First, imagine a bat. It’s small. It’s brown or black (or maybe gray, silvery, or reddish, but probably brown or black). Its wings are large and leathery. Its body is tiny and covered with the softest fur imaginable. Its ears and nose are prominent. If you see it resting, you can easily mistake it for a dried leaf, if you notice it at all.
Now imagine a dozen bats huddled together. How about a hundred bats? A thousand bats? Can you imagine 23,000 bats? (Are you ready to scream?)
If I asked you to describe 23,000 bats, you might say “creepy,” “spooky,” “horrifying,” or “just get me out of here.” But would the word “vulnerable” come to mind?
Scott Darling, of the Vermont Fish and Wildlife Department, has been in the cave where Vermont’s largest known population of hibernating bats spends the winter.
What’s it like inside a cave with 23,000 bats? “Pretty quiet,” he says. There is a reason for that.
Each winter, some of the species of bats known in our area (there are nine species known in Vermont, seven in New Hampshire) migrate. Yes, like birds, they fly south for the winter. Others, though, hibernate. Some species (such as the big brown bat) have learned to use attics, basements, and outbuildings. But many bat species hibernate in caves – and have taken to hibernating in mines as well.
Not any old cave will do. It’s got to be the right temperature, and that temperature has to remain constant throughout the winter. The air circulation has to be just right. And, the cave must be undisturbed throughout the winter.
Each time a bat wakes during the winter, it uses the amount of energy that it would expend in two to three weeks of hibernating. The bats in our area eat only night-flying insects, so there is no extra food available in winter. (When was the last time you got a mosquito bite in February?)
It’s easy to do the math: wake up too many times in winter and a bat will starve to death. So, the bats stay quiet.
“They are just hanging out, really,” says Darling. “They are in a state of torpor. They may wake up five times in the winter. The torpor is strictly an energy-saving device.”
Bats are known to congregate for hibernation in eight places, or hibernacula, in New Hampshire, says John Kanter, of the New Hampshire Fish and Game Department. All of the New Hampshire bat hibernacula are in mines, he says. New Hampshire’s geology just doesn’t do caves the way Vermont’s does.
About half of the mines that are known bat hibernacula in New Hampshire are in the Upper Valley, he says. Each bat hibernacula in New Hampshire supports about 500 bats.
Wildlife biologists in both Vermont and New Hampshire survey known bat hibernacula once every few years. They wait several years between surveys to keep the disturbance to a minimum; but why survey at all?
“It is a place where we can get a consistent pulse of what is going on, since bats are so hard to count when they are dispersed across the landscape,” as they are in summer, says Kanter.
In New Hampshire, the small-footed bat is a conservation priority. In Vermont, it’s the federally endangered Indiana bat that causes the greatest concern. Both species are known to hibernate in the Connecticut River valley.
Darling notes that the winter surveys don’t get an absolute number of bats in an area. There are too many bats squeezed into crevices and hidden in parts of the cave where humans don’t fit. Still, scientists can get an idea of the general trend of a bat population.
Unfortunately, worldwide, that trend has been downward. In North America, the hub of bats’ vulnerability seems to be caves.
“From our viewpoint, hibernacula are vital to bat survival in a region. If you have any number of bats in a mine or cave, it is very important that it be conserved, particularly from any blockage or closing of the cave openings,” says Darling.
The solution is in recognizing bat hibernacula and creating conservation plans. Darling says this is just what two private landowners in Vermont did this summer. They received grants to gate their bat caves against human access and, with the Department, created cave-management plans.
The good news, says Darling, is that today there is wider recognition of bats’ significant role in the natural environment, and therefore more funding is available for bat research.
No matter how much we learn about bats, the thought of bats in a cave may still give us the chills. But if you are spooked, it may help to remember that in our area, especially in winter, it’s the bats who are vulnerable.