I began working with bats on September 5, 1964, when I was 20 years old. I initiated the first focused study of freeliving bats away from roosts in Nevada using the newest technology then available: mist nets, nylon or polyester mesh nets suspended between two poles like a volleyball net, used to capture bats without harming them.
I was a student involved in desert pupfish research and only started bat work as a class project. Not knowing the literature, I continued to net bats through the fall and winter and only after the fact pieced together that bats weren’t supposed to be active in north temperate regions in the winter! I was hooked.
The Anabat system uses a broadband electret microphone. This specific system will only display the harmonic with the greatest energy—recording only the loudest calls. Different species produce calls at different energy levels, therefore quiet species may need to get really close to the microphones (within three meters) while others are quite loud and can be detected at distances greater than 100 meters.
For the Vermilion Cliffs study, we chose four long-term sites for data collection, all night, every night. To get greater detail for such a large area on a limited budget, we also deployed five mobile units at two-week intervals. By shifting the locations of the mobile units, we were able to sample the range of habitat types across the 198,500 acre Paria Plateau.
Although recording is continuous from sunset to sunrise, the periods of silence are not recorded. The algorithm we use is based on sound detected and attributed only to bats. After one or more calls are detected, a period of silence greater than five seconds triggers those calls being saved to a digital file. Continuous calls result in increasingly larger files until the 15-second mark, at which point that block of data is saved to a file. We end up with thousands of small files no longer than 15 seconds each.
There’s no such thing as a typical night. Bat activity varies from site to site and from night to night at any location. An interesting night might be discovering a recording of a new species at a study site. A really interesting night might be a new species for the whole state, or a new behavior for a particular species. We are easily entertained.
I don’t actually listen to recordings. It’s counterintuitive, but acoustic identification of bats is performed visually. It requires sitting at the computer, scanning each and every data file visually and entering a species identification code for each species.
As of July 2015, I’d examined 54,225 files from the four stationary sites and 457,568 files from 25 mobile sites—about 130 hours of work. I’m about halfway through two years of data.
I’m locked at my computer, staring intently at the screen in total concentration.One cannot do this for long periods of time. Usually, I can do about 1600 files per hour and I have done as many as 11,500 files in a single day. One season, I processed over 750,000 files and it took several months before I could sit down and maintain concentration for more than a half hour at a time!
Simply put, I use the time and frequency of the call to identify the bat species, then look at the frequency range and shape of calls to tell species apart. In reality, it is much more complex—I have to look at the range in variation and how calls change depending on surroundings and specific behavior.
Each species has a distinct vocal pattern, but it’s common for individual species within a given genus to have similar shapes but different frequency ranges. Bats species that are closely related often sound more alike. Larger species within a genus might use vocalizations at lower frequency ranges than small species—imagine a tall person with a deep voice and small person with a high-pitched voice. When this occurs, each species would differ from the next in size by about 8-10 kHz.
We’ve recorded 19 species of bats over the first year of the study. Among these are the smallest and largest species in the United States: the parastrelle (Parastrellus hesperus) and the mastiff bat (Eumops perotis), respectively.
There are three tree-roosting migratory species: the silver-haired bat, red bat, and hoary bat. There are also three related species that may be considered rare or of special concern: Townsend’s bigeared bat, the spotted bat and Allen’s big-eared bat.
Species diversity is high within the monument, primarily due to habitat diversity, specific habitat mosaic, and proximity to the Colorado River drainage. Why are bats an important conservation focus? Bats are the only mammals that fly. They fill a wide variety of feeding niches, similar to that of birds, and reflect, essentially, the night shift. They are long-lived with low reproductive potential and tend to roost in large numbers, which makes them vulnerable to vandalism and other disturbances, and slow to recover. In the desert Southwest, they are important cyclers of energy and nutrition. Among the insectivores, they play a big role in pest control, particularly in forests.
Final results of the study, a partnership between the Grand Canyon Trust, Friends of The Cliffs and the Bureau of Land Management, are expected in early 2016. The study is made possible by a grant from the National Landscape Conservation System.
"We’ve recorded 19 species of bats over the first year of the study. Among these are the smallest and largest species in the United States: the parastrelle and the mastiff bat, respectively."
