Catching A Texas Batnado
Bracken Cave, 20 miles outside of San Antonio, is the summer home to 15 million Mexican Free-tailed bats. Each night, the bats swarm out of the cave in a “batnado“ in search of food.
Fran Hutchins, director of Bat Conservation International’s Bracken Cave Preserve, talks about how the millions of individuals form a colony and the conservation efforts to preserve this colony in the face of housing developments and the encroaching city.
IRA FLATOW: This is Science Friday. I’m Ira Flatow, coming to you from the Tobin Center for the Performing Arts in San Antonio, Texas.
Yes. You know, San Antonio has a lot of nicknames. You can call it the Alamo City, Military City, home of the Spurs. I’m going to add another one. I’m adding another one to the list called Bat City, USA. And that’s because every summer, millions of Mexican Free-Tailed bats cross the border.
And they fly up, and they go through this little [INAUDIBLE], this cave that they like to occupy. They wing their way to Bracken Cave, located just north of San Antonio. And that makes this town home to one of the largest bat colonies in the world. We’re talking 15 to 20 million bats in the Bracken Cave. Our team went to the cave and brought back a sound portrait of their visit.
SPEAKER 1: We are at Bat Conservation International’s Bracken Cave Preserve. The mouth of the cave is at the bottom of a 80 foot deep sinkhole. If you look right down into the darkness below that second arch, you can see– it looks like the air’s moving a little bit. Those are the bats swirling right there in the darkness. And you’ll catch a glimpse of them every now and then.
So that vortex is going to move out to the mouth of the cave. And then once it’s in the sinkhole, it has the spiral upwards to get to the top of the trees. And then they stream away, and this river of bats in the sky.
SPEAKER 2: It smells like kind of a wet dog.
SPEAKER 1: And that smell of guano, it’s all over the floor. The guano is like over 75 feet deep.
SPEAKER 2: Is it happening?
SPEAKER 1: Here they come. And it pulses. It gets really strong, and then it kind of pulses down and pulses up, so as they come out. That’s a Swainson’s hawk. See that raptor down range there on the outside of that big tree? They just catch them with their talons. Yeah, they’ll just dive in and grab them, or eat them right on the wing.
EDITH BERGQUIST: I’m Edith Bergquist, and I’m a volunteer at Bracken Bat Cave. And I’m also a master naturalist, and we live nearby.
SPEAKER 2: Can you see them by near your house?
EDITH BERGQUIST: We can see them from our back porch.
SPEAKER 2: What’s it look like back there?
EDITH BERGQUIST: At first, it looks like smoke coming out. And of course now, we know what it is. But eventually, they actually come right over our house.
SPEAKER 1: A lot of bats in the cave.
EDITH BERGQUIST: Yeah.
SPEAKER 1: It takes them about 3 and 1/2 hours to clear out, empty the cave. We’re going to be home in bed, or home, anyway. Yeah, this is like 1%.
IRA FLATOW: Joining me now for the rest of the tour is Fran Hutchins. He’s director of the Bracken Cave Preserve, part of Bat Conservation International. Welcome, Fran.
FRAN HUTCHINS: All right, thank you.
IRA FLATOW: Now, the bats fly in from Central and South America, right?
FRAN HUTCHINS: Yeah, and Mexico, yes.
IRA FLATOW: Yeah, and they could stop anywhere they want to. Why the Bracken Cave?
FRAN HUTCHINS: Well, it’s the perfect setting for the bats. It’s a large, cavernous space. You can kind of think of it like a giant incubator.
IRA FLATOW: You know, we all talk about people having their own man caves at home.
FRAN HUTCHINS: Right. [CHUCKLES]
IRA FLATOW: But Bracken is what you call a mom cave. What does that mean?
FRAN HUTCHINS: We have somewhere around 8 to 10 million pregnant females that move into the cave in mid-March. And then in early June, everybody has a baby. A baby bat’s called a pup. And so that’s when our population jumps up to around 15 to 20 million.
IRA FLATOW: Wow. Wow. And the bats live in Bracken Cave. These bats are called Mexican Free-Tailed bats.
FRAN HUTCHINS: Right.
IRA FLATOW: Well, tell us about them. What are their habits? What do they eat?
FRAN HUTCHINS: Well, they’re insectivores. So they’re really important to our local farmers and us because of all the insects that they eat. And Bracken Cave is one of a dozen large colonies of bats that we have in the Texas Hill Country.
IRA FLATOW: So you’re saying– when you say they’re important to the farmers, because they eat so many insects– what, mosquitoes and other insects?
FRAN HUTCHINS: This particular bat is going to target a lot of our agricultural pests, [INAUDIBLE] So tonight, right now, when these bats come out for dinner, they’re going to eat about 147 tons of bugs. And that’s every night of the week.
IRA FLATOW: Wow. So that’s less pesticide then.
FRAN HUTCHINS: Right. It saves the crop damage and less pesticides that the farmers are having to spray on our crops.
IRA FLATOW: That’s really great. Now you say there are millions of bats in the cave. How do they organize themselves? Do they create little groups? Tell us what goes on inside the cave.
FRAN HUTCHINS: This colony is made up of thousands of smaller colonies that have migrated into Bracken Cave. We can’t really tell the difference between one colony space and another, but they know. You’re looking at around 500 bats per square foot, roosting on the walls of the cave. So a sheet of notebook paper sized space can have a few hundred bats hanging from it.
IRA FLATOW: How big are the bats? About the size of your hand?
FRAN HUTCHINS: About as big as my two thumbs together with a 10 inch wingspan.
IRA FLATOW: So they have to think about dinner every night when they go out, right?
FRAN HUTCHINS: Yes, these are hungry pregnant female bats.
IRA FLATOW: And tell us– I heard it described that when they leave the cave, they swarm into something called a batnado.
FRAN HUTCHINS: Right. So yes, we have literally a batnado in the sinkhole as they spiral up because they got to get out of the cave and out of that sinkhole to be able to fly downrange. Because they’re heading out about 60 miles to feed.
IRA FLATOW: They go 60 miles.
FRAN HUTCHINS: And Mexican Free-Tailed bat will head out about 60 miles and can feed up to around 10,000 feet in altitude. Just depends on where there’s large insect migrations. They’re flying into those insect migrations to feed on them.
IRA FLATOW: Now do they send like a scout bat out first to see if it’s the right time, the right weather?
FRAN HUTCHINS: No, they just–
IRA FLATOW: Or how does that work?
FRAN HUTCHINS: We’re not sure what the trigger is. And being a scout bat’s really not a good thing because you’re the only bat out there. And the hawks, it’s easy pickings if you’re just a lone bat. So they all come out together, and that stream in the sky makes it more difficult for the hawks to get them.
IRA FLATOW: And the swarm or the stream is so big that you can see it on radar.
FRAN HUTCHINS: Right, they’ll show up on Doppler radar as a cloud forming over the cave.
IRA FLATOW: Entire colony goes out for the night. The whole colony goes out.
FRAN HUTCHINS: Right.
IRA FLATOW: And next question is how do they get back in, I mean, if they’re all out?
FRAN HUTCHINS: Right, yeah. And yeah, so they’re coming in from hundreds of feet up in the air. So they’re coming in from about 360 degrees, but primarily south, southeast. So directly above the cave, they tuck their wings and dive bomb into the cave. It literally rains bats in the morning.
IRA FLATOW: How fast are they going?
FRAN HUTCHINS: We’ve clocked them at over 40 miles an hour in the dive.
IRA FLATOW: That’s a 40 mile an hour dive from 10,000 feet.
FRAN HUTCHINS: Yep. And they make a– the air going over their skin makes this buzzing sound as they’re diving in. Because it’s faster to dive in because the hawks are there for breakfast, too. So they want to get away from the hawks as fast as possible. They’ll come in right behind and just pluck them right out the sky.
IRA FLATOW: Wow. Let’s go to the– we have a– yeah, the microphone there. Yes, sir.
AUDIENCE: Yeah, I’ve heard about a lot of bat colonies being decimated in other parts of the country by this white nose disease, a fungal disease. Is that something that’s coming down to Texas?
FRAN HUTCHINS: Right, there’s a disease called white nose syndrome that’s affecting our hibernating bats. We do not have the disease in Texas yet. But we do have the fungus that causes the disease. And we discovered it this past February in Bracken Cave. But this disease affects our hibernating bats.
But basically, bats have two choices when they migrate. They can migrate out of an area to when it gets cold, or they hibernate like bears do. So our hibernating bats, this particular fungus thrives in cold temperatures. So it gets on their skin and eats into their skin. And it’s an irritant, and it wakes the bats up out of their hibernation cycle. And they burn off their fat reserves and starve to death.
We don’t have the disease here in Texas, but we do have the fungus that’s causing the disease. And Bat Conservation International is doing a lot of research right now on what we can do to mitigate the effects of this disease.
IRA FLATOW: Now you are director of the Bracken Cave Preserve. The cave was under possible development at one point, right? Developers wanted to go in and maybe effect the housing. They might have put up, might have affected the health of the colony.
FRAN HUTCHINS: Right. Yeah, it’s actually we were rolling on our five year anniversary. Halloween in 2014, we purchased the land away from a developer so they wouldn’t build homes.
IRA FLATOW: And this is something that people are doing around the world trying to conserve the bat population.
FRAN HUTCHINS: Right, and one of the biggest threats to bats is loss of habitat. So Bat Conservation International is working in a number of different countries to purchase caves to help protect them from humans, from development, so that our bat species can survive.
IRA FLATOW: Over here, yes, sir.
AUDIENCE: Actually, both of the things I was going to bring up were already talked about.
IRA FLATOW: All right, sit down. We’ll get some–
AUDIENCE: But no, no, no. As a reference, I was going to ask more about the pollination that the bats do, not necessarily the Free-Tails because they’re insectivores, but the pollination that other bats and other bat species do as well.
IRA FLATOW: Yeah, that’s a very good– I’m glad you didn’t sit down because that’s a good que– they are pollinators. We think–
FRAN HUTCHINS: Right, so besides being insectivores, a lot of our species of bats are pollinators, basically the nighttime version of honeybees. So those plants that bloom at night need bats to pollinate them. So there’s over 500 species worldwide that plants that humans rely on that bats pollinate.
So for example, cocoa– if you like chocolate, you can thank a bat. Bananas– so we need our wild bananas to keep our store bought bananas disease resistant. So bats pollinate bananas. And a very important, especially on Fridays, is tequila.
So bats pollinate agave, and we need those wild agave so that we have the farm raised agave for tequila production.
IRA FLATOW: We have time for one more question. Yes.
AUDIENCE: You mentioned these are all females. Where are the males?
FRAN HUTCHINS: Yeah, so the male bats are here, too. Most of the mating goes on in early February before they migrate. And we have bachelor colonies. So for example, if you’re over near the Pearl Brewery at Camden Street, that’s about 60,000, 50,000, 60,000. And so that’s a bachelor colony. The males, they can’t feed the young, so they’re just in the way.
IRA FLATOW: And that’s where the term man cave comes from.
FRAN HUTCHINS: That was the original man cave.
IRA FLATOW: I get it.
FRAN HUTCHINS: Yeah. See?
IRA FLATOW: I get it. Fran Hutchins, you filled in everything we wanted to know. Director of the Bracken Cave Preserve, part of Bat Conservation International, thank you for taking time–
FRAN HUTCHINS: Thank you very much.
IRA FLATOW: –to be with us today.
After the break, how the things people do, from ranching to bird feeders, can affect bird populations. Taking us to the break, our musical guests for the evening, [INAUDIBLE].
I’m Ira Flatow, and this is Science Friday from WNYC Studios.