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.

A year later I found new information about where the mine might be and decided to try again. Galen and I walked directly to a place with a network of old roads at the base of a series of bedrock cliffs that appeared to have been worked by miners. There was no evidence of rock drilling or talus, just shallow caves (Figure 2) in vertical rock walls that seemed to follow quartz veins. Ores of precious metals in Vermont are often found along veins of quartz, and there were lots of quartz veins near the caves (Figure 3).

Figure 2. One of many shallow caves we thought might be prospecting efforts to expose quartz veins which the prospectors hoped would be associated with metal ores. October 11, 2021.
Figure 3. A quartz vein through a bolder which had fallen off the cliff. Quartz veins usually don’t get prospectors excited unless they are a foot or more thick. October 11, 2021.

None of the caves seemed to penetrate the rock more than several feet so there was no indication that they would be suitable for bat hibernation. But we had with us something I did not have a year earlier. We had two AudioMoths, devices capable of recording the ultrasonic calls of bats. We left an AudioMoth near the putative mining activity and another in the nearby meadow 100 feet below (Figure 1). Both were programmed to listen for sounds on and off during the night and to record for at least 10 seconds after each loud sound. The AudioMoths listened every night for eight days (Figure 4). In the middle of October Vermont’s bats have changed their routine and may be swarming near their hibernation sites and breeding before they begin hibernation. Near a hibernation site it might be possible to observe (or record) many bats flying at night.

Figure 4. Number of audio events produced by two AudioMoths: one at the putative mining area (upper) and one at a nearby wetland meadow (lower). AudioMoths listened for three hours and twenty minutes every night for eight nights. Bat species were identified based on call frequency ranges (Myotis is probably little brown bat or Indiana bat). There are no data for the last two days in the meadow because on October 14 I swapped the microSD cards and the new 4GB card filled up with noise from rain and wind on October 15 and 16.

The recordings on both AudioMoths produced 1172 files from more than 26 hours of listening over eight days. Only 32 of these audio files were of bat calls (Figure 4). Only two of the bat calls were recorded near the shallow caves. Most of the bat calls were recorded in the wetland meadow which should be better feeding habitat for bats than the forested mining area. But the number of calls recorded was a fraction of what might be expected on a summer night in good habitat. On October 14 Galen and I visited the site after dark and listened for bats with an Echo Meter Touch 2. A phone app identifies bat calls picked up by the ultrasonic microphone plugged into the phone’s USB port. Confirming the AudioMoth results, we heard no bats near the mining area and a few (silver haired bat, Myotis sp.) in the meadow.

These results suggest that a few bats were still active but there was not any bat swarming happening near the mining site. So I decided to move the AudioMoths about a half mile to a site where another mine was supposed to be. On October 19 I retrieved both AudioMoths, swapped the SD cards and batteries, and deployed them at the new site.

Figure 5. The second putative mining area was in this handsome ravine with an impressive old forest of yellow birch and sugar maple.

The second mining area had old roads switch-backing up a steep slope to vertical cliffs of bedrock. This looked promising, and the caves at the base of the cliffs were much bigger than at the first area (Figure 6). But the caves were still only several feet deep and there was no direct evidence of mining. I left one AudioMoth at the base of the cliffs and another down the hill next to a small pond.

Figure 6. There were lots of overhangs and shallow caves at the second site, but I found no real caves or convincing evidence of old mining. This bedrock is quartzite or something like it.

That day I got an email from a friend with a screenshot of Vermont’s LIDAR topographic map. This map was of another area but it prompted me to inspect the LIDAR map of my hibernaculum search area. The LIDAR maps showed no unusual topography at the sites I had visited, but nearby were some pits and mounds that looked unnatural. I made GPS waypoints of the pits and labeled them “Diggings.” I waited three nights to give the AudioMoths a chance to record whatever was calling at the second site and then returned with Galen to retrieve them and explore the Diggings.

Figure 7. Number of audio events produced by two AudioMoths: one at the second putative mining area (upper) and one at a nearby pond (lower). Each AudioMoth listened for three hours twenty minutes per night for three nights in mid-October, 2021.

Data from the second site confirmed the result from the first site. Bats were not doing much either in the woods near the cliffs or near a pond (Figure 7). When we retrieved the AudioMoths from the second site Galen explored higher on the cliffy hill but found no bigger caves or evidence of mining. We set off up the ravine for the Diggings.

Figure 8. The Grotto.

We were immediately impressed when the phone told us we had arrived at the waypoints for the Diggings. House-sized blocks of quartzite broken from the cliffs over the millennia were piled along the trail. The jumble created “caves” deeper than anything we had seen so far. We found ourselves in a small gap in the forest surrounded by steep slopes and tall, dripping quartzite cliffs. Ferns covered the ground and the boulders and we began to refer to the place as The Grotto. Then Galen’s face froze as he looked over my shoulder. He found no words so I turned around to look into a perfect round mine entrance almost as tall as I am and deep enough to be dark inside (Figure 9).

Figure 9. A mine entrance at The Grotto. Several drill holes (for inserting explosives) are preserved in the walls of the adit.

The adit was only about 25 feet long and appeared to have led to a vertical shaft that was now filled in. There was no way for bats to get deep enough to hibernate. We had finally found a mine, but it was not a viable hibernaculum.

Figure 10. The roof of the adit at the entrance. The orange rock is an exposure of iron-stained quartz about two feet across. This might be the exposure that prompted the miners to excavate the adit. In the upper right quadrant is a longitudinal section of a drill hole used to insert explosives. Other drill holes are present confirming that this is not a natural cave.

Across The Grotto were two deeper caves. These were formed by huge fallen blocks of quartzite and had no evidence of drilling. These could have been explored or cleaned out by miners but might be unrelated to the mining activity. They were both at least 30 feet long and we did not follow them far enough to learn whether the horizontal tunnel went a lot farther. These might have been deep enough to protect bats over winter but we didn’t see any bats or guano.

Figure 11. A cave at The Grotto formed by huge fallen blocks of quartzite. This cave made a right angle turn 30 feet in then extended another 25 feet. We did not go to the end to see if it turned and continued because we have read Tom Sawyer and knew that running into Injun Joe might not go well.

I had seen enough and started to deploy the AudioMoths. This time I had three AudioMoths and hung them around The Grotto within a couple hundred feet of each other. As usual, Galen climbed up to explore the base of the cliffs. As I was swapping batteries and SD cards, Galen called down to report three findings. He calmly listed: a deep pit, an iron bar inserted into the bedrock, and a sign that said, in effect, “This is the cave you are looking for.”

Figure 12. Galen found this sign next to a deep pit with a tiny entrance at the bottom. This confirmed that we had found the rumored bat hibernaculum. The logo of the US Fish & Wildlife Service suggests that the federally endangered Indiana bat was known to hibernate here.

I thought he must be joking, but I joined him and there it was. And so ended a 13 month search for the elusive Indiana bat cave. I finished deploying the AudioMoths, one of them right next to the signed mine pit.

Five days later I returned at dusk to retrieve the AudioMoths and listen for bats with the Echo Meter Touch 2. I listened for an hour after dark and heard one bat call and it was not an Indiana bat. This slim result was confirmed by the data on the three AudioMoths (Figure 13).

Figure 13. Number of audio events produced by three AudioMoths: one at the closed mine entrance with the white-nose syndrome sign (upper), one 150 feet away across The Grotto, (middle), and one 200 feet away across The Grotto (lower). Two of the AudioMoths (upper and lower) stopped collecting data (“no data”) probably because the microSD card filled up (lower) or otherwise failed (upper). These two AudioMoths had cheap 4GB cards.

All three AudioMoths recorded what might have been a single Myotis bat on October 22 (Figure 13). Then at least two bats were recorded on October 26. Otherwise, only noise from rain, wind, and who knows what was recorded. This result does not support the idea that lots of Indiana bats are using the mine they are known to have used in the past. There could be bats in the mine that we never heard, or the bats might have flown at times the AudioMoths were not listening. But maybe the entire colony of Indiana bats using this mine succumbed to white nose syndrome. Or maybe the few Myotis we recorded are Indiana bats and will spend the winter in the mine and maybe next year there will be more.

Figure 14. Galen also found this entrance to what appears to be another mine adit near The Grotto. There were no drill holes or other evidence that it was made by miners, but it must have been made by miners. The horizontal cave extended 20 feet and turned 90° and continued.

Certainly a lot of people know about these mines, but it was fun to find them without any help. (Give it a try!) It was also good to learn that you should not use cheap microSD cards in AudioMoths. The configuration I used works well for recording bat calls but it seems that noise from rain and wind will always offer the threat of prematurely filled SD cards and thousands of non-bat audio recordings to winnow out. The configuration I used for all of these recording sessions is here (human readable) and here (machine readable).

Figure 15. Sonogram of a Myotis call (left) at The Grotto on October 22, 2021 at 6:34 PM recorded by an AudioMoth. The leftmost swoop is a single chirp of the bat and the other chirps are eight examples of little brown bat (Myotis lucifugus) calls from different places in eastern North America. Calls of Indiana bats are highly similar to calls of little brown bats and often the two cannot be distinguished. Screen capture from SonoBat with analytical data at bottom.

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