When newly hired meteorologists get to San Antonio, the first thing they have to learn is that the heavy showers they see forming on radar northeast of the city every summer evening have nothing to do with rain.
That’s because radar isn’t picking up rainfall. It’s picking up bats taking flight. Bat Conservation International’s Fran Hutchins said there are so many, if you’re close by, you can even hear them.
“We call it a 'batnado.' It's so intense,” he said.
Hutchins stood in a bowl-shaped setting with concrete benches facing southward as the rocky ground slipped downward toward a cave.
“We're sitting here at Bracken Cave Preserve, and Bracken Cave is the home to the largest colony of bats in the world,” he explained.
Just about five miles northeast of the city limits, a few hundred yards from Natural Bridge Caverns, Bracken Cave is an eye-shaped hole in the ground that’s about 650 feet wide and 117 feet deep. Hutchins said it would be a lot deeper if it weren’t for the guano—or bat excrement.
“That guano has accumulated over thousands of years to where it's almost 100 feet deep,” he said.
How many bats does it take to generate 100 feet of guano or to impersonate a thunderstorm? Hutchins said they’ve done the research, and the numbers are staggering.
“We have mommy and baby bats in the cave. We started having babies about 10 days ago, so our population went from eight to 10 million pregnant females, up to around 20 million mommies and babies,” he said. “The babies will start flying in a couple of weeks.”
The males roost elsewhere, like under Austin’s Congress Avenue bridge, or San Antonio’s I-35 downtown. Males roosting separate from the females is just the way Mexican free-tailed bats operate. Hutchins thinks vampire movies pretty well tainted Americans’ views on bats and that we need a collective reset to understand all the good the world’s 1400 species of bats do.
“There are fruit bats [that] are important for seed dispersal. We've got pollinating bats, which are basically the nighttime version of honeybees,” Hutchins said. “So there's over 500 species of plants worldwide that bats are responsible for pollinating—and one that a lot of us are familiar with are agave. That's where we get tequila mezcal so that's important.”
There’s even one bat whose favorite treat is scorpions.
“Pallid bats love scorpions, which is great. A scorpion to pallid bats are like candy...[T]hey can hear a scorpion run across the ground from about 15 or 16 feet away, and they just fly over and just jump on top of it and start eating it.
The main thing done by these tens-of-millions of Mexican free-tailed bats each night is dining. Hutchins said each night they eat their body weight in bugs.
“So what's really cool with these bats eating their body weight in bugs, which are primarily agricultural pests. Traveling about 60 miles one way to forage over where we have our corn and cotton and sorghum crops,” he said. “They’re going to fly 60 miles, and up to 10,000 feet in altitude, they're going to eat their body weight in bugs, which is over 150 tons of bugs tonight. And they're going to come back, nurse their young and do this all over again tomorrow.”
They will, of course, be passing some of those digested bugs onto the floor of Bracken Cave, and that 100 feet deep space of guano at the cave bottom used to be a harvestable commodity in centuries past.
“The Marbach family protected this cave for hundreds of years, mining the guano because it's a great fertilizer,” Hutchins said. “It also is used in gunpowder production. It's really high in nitrates. So during the Civil War, they used the guano to make gunpowder.”
Bracken Cave volunteer Edith Bergquist lives nearby.
“As the crow flies, maybe a quarter mile. Our house backs up to this property,” she said. “When we moved down here 19 years ago, we didn't even know this was here. A neighbor told us that there was a bat cave. So we immediately went to the website and signed up and came out to see them and we just became passionate about bats.”
She and her husband soon saw the Batnado of millions of bats swirling up and flying right over their property.
“It really wasn't scary at all ... We were just amazed that we were so close, that we could see it that well,” Bergquist said.
Hutchins says the cave doesn’t have a well-documented discovery story.
“Sometime when San Antonio was settled, the story goes that some people were in the area, and they saw smoke in the sky,” he said. “And they were following the smoke, and the smoke was actually bats streaming away at the top of the trees. And following that smoke is how they found the cave and the bats.”
@texaspublicradio When newly hired meteorologists get to San Antonio, the first thing they have to learn is that the heavy showers they see forming on radar northeast of the city every summer evening have nothing to do with rain. That’s because radar isn’t picking up rainfall. It’s picking up bats taking flight. Bat Conservation International’s Fran Hutchins said there are so many, if you’re close by, you can even hear them. “We call it a batnado. It's so intense,” he said. Just about five miles northeast of the city limits, a few hundred yards from Natural Bridge Caverns, Bracken Cave is an eye-shaped hole in the ground that’s about 650 feet wide and 117 feet deep. Hutchins said it would be a lot deeper if it weren’t for the guano—or bat excrement. How many bats does it take to generate 100 feet of guano or to impersonate a thunderstorm? Find out at tpr.org 🎥 Gideon Rogers, Jack Morgan / TPR
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The cave and the surrounding area were under threat back in the '90s.
“Bat conservation International got involved in 1992 when we purchased the five acres of the cave and that started the protection of land in this area,” Hutchins said. “And now between the Nature Conservancy and Bat Conservation International and City of San Antonio or Edwards Aquifer authority, there's almost 6000 acres of continuous green space all the way from Highway 3009 all the way to Highway 281.”
Christie Minor is a Project Coordinator at Bracken, which itself now contains a sizeable acreage.
“The Preserve is 1500 acres. So we do a lot of other things. We do tree surveys, where we walk designated areas of the preserve and tag all of our old growth trees to kind of inventory them,” she said.
Bracken’s baseline for information gathering and processing is science-based. She said it’s funny though, that not all Batnado visitors start out as bat fans.
“I find it really exciting when it's people who show up and say, ‘Oh, my friend dragged me here’ and blah, blah, blah. And then the bats come out and they're like, ‘Okay, I get it!’” she said.
Minor notes that the experience of watching the Batnado arc up and out of the cave and turn towards the south is incredible to just about everyone. And peoples’ incredulity at the sight is part-n-parcel of their understanding the need for its protection.
“You don't want to protect something that you don't know anything about or that you're not inspired by, so I think it's a really good experience for people to have,” Minor said.
Fran Hutchins thinks it’s particularly fun to see jaded guests' veneer crack.
“That's one of the cool things about having guests out here, especially the kids,” he said. “You got a 9-year-old, 10 year-old sitting here watching bats…so they put their phone down. And they're watching this amazing spectacle of nature. The tornado of bats is 20 feet away. You can feel the wind from the vortex. And the sound of wingbeats is like rain on the metal roof of your house.”
In a world full of computer screens and AI-augmented realities, watching 10 million bats emerge from a cave is about as real as it gets.
“When you have a nine-year-old turn to their parents and say, ‘Hey, this is cool’…I mean, you've accomplished something!” he said.
What goes up must come down, and the same goes for the bats. After an entire night of dining on bugs and a 120-mile round trip, Hutchins said the return is the most dangerous time for the bats.
“Around 5:00 in the morning, the bats start returning, so we see the reverse Batnado. Just a little bit before 6 a.m. we get first light. And that's when the hawks show up for breakfast, and spiraling back in takes too long,” he said. “So the bats change their behavior, and it literally rains bats, because they're diving in at over 30 miles an hour to get into the cave as quickly as possible. So the bat hawks don't eat them for breakfast.”
The hawks are there in smaller numbers in the evenings, too. We saw one fetch his dinner from the Batnado leaving the cave.
“Some people have lived in San Antonio all their lives and don't realize we have this amazing natural wonder here,” Hutchins said.
The Bracken Cave Batnado is available for public viewing most days. Learn more at Batcon.org.
