Sunday, April 27, 2008

More bat music

Recently I had the opportunity to visit Kim's house here in San Francisco, where bats have taken up residence between houses and even in a bat house attached to the outside of her house. I believe the bats were all Mexican Freetails (Tadarida brasiliensis.) Freetails tend to fly high and fast, and their echolocation calls are usually a long, loud call at a single frequency, around 30 Khz. It's pretty variable, though, when they're flying lower or in cluttered environments like a narrow back yard. They adapt the call by varying the frequency, e.g. starting at a higher frequency (50Khz) and then going down. We call those FM, or frequency modulation, calls. By using a range of frequencies they get more detailed information about what's around them. The downside to a higher frequency is that the call doesn't carry as far. The equipment I use records their calls and then transforms them by slowing them down and bringing them down to a frequency that we can hear. I set up my bat recorder that night in Kim's back yard and the next morning when I retrieved it and uploaded the files to my computer, I got lots of calls. Some were the traditional Freetail single-frequency foraging calls. But many were very interesting and unusual. Many were probably those modified FM calls while flying in the back yard area, but some were also possibly social calls. These included some calls that rise in frequency, or rise and then fall. Previously I posted individual recordings, but this time I collected a number of the call files into a single MP3 file for your listening pleasure, linked below. At the very beginning you can hear some feeding buzzes, which sound like a soft rattle. That's the bat actually approaching a bug it wants to catch. After that, be careful as some of the calls can be rather loud. The whole "song" lasts 3 minutes 44 seconds. It ends up being a lovely song, no? Freetail chorus from Kim's backyard


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