audiomoth – Long Driveway

Indiana Bats and the Grotto of Doom

More than a year ago I took a hike along the Green Mountain Escarpment (Vermont) in search of an old mine. I had read in an old report that Indiana bats hibernated in a mine in the area, and that was enough of an excuse to go exploring. I watched a bear and two cubs for 15 minutes but never found a mine. It was a pleasant hike along old logging roads through private property. No one lived on the several properties I traversed, none of which was posted.

Figure 1. This six acre wetland is at the base of the steep, rocky slopes I was exploring for a mine. Although I found nothing on the slopes resembling a bat hibernaculum, this wetland looked like excellent foraging habitat for bats. Plants in bloom include Canada goldenrod, jewelweed, boneset, joe pye weed, and New England aster. September 11, 2020.

More noise, fewer bats

After 12 nights of recording bat calls near an Indiana bat maternity roosting colony, we deployed the AudioMoths for a week at the vernal pool where we recorded bat calls in August. Instead of putting two AudioMoths at the vernal pool, we put one by the pool and two in the forest surrounding the pool. One of the forest AudioMoths recorded nothing (a battery was inserted backwards), so we got data from only one non-pool AudioMoth.

Figure 1. Vernal pool MLS619 (Bridport, VT) on September 23, 2021 at the end of a week of recording bat calls. There was no standing water in the pool but the liquid mud was 6 inches deep. This might be as low as the water level gets in the pool this year (it was dry and firm last October). There were a few inches of water in the pool at the beginning of the AudioMoth recording on September 16.

Maternal roosting

Indiana bats live throughout the US Midwest and into New England. In winter they gather in a small number of caves where as many as 50,000 bats may hibernate together. This makes the population vulnerable to vandalism and since 1967 the Indiana bat has been on the US endangered species list. It was listed as Vermont’s first endangered species in 1972. Communal hibernation also makes bats vulnerable to the spread of white-nose syndrome and Indiana bat populations have declined moderately since the disease appeared in 2006.

Vermont is at the northeastern edge of the Indiana bat’s range where it has been observed foraging and raising young throughout the southern Champlain Valley. About 10 maternity roosting colonies where females raise their pups have been documented in Addison County. Female bats select forested sites with large trees and spend the day under loose bark with their single pups and forage at night for flying insects within two or three miles of the roosting trees.

Figure 1. A shagbark hickory tree (*Carya ovata*) near the site of a known maternity roosting colony of the Indiana bat in Addison County. Female bats and their pups spend the day under the loose bark on living and dead trees of this and other species.

Sound data

Part of the Vermont Center for Ecosystem Studies Vernal Pool Monitoring Project includes audio recordings to document the first calls of frogs at the pools. This year they started using AudioMoths to make the recordings. AudioMoths are open-source data loggers with a low-power sleep mode, real-time clock, microSD card slot, MEMS microphone, and circuitry to support audio capture. AudioMoths can be configured to automatically save audio recordings on a custom schedule. The AudioMoth software and hardware are well thought out and the audio quality is very good (see audio file below). The AudioMoths were deployed at about 50 Vermont vernal pools in weather-tight cases (Figure 1) and recorded for a few short sessions every night for several weeks in the spring.