Acoustic research

Bat Calls TABR

One of the things that first attracted me to studying bats was that it was technology-intensive.  We can’t see them, because they fly mostly at night, and we can’t hear them, because they use ultrasonic calls for navigation and foraging.   It seemed like a good way to leverage my tech background, and plus it’s just really cool.  My first scientific introduction to bats was at the San Francisco State University field campus in the Sierra Nevada mountains, at a course on Ecology of Bats of California led by Dr. Joe Szewczak.  In addition to being a biology professor at Humboldt State University, Joe also created the bat acoustic software product Sonobat.   In addition to learning about bats in general, that week I learned the basics of acoustic monitoring and have been using ever since.

bat detector inside prepainted mustard

Bat detector inside prepainted mustard container

Until recently, I used Pettersson D240X time-expansion ultrasonic bat detectors.  Connected to a small MP3 recorder, it is possible to deploy this system and capture recordings of bats over a whole night.  This is the equipment I used to collect data for my master’s thesis.  Because it is not waterproof and I had to leave them out all night, part of my research was to design a casing that would be waterproof, introduce minimal acoustic interference, and also be secure recording in urban parks.  After some trials of different models, I used gallon-sized mustard containers adapted with a PVC elbow in the side, with the detector microphone aimed down into the elbow to catch reflected noise.  A coat of camo paint and a metal cage completed the human-proofing.  My end result was successful with no loss or damage of equipment.  While there was some acoustic interference, in general most files were adequate for qualitative analysis.  Since that time, the wide availability of automated species identification software such as Sonobat 3.0 requires higher quality recordings, so I would not recommend using this configuration for future projects.

SM2 adapted for high altitude recording

SM2 adapted for high altitude recording

More recently I have been using SM2BAT detectors from Wildlife Acoustics.These terrific units offer lower-cost and higher-quality recordings in a water-proof casing and omnidirectional microphones, showing that technology advances happen even in the small, not-for-profit world of ecological sciences. I’m successfully using a modified ultra-light version of these units for high-altitude recording with the  Helikite. In addition to the SM2 detectors, I also use  Anabat frequency-division detectors for tracking bat activity around the cave. These detectors are perfect for this application because they can run unattended for long periods and data collected is very compact and easily examined.

In addition to using recordings for scientific research, I’ve discovered that time-expanded recordings of bat calls, especially bat social calls, are beautiful to listen to.  Here’s a compilation of recordings of Brazilian free-tailed bats made in San Francisco.

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