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 maternal 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 maternal 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.

I have listened for bat calls at night using a gadget for my phone which captures the ultrasonic calls and transforms them into lower frequency sounds I can hear. The phone app also attempts to identify the species of bat calling (Figure 2). Some bat species including big brown bats, silver-haired bats, and little brown bats can be heard almost everywhere on summer nights in Addison County. Very occasionally the phone identifies a call as an Indiana bat. The calls of the Indiana and little brown bat are similar, so I was never certain whether the few Indiana bat calls are in fact mis-identified little brown bats. Maternal roosting offered a way to test the app.

Figure 2. Screenshot from the Echo Meter Touch 2 phone app of one of the bat calls the app identified on August 28 as an Indiana bat (Myotis sodalis, aka MYOSOD). The spectrogram (or sonogram) is a visual representation of the time course (milliseconds, horizontal axis) of the sound frequency (kHz, vertical axis). Each vertical green line is a “chirp” of the bat.

On an August evening I visited one of Addison County’s southernmost known maternal roosting colonies of the Indiana bat. I had only a general idea where it was supposed to be, so I was looking for big trees and trees with loose bark, and I was also using the phone to listen for bats. I hoped to deploy two AudioMoths where Indiana bats were active to record bat calls through the night. The AudioMoths were set to start recording at 7:15 PM so when it was time I tied them to trees in areas that looked promising even though I had not heard any bat calls or seen any perfect looking roosting trees. I kept exploring and at 7:28 PM near some huge sugar maples I heard the first bat call of the evening. The phone app identified it as an Indiana bat and in the next 10 minutes identified 13 Indiana bat calls (e.g., Figure 3), more than it had in the year I have owned it. No other species were identified. I assumed I had not only found the roosting colony but demonstrated that the app could indeed distinguish Indiana bats from little brown bats.

Figure 3. Sonogram of a bat call displayed in the program Kaleidoscope. This call was recorded by the Echo Meter Touch 2 phone app near the Indiana bat maternal roosting colony on August 28, 2021. The app identified this bat as an Indiana bat. The vertical axis is sound frequency (kHz) and the horizontal axis is time (milliseconds). The downward swoop of more rapid and lower frequency (pitch) calls is a feeding buzz as the bat zeroes in on its prey. The call then continues with more typical echolocation chirps.

I left the AudioMoths where they were and returned two days later to move them closer to the newly discovered bat hot spot (Figure 4). I listened again for bats with the phone app and heard more Indiana bats but this time also heard lots of big brown bats and a couple of little brown bats. Unlike the phone app, the AudioMoths do not identify bat species, so when using AudioMoth data I cannot reliably discriminate between Indiana bats and little brown bats which have similar calls. Big brown bats and silver-haired bats are also difficult to tell apart, but are easy to distinguish from Indiana or little brown bats.

Figure 4. The second recording site near an Indiana bat maternal roosting colony. Some sugar maple (Acer saccharum) trees here were 30 inches in diameter and probably 150 to 200 years old. These trees do not have very shaggy or loose bark but might be suitable for roosting trees.
Figure 5. Results from AudioMoth recordings on the nights of August 28 and 29, 2021 near an Indiana bat maternal roosting colony in southern Addison County. Two AudioMoths (top and bottom) recorded for two nights (left and right). Each audio recording was triggered by some sound (amplitude threshold of 10%, or -26dB, or 3328), and I looked at a sonogram of each recording to determine whether or not it was a bat. Some of the “Likely Indiana bat” events might be little brown bats and some of the “Likely big brown bat” events might be silver-haired bats.

At the first recording site bat calls triggered less than half of the recordings (Figure 5, gray bars are recordings triggered by wind, rain, other animals, nearby human activity, or other bumps in the night). The substantial differences between the two nights and between the two AudioMoths, which were only 200 feet apart, suggest that many more nights of recording at many different places in this area would be required to learn much about these bats. I tried to design this recording effort to capture Indiana bats leaving the daytime roosting site at dusk to forage for insects, and then returning at dawn to the roosting trees. There is some indication of that pattern, but it is quite weak in these data. After moving the AudioMoths to the new location, they recorded for 10 nights before I retrieved them (Figure 6).

Figure 6. Results from AudioMoth recordings on the nights from August 30 to September 8, 2021 near an Indiana bat maternal roosting colony in southern Addison County. Two AudioMoths 150 feet apart (top and bottom) recorded simultaneously. During four sessions each night, recordings were triggered by a sound amplitude threshold of 8% (-22dB or 2816). Recordings not triggered by bats are not shown.

Although the new locations of the two AudioMoths were only 150 feet apart, each AudioMoth appears to have recorded different bats (Figure 6). The AudioMoth in a low area seems to have recorded lots of Indiana bats and the AudioMoth on top of a rocky ridge recorded mostly big brown bats. This could be because big brown bats were flying through the upper forest canopy (I watched them do this) and Indiana bats were flying lower (I did not observe them doing this). The AudioMoth on the high point might have been within audio range of the forest canopy while the lower AudioMoth was not. The lower site definitely seems to have had substantially more Indiana bat activity.

Figure 7. The same data as in Figure 5 but sorted by time of night (Dusk = 7-8PM, Late evening = 10:30-11PM, Early morning = 2-2:30AM, Dawn = 5:30-6:30AM).

Sorting the results by time of night could reveal whether bats were leaving the roosting site at dusk and returning at dawn (Figure 7). There is some weak support for this, but if there is such a “rush hour” my pre-programmed recording sessions might have mostly missed it. A stronger pattern in these results is that there was less bat activity at 2:00 AM (“Early morning”) than at other times. And also that there is a lot of variability in what bats do.

For the 10 day recording effort I changed the amplitude threshold from 10% to 8% so quieter sounds would trigger a recording. This was a good threshold for this deployment and there were few non-bat recordings on most days (Figure 8). On the nights of September 5 and September 8 there were several dozen false triggers at both AudioMoth locations. These were due to rain which was heavy on those nights and resulted in near-continuous recording for some sessions (and lots of extra files to examine).

Figure 8. The same data as in Figure 5 with the addition of all non-bat triggers of recording. The amplitude threshold of 8% resulted in excessive false triggers only when there was heavy rain.

As is often the case, this exercise taught me more about how to use an electronic gadget than it did about biology. But it is good to be reminded that the endangered Indiana bat is carrying out its complex family life in healthy forest patches scattered among our houses and farms. Those forests are not just trees.

You can download the AudioMoth configuration file I used in both AudioMoths here.

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