I am sitting in the dark at the mouth of a cave. My knees are drawn up, and my heels are hooked over a slab of a rock to stop me from tumbling forward on the steep slope. If I stretched out my arms, my hands would touch the wide slats of a metal gate across the cave’s entrance.
I can see nothing. Squeaks rise up from the cave, as does icy cold air. All around me I can hear the flap of wings, like small flags snapping in the wind.
But there is no wind in this sheltered cave-mouth. I feel a puff of air as a wing grazes my left cheek. Then someone above, outside the cave, turns on a light. A puff of air in my right ear makes a gentle “whoof,” and a bat flies directly in front of my face, too close to focus my eyes on and gone too quickly for me to react.
In the dim light I can see the six other people who are sitting in front of the gate in the cave-mouth with me. On the far side of the group from me is Vermont Fish and Wildlife biologist Scott Darling.
And I can see bats. Dozens of them, weaving among us and each other. Slipping through the slats in the metal gate in a gliding swoop to and from the cave below. They flap above us in the random pattern of a milling crowd
Darling has been studying Vermont’s bats for six years. He has brought a group of us on a Nature Conservancy field trip to the Dorset Bat Cave on Mount Aeolus in Dorset, Vermont. He speaks in a quiet, steady voice. It’s like listening to a ghost story as I watch the bats flap and swerve.
The Dorset bat cave is the largest bat hibernaculum, or hibernation location, in New England, Darling says. The latest mid-winter survey counted 23,000 bats in the cave. Tagged bats from as far away as Cape Cod have been found here. But on this hot September night, the dozens of bats flying around us are just a fraction of the bats to come.
Bats start arriving at their hibernation sites in the middle or end of August. Each night the bats fly in a swarm in front of this cave entrance and also the entrances of other caves or mines where bats hibernate.
The swarm is a mating ritual. Eventually, some bats leave the swarm in pairs to mate. The squeaking sound coming from the cave is the bats’ social call, Darling tells us. It’s the sound of bats getting to know each other. While mating occurs during the fall swarm, the females won’t become pregnant until spring.
The bats in the swarm won’t always be the same bats as the night before, says Darling. “Some bats may visit the cave one night, and then not return again for several days. Bats even visit more than one cave or mine entrance in one night.”
The swarming season probably peaks in early to mid-September, he says, and declines as the evening temperatures cool off in October.
Six of Vermont’s nine species of bats hibernate. Those six also swarm. The federally endangered Indiana bat, the state threatened small-footed bat, the little brown bat, the big brown bat, the northern long-eared bat, and the Eastern pipistrelle have all been found in the Dorset cave.
The Dorset cave itself is a natural wonder. Mount Aeolus, also known as Green Peak, is studded with marble quarries. The cave is a “solutional” cave, formed as water dissolves the soft, calcium-rich rock. It is the largest cave of its kind in New England.
The Nature Conservancy has preserved the Dorset cave and about 250 surrounding acres. The cave is crucial to the health of bat populations in the region, because so many bats winter there, and because bats are drawn there from all over New England and New York.
The area surrounding the cave is also important habitat. While female bats may travel far to join a maternity colony - or group of female bats that raise their babies in one protected place - the males tend to stay in the forests surrounding their hibernation caves for the summer.
The Nature Conservancy runs several day-time field trips to the Dorset cave each summer. The nighttime hike that I went on is a rarer occurrence. The day-time hike is a rugged one that does not include getting up close and personal with the bats, as the people on my trip did, but often includes a closer look at the cave. For the many people who admire these insect-gobbling mammals from a distance, but aren’t crazy about having them flutter around their heads, that’s a good thing.