SALISBURY — After a tremendous fire destroyed the Swamp Road covered bridge over Otter Creek on the Salisbury-Cornwall line on Sept. 10, 2016, naturalists feared that the large colony of little brown bats that made their home in the structure would disperse to the four points of the compass.
But recent reports from the scene say that quite the opposite is the case.
“When the bridge burned down, everyone thought the bats in the area were just ‘gonzo,’” said Dylan O’Leary, an AmeriCorps land steward and outreach coordinator for the Nature Conservancy. Now O’Leary and his fellow volunteers’ work has shown that the bat population at the site is actually plentiful.
According to Nature Conservancy statistics, the number of little brown bats counted in the area was only 97 in 2012, and rose to 181 in the summer of 2016, before the fire. The census fell to 149 the following summer, but bounced back to 272 in 2018 and 362 in the largest of five counts this July.
Every Monday evening for the month of July, armored with bug spray, O’Leary and his fellow Fish and Wildlife volunteers met at the outlet past Swamp Road Bridge in Salisbury just before sunset. The four spent an hour helping to evaluate the health of bats, especially little brown bats, by counting the bat population.
The concern for Addison County bat residents began even before the incineration of the Swamp Road covered bridge. Since 2006, White-nose Syndrome has resulted in the loss of 5.7 million bats in the Northeast.
The disease was first discovered in Vermont in 2008 and has especially targeted the state’s two most common species: the little brown bat and the northern long-eared bat. The population of these bats has decreased over 90 percent since the discovery of the disease.
Bats contract White-nose Syndrome while hibernating during cold months because a fungus is able to attack their skin in their inactive state. The fungus can cause bats to become more active than their norm, resulting in the bats burning fat that they need to make it through the winter. In an effort to survive, the diseased bats may leave their lair in the winter in search of food and water, and if they are unsuccessful, they will often freeze or starve to death.
With the dangers of White-nose Syndrome looming, Alyssa Bennet, Vermont Fish and Wildlife’s small mammal biologist, began monitoring the health of bats all over the state, including along Swamp Road, not long after the disease was discovered in Vermont. Brian Carter and Leah Keller, two volunteers present Monday night, have been monitoring the Salisbury bat population since 2014.
Because the covered bridge’s location in the swamp was the perfect home for bats, scientists feared that its loss three years ago — on top of the devastation wrought by White-nose Syndrome — would be particularly important to the wellbeing of the flying mammals.
“This is such an ideal habitat: a marsh full of bugs, the river, and coverage,” O’Leary said Monday, pointing to the trees above him.
The bats are evaluated in the month of July because pups are beginning to fly at this time. Typically, bats mate in the fall, spend the winter in caves, emerge from hibernation pregnant, and then find their way to bat boxes that serve as “maternity roosts.” By July, when pups are able to fly on their own, volunteers can record a full evaluation of the local population.
Bat boxes are built specifically for the species they are catering to, so along Swamp Road little brown bats are in need of boxes on the smaller side. The boxes in Salisbury are a foot and a half by three feet; amazingly each can accommodate around 130 bats or more at once.
Each Monday evening last month, volunteers were posted at each of the three bat boxes along Swamp Road where they sat, swarmed by bugs, and waited for the first sign of flight. Each volunteer clicked a button on a counting device that recorded every time a bat left the box.
The volunteers recorded the time of the first flight, which is typically around sunset, and then continued to count until a bat has not emerged for 10 minutes. O’Leary noted that the bats tend to fly out in bursts, and then there will be a one or two minute pause before another burst.
By the end of July, the female bat population was already beginning to disperse and leave the bat boxes after the summer to mate with male bats, that do not stay in the bat houses over the summer. Just between the second and third weeks of July, the bat count went down by 60. On this final Monday night, July 29, volunteers counted 110 bats in one box, 79 in another, and 9 in the last.
As a result of their hard work, the Vermont Fish and Wildlife volunteers have found that the bats in the area are proving to be tolerant to the White-nose Syndrome fungus. O’Leary said that the volunteers are hopeful that the bats will continue to pass on their good genes and produce healthy pups.