SAN ANTONIO -- Bat Conservation International's Bracken Cave is home to the largest colony of bats in the world, and offers visitors a rare opportunity to see millions of Mexican free-tailed bats emerge from their natural cave roost. 

"We're talking somewhere in the neighborhood of 20 million Mexican free-tailed bats," said Fran Hutchins, director of the Bracken Cave Preserve.

Mouth of Bracken Cave (Photo: TPWD)

Located in southern Comal County, about 20 miles northeast of downtown San Antonio, Bracken Cave is a 100-foot-wide crescent shaped opening in the earth that lies at the bottom of a sinkhole.

BCI said it first purchased the land on which the cave is located in 1992. Since then, the organization and its partners have continued to purchase land around the cave to conserve not only the bats, but also the many other native and endangered species found on the Bracken Cave Preserve.

"Think of the cave as a giant incubator. Ceiling temperatures right now are 102 to 104 degrees and 98 percent humidity. So, it's a very hot environment inside the cave, which is great if you're a baby bat," said Don Bergquist, a volunteer with Bracken Cave Preserve.

Mexican free-tailed bat (Photo: U.S. Fish and Wildlife Service)

BCI says the bats of Bracken Cave comprise the largest known bat maternity colony in the world, producing millions of young each year.

There are only a small number of suitable sites in Texas for large maternity colonies, which require high humidity and temperature levels, according to Texas Parks and Wildlife.

In March and April, expectant females return to the cave after wintering in Mexico.

"When they come to us from Mexico, we think that they roosted in various places. But when they come here, they come into the maternity colony as one big group," said Bergquist.

Male bats do not help in raising the young and form smaller bachelor colonies elsewhere.

In the latter part of June, females give birth to a single pup, nearly doubling the cave's population.

BCI says that as the bats give birth, the cave's walls become densely packed with pups, clustering tightly at up to 500 per square-foot. By late July, the young bats are ready to test their flight skills outside the cave, and join their mothers to feed on insects.

To find sufficient food, the bats often emerge up to three hours before sundown, creating one of the most spectacular sights in all nature, an event spanning several hours, according to BCI.

"We're not sure what the trigger is, but some time before it gets dark, the bats will go out into the bottom of the sinkhole. We're literally going to see a vortex, or a 'batnado,'" said Hutchins.

The plume of bats is dense enough to show up on airport and weather radar.

Mexican free-tailed bats are the most common bat found throughout Texas.

In 1995, the Texas State Legislature named the Mexican free-tailed bat as the "state flying mammal."

TPWD officials say the bats can fly up to 100 miles round trip in an evening looking for food. They are built for speed with short fur and long narrow wings and can fly up to 60 mph with a tail wind.

Mexican free-tailed bats are an essential predator of corn earworm moths and other crop pests, including ants, beetles, and leafhoppers, according to TPWD. The Bracken Cave colony alone is estimated to consume more than 100 tons of these moths every summer night.

"There are a lot of myths out there about bats. For example, a lot of people think bats get in their hair.  A lot of people think bats are blind. We have over 1,300 species of bats worldwide, but none of our bats are blind," said Hutchins.

TPWD officials said the bats have been observed flying at up to 10,000 feet while searching for food.

"This colony of bats will, with the amount of insects they're going to eat, this colony is going to eat around 140 tons of bugs. A world without bats is a world full of biting insects," said Hutchins.

When the first cold fronts work their way through the Texas Hill Country in late October to mid-November, the Mexican free-tailed bats begin their migration back to Mexico.

BCI stewards the entire 1,500-acre property around Bracken Cave, rehabilitating the land by removing invasive vegetation and reviving native plants.

Although the land lost some of its original plant and animal diversity as a result of ranching and other commercial uses, TPWD says the area is being managed as a nature preserve, and is home to many bird species, including endangered golden-cheeked warblers.

The Bracken site has also played a role in Texas and U.S. history. Confederate soldiers mined the cave in the 1860s for guano -- bat droppings -- to manufacture black gunpowder, during the Civil War. 

The guano was later used as a rich fertilizer for croplands across the U.S. 

Photo: TPWD

To visit the site on a member night, membership in BCI is required and starts at $45. Reservations to attend are required for BCI members.

Bracken Bat Flight tours also offers guided viewings several times a week, April through October. The tour begins at Natural Bridge Caverns with an educational briefing on bat species, behavior, and habitats prior to the flight viewing at Bracken Cave.

The cost per person starts at $37.99, with a portion of the proceeds supporting stewardship and conservation of the Bracken site. For more information and to make reservations, visit: www.brackenbatflight.com. 

For more information about Bracken Cave, and how you can help, visit www.batcon.org/Bracken.

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