The Bats of Bracken

…one of the most spectacular wildlife events that you can see anywhere…It’s the largest congregation of bats in the world, and they come out of this cave by the millions.

~Mylea Bayless, a senior director at Bat Conservation International

Here is another long overdue post from a wonderful trip to Austin, Texas, a few months ago. Melissa had a presentation at a national conference, and I was lucky enough to tag along on the front end. Our gracious hosts are long-time friends of her family, so we had a great tour of this beautiful city, its food, and nearby natural wonders. I have been lucky to see some incredible wildlife spectacles in my wanderings – the overwintering reserves of monarch butterflies in Mexico; thousands of reef fishes in all colors of the rainbow in the Virgin Islands; bison herds in Yellowstone; huge flocks of sandhill cranes and snow geese at Bosque Del Apache National Wildlife Refuge in New Mexico; and fields full of black bears and tundra swans back home in North Carolina. On this trip, I added another, the bats of Bracken Cave.

Bracken Cave

The unassuming entrance to Bracken Cave (click photos to enlarge)

We had heard about the bats in Austin and at Bracken (closer to San Antonia) from our hosts’ son, Skip, a professional wildlife filmmaker, when he visited us last year. We joined Bat Conservation International (BCI) before our trip in order to qualify for an overnight camp-out near the cave. This allowed us to witness both the evening departure and early morning return of the millions of Mexican free-tailed bats that call the cave home for several months each year.  I wasn’t that thrilled with the idea of camping in Texas in early September, but we decided it would be worth it for this. And it turned out just fine, with mild temperatures, a wayward wild turkey that hung out with us, and a great group of fellow nature nerds with which to share a campfire.

Shortly after arriving, we walked down to the seating area near the cave for an introduction by one of the volunteers that help manage the site. Here are just a few of the amazing facts he shared about this incredible place:

  • There are an estimated 15-20+ million Mexican free-tailed bats (Tadarida brasiliensis) using the cave. Most of these bats spend the winter in Mexico (~1000 miles from this cave), where they mate before returning to Texas. Pregnant females form huge maternity colonies at places like Bracken Cave that have the right temperatures and humidity for raising their young (male bats form smaller colonies elsewhere).
  • Bracken Cave has one of the largest concentrations of mammals anywhere in the world.
  • The cave is a birthing place for millions of bat babies (called pups) every year with concentrations of up to 500 pups per sq. ft. hanging on the cave ceiling. Somehow, using a combination of sight and sound, mothers returning from a night of foraging manage to find and nurse their own pup amid this chaos. Pups are about 1/3 the weight of the mother (the equivalent of a human mother giving birth to a 40 lb. baby).
  • Each female bat gives birth to one pup per year with only about 50% of the pups surviving their first year. Once they reach adulthood, these bats may live up to 15+ years in the wild.
  • Mother bats must leave the cave each night to feed on night-flying insects to sustain themselves and their growing young.  They fly up to 60 miles away from the cave each night, consuming huge quantities of flying beetles, winged ants and moths. Scientists estimate the Bracken bats eat 140 tons of insects every night. The value of this to Central Texas is huge in that one study showed the bats save the just the region’s cotton growers an estimated $741,000 per year in pesticide costs and crop damage.

The viewing at Bracken is controlled by BCI and limited to small groups a few nights per week. They are doing a great job of ensuring the bats keep returning to Bracken Cave. The site and surrounding landscape has been threatened by developments but recent partnerships brokered by the Nature Conservancy between business, politicians, and conservationists, have set aside more land to provide a buffer for the bats.

As we sat and listened, we saw some of the many predators that stalk the bats as they exit – a skunk, two coachwhip snakes, a couple of hawks, all seeking an easy meal provided by a nightly cloud of bats, that now included the inexperienced young. The air was still and had a slight pungent aroma of bat guano, which was once harvested from this cave for fertilizer.

The show begins slowly, a few bats fluttering out of the dark hole. People start pointing and you can hear some excited chatter. In the clip above, the wind was blowing so there is a bit of sound distortion.

Bats blurred coming out of entrance 1

Bats emerging from the cave entrance

The sound of thousands of bat wings flapping is a bit like running water, and it fades and grows depending on where the swarm circles in relation to our viewing area.

Bat silhouettes above entrance 1

A cloud of bats in the sky above Bracken Cave

After circling the cave for a few minutes, the swarm begins to form, circling around higher until it trails off like a stream of smoke on the horizon.

Mexican free-tailed bats flying out of Bracke Cave

The exodus continued for a few hours

It became very quiet, except for the whir of wings, as we all sat there in awe at this spectacle, trying to comprehend the sheer numbers of animals emerging from the ground before us.

Bats in late sunlight

A bat storm

Bats blurred coming out of entrance

The beauty of bats in motion

Bat caught on cactus

The cacti surrounding the cave claimed a few bats

Snake at entrance

As so did a couple of snakes patrolling the entrance (one is in the lower right in photo above)

We began to look at other details in the scene as the seemingly endless stream of beating wings continued – a bat caught on a cactus spine; snakes grabbing bats that hit the ground near the entrance; a peregrine falcon and a Cooper’s hawk that each nabbed a bat mid-flight; missed attacks by a red-tailed hawk. It apparently isn’t easy being in such a dense crowd, especially if you are an inexperienced youngster.

Bats and people at Bracke Cave

People viewing the bats from above the cave entrance

Several people eventually walked the designated path to the other side of the cave entrance to experience that view, providing me with a unique perspective of two groups of mammals.

As darkness settled, the stream of winged creatures continued to emerge and fly off to hunt. I finally walked over to a point in the path where the exiting swarm was flying directly over my head.

Absolutely stunning! Be sure to turn your sound up for the roar of the wings and the clicking noises. And a reminder that this scene continued for a few hours! We finally retreated to our campsite and enjoyed some good conversation before retiring to our tent for the evening.

We were up early the next morning to witness a very different event – the return of the bats to the cave. By the time we arrived (at the first hint of light), the bats had already been returning for a couple of hours. There wasn’t a huge swarm like the mass departure of the night before, but rather a continuous stream of small groups of bats. They zigzagged down into the cave at rapid speed from high in the sky, perhaps to avoid predators. The sound of plummeting bats was like that when you rapidly swing a short section of rope above your head – whup, whup, whup. After a couple of hours, the numbers dwindled, and then finally stopped. We packed up and headed back to Austin to witness another bat spectacle.

Crowd in Austin watching bats

Crowds gather each evening in Austin to watch the bats

It turns out that among the many cool things about Austin (breakfast tacos, live music, art) it also is home to the largest urban bat colony in North America. About 1.5 million Mexican free-tailed bats call downtown’s Congress Avenue bridge home from mid-March to early November. It has become a nightly tourist attraction for hundreds of visitors.

Bats at bridge in Austin

Bats flying out from the Congress Ave. bridge with viewers lining the bridge and the surrounding shoreline

Bats have apparently lived in Austin for many years but did not start using the bridge in large numbers until some renovations in 1980 that created 16-inch deep crevices running the length of the underside of the bridge. Bats use the cracks that are 3/4 to 1 1/2 inches wide where temperature and humidity conditions are just right for raising their young.

Dense cloud of bats

A spectacle definitely worth experiencing

It was quite a contrast from the quiet and solemn feel of Bracken Cave (above), but it was very cool to see so many people congregate for a natural event. In fact, I can’t think of too many other places on the planet where this many people regularly gather to witness such a spectacle of wild creatures. Kudos to the people of Texas that have helped protect these amazing concentrations of mammals that allow us to experience such moments of awe in the natural world.








10 thoughts on “The Bats of Bracken

      • Down the hill from my home, there was a hole in the ground that bats came out of. It was odd at first. One day, I watched it as bats just continued to come out. I had never paid much attention to it before . . . because I just left when I saw the bats . . . promptly! Anyway, they just kept coming out of the hole! There were so many of them, and they were still coming out when I left! I can’t help but wonder where that hole goes, and what is in there!

  1. Fascinating! Thanks for sharing this. Bats are such important creatures and, well, ‘different’ I suppose in terms of the wildlife that people commonly think about, although of course they actually make up a large proportion of all mammals. I visited the so-called ‘bat volcano’ in Mexico when I was working on a research project there, where huge numbers of bats come up like a tornado out of a limestone cave. I will definitely be looking for bats if I ever get to Austin!

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