are most common.

In most bats, the tail is contained within a membrane that joins the thighs. But in these bats, the tail extends beyond that membrane. A mature bat weighs half an ounce and is about three and a half inches long, with dark brown or gray fur. Their appearance is unassuming. But their flight sets them apart.

Each spring, the bats travel to West Texas from caves in northern and central Mexico. They can reach speeds of 60 miles per hour in their journey. And they regularly fly at altitudes of more than 10,000 feet – higher than any other bat.

The bats do their breeding in Mexico. When they arrive in Texas, they’re opportunistic in selecting homes, Michael Nickell, museum scientist at Midland’s Sibley Nature Center, said.

“They’re very adaptable,” Nickell said. “They find roosting places not only in caves, but underneath bridges and railroad trestles. They will find their way into human habitations. Wherever they can find a little nook or cranny to spend the daytime hours in, they find it. What’s really amazing are some of the caves that house these maternal colonies – they can actually number into the millions of bats.”

Famous colonies are found in Central Texas, at the Congress Avenue Bridge in Austin and at Bracken Cave, near San Antonio, where 20 million bats roost each year. Major colonies in our region include Carlsbad Caverns, and the Clarity Tunnel, an abandoned railroad tunnel at Caprock Canyon State Park, near Lubbock. Hundreds of thousands of bats summer at each location.

While males may roost alone, females prefer to spend the daytime hours together, in maternal or nursery colonies.

In June, each female gives birth to a single pup. The pair remain attached for up to an hour, as mother and pup learn one another’s scent and voice.

That connection is crucial. In a dense colony, hundreds of bat pups can roost in a single square-foot. A mother finds her pup multiple times each day to nurse. Scent and sound are key.

By four to five weeks, the young bat has reached its adult weight – and is ready for its first flight. In echolocation, bats emit an ultrasonic pulse. Through the echoes, they gather detailed information on their surroundings. The young bat’s echolocation skills are put to the test in its first flight. The bat must dodge mid-air collisions with its peers, often multiple times each second.

At dusk, the bats fly from their roosts. The bat flight at Carlsbad Caverns National Park draws crowds of onlookers on summer nights. Nickell said the flight at Clarity Tunnel is equally impressive.

“When the bats started coming out, you could feel the breeze – the breeze would actually be blowing your clothing,” Nickell said. “It was absolutely amazing. We sat there watching this bat flight for a solid hour, and it had not ended.”

The most spectacular flights occur in late summer and early fall, as the young join in the hunt. The bats disperse into the night – to feed on every manner of flying insect. A Mexican free-tailed bat can travel 50 miles in a night to feed, and can easily consume its weight in insects each night.

“It’s tons and tons and tons of insects,” Nickell said. “Many of these insects that they feed upon are going to be agricultural pests, and so, really, bats are one of the farmer’s greatest allies.”

The bats’ West Texas stay ends with October’s first chill. The bats will take to the air in flocks, and make a swift journey to their wintering grounds.

But on summer nights, keep your eye out for that distinctive, erratic flight. Odds are, it’s a very hungry Mexican free-tailed bat.

Nature Notes is underwritten by the Dixon Water Foundation and is produced by Marfa Public Radio in cooperation with the Sibley Nature Center in Midland, Texas. This episode was written by Andrew Stuart.