Summer School: How to count little brown bats, who are making a comeback in Vermont
On a warm night in July, state bat biologist Alyssa Bennett stands in front of an old wooden barn near Milton. It’s twilight, and fireflies are starting to blink.
We’re here tonight to find a maternity colony of little brown bats. If that sounds cute, it is: It’s a bunch of female bats hanging out with their babies, or pups.
An adult little brown bat is about the size of a thumb. It’s one of the nine bat species that are native to Vermont.
Bennett gestures to some boxes attached to the barn.
“We have six bat houses up,” she says. “We have another two on the backside in case the weather’s too hot and they want to move into the shade.”
We stand right underneath one of the boxes and listen.
“This is like the morning for them, so they’re waking up and stretching,” Bennett says softly. “They’ll groom a little bit, and you’ll see their wings flutter out, even while they’re in the bat house, so you’ll hear some ‘flap, flap, flap…’”
After a few minutes of rustling wings and the occasional audible squeak, bat after bat begins to drop out of the boxes and into the gloaming, like tiny hang gliders.
Soon they’re all around us, hunting bugs. In fact, while female little brown bats are nursing their young, they can eat their body weight in insects every night.
This colony is doing well. But just a few years ago, biologists like Bennett weren’t sure there would be little brown bats here in Vermont much longer. In 2011, the bats were placed on Vermont’s endangered species list, along with four other bat species. Little brown bat populations had dropped by 90% in just a few years.
A fast-moving fungus is the primary cause of their demise. White-nose Syndrome eats away at the bats’ wing tissue, which is only a few cells thick. It creeps in when they’re hibernating and their immune systems are partly shut down.
As we step away from the barn, Bennett pulls out two sets of fuzzy taxidermied brown bats, one big and one small.
The big one — a big brown bat — has wings that are smooth. But the small one has wings that are torn and full of holes.
“You’ll see on a normal healthy bat, which this big brown bat was, I think before a cat got it, its wing tissue looks pretty good," Bennett says. "And if you look at the little brown bats here, which is a species that is really susceptible to white-nose syndrome, you can see all this area on their wings that almost looks like it’s shriveled up."
After losing nine out of every 10 little brown bats in Vermont in the years after white-nose syndrome arrived here, Bennett says biologists feared the worst.
That’s why they were shocked to discover that little brown bat populations here seem to be stabilizing.
Bennett says they’ve got some ideas about why: “One of the hypotheses is called the 'fat bat hypothesis.' So that means that the bats that made it through white-nose syndrome are fatter going into the winter — just naturally fatter."
Bennett says that may give them an evolutionary edge. Little brown bats hunker down in caves through the winter. And looks like that extra fat lets them bounce back faster in the spring, so they can heal and get to work prepping for the next long Vermont winter.
There’s also some thought that different hibernation habits play a role in which bats survive. In either case, Bennett says this is very good news for people too. Researchers have estimated that bats save the United States’ agricultural system billions of dollars every year in pest management.
Back at the barn, Bennett pulls out an iPad with a bright orange microphone hooked up to it. Lights and lines blink across the screen. After a few moments of quiet, it starts chirping out strange sounds. She’s using the app to translate the high-pitched calls bats use for echolocation into our hearing range.
When they hunt, bats bounce sound off of objects, similar to the way a boat uses sonar. These sounds are too high for human ears to hear on their own.
“Their echolocation is actually so wonderful that they can tell the texture of an object — if it’s a moth, what kind of moth is it?” Bennett says.
Sometimes scientists will count bats by recording them this way, and sorting the calls they hear by frequency and species.
But Bennett says the most important tool she has for tracking bats is you and me. Every summer during the last two weeks of July, Vermont Fish and Wildlife’s Got Bats? program rallies community scientist volunteers to go out and count bats at colonies around the state. Anyone from little kids to grandparents can participate.
“You get out at sunset, you get comfortable in your chair, and you stare at your attic or chimney or bat houses and barn, wherever these bats are roosting, and you count,” Bennett says.
Little brown bats love to gather in really large numbers. A typical colony can have 100 to 500 bats, so Bennett recommends getting a hand tally counter, which costs a couple of dollars.
Bennett says it’s best to look towards the lightest part of the sky as you count the bats leaving their houses.
Once you’re done, you can enter that number in a data form online. And if you have 10 or more bats in your house, Bennett would love to hear from you.
In fact, she says without these community scientists, we might not even know that little brown bats are doing OK.
“Without that information, we wouldn’t be able to then see what we are right now, which is that this bat has really stabilized in Vermont,” Bennett says. “And in some places, we’re actually seeing colonies grow in size over the last decade or so.”
You can learn more about how to sign up to be a bat monitor or report a colony online, here.
All summer long, Vermont Public reporters are learning how to do something. Have an idea? Send it to us here.