Naturespeak A naturalist's view of the world

July 28, 2008

The Crack of the Bat

Filed under: Uncategorized — wykes @ 7:22 pm

  To any baseball fan, one of the most satisfying sounds in the world is the resounding “crack” of a ball coming off of a home team bat. This is a sound that reverberates around the stadium and prompts the faithful to rise up and believe. Wooden baseball bats, however, are not the only bats that make warm summer nights sounds.  The mammalian bats of Michigan also fill the night with resonating tones.  In addition to being a Detroit Tigers fan, I also happen to be a big fan of the Microchiropteran Bats, and their sounds can bring me to my feet as quickly as any Louisville Slugger can.

  There are over 800 kinds of Microchiropteran bats in the world – the closer you get to the equator the more kinds there are.  We happen to have nine species that can be found in our state ranging from the tiny Pipistrelle to the bird-sized Red Bat. These bats all use echo-location to locate their food (the Megachiropterans, by the way, don’t).  

  Echo-location allows these night-flying creatures to literally “see with sound.”  They let out short sonar bursts and listen for the returning echoes to tell them about the landscape and prey species ahead of them. Since their hearing range is far above ours, most of their sounds are well above our capability to naturally detect them – a sound of silence you could say. Bats echolocate in the 20 to 200 kilohertz (kHz) range while we function in the 20 hertz to 15 kilohertz level.

  Last night, there being no Tiger game on, I ventured out into my backyard about a half hour after sunset to listen in on some bat noises. This is the prime time to see bats as they begin their night-time flight and the night sky is still bright enough for us to see them.  I was equipped with a Bat Detector – an electronic device that converts the high range bat noises into lower range human sounds. The detector can be tuned to species specific kHz levels and is a useful tool to “see” invisible bats with audible sound.

   It only took a few minutes before the detector lit up with sound. I dial-tuned the device and found that the noises were best heard the 29 KHz level (see above). This alone would be a good indication that the noise makers were probably Big Brown Bats, the most common bat in southern Michigan.  As soon as my eyes adjusted to the dark, I could see the ghostly forms flitting about overhead and indeed, they appeared to be Big Browns (see here- note the large ears).

  Unfortunately, the other thing that only took a few minutes to occur was the ascent of the mosquito hordes from the depths of hell. The detector was sensitive to the brushing sound of my hand whisking off blood-thirsty pests, so my audio results were compromised. I was able to stick it out just long enough to record a few of the echo-location sounds before lack of blood forced me inside.

  Listen to my recording (Recording) and you’ll hear what I heard. There were several bats flying about, so you are listening to several individuals taking turns in the air space above my head.  There is an amazing amount of information contained in those clicks and pings. The pulses are emitted through the mouth and nose at a noise level equivalent to a smoke alarm (110 decibels), but at such a high frequency we don’t sense it. Believe it or not, Big Browns are classified as “shouting bats” because of this.  

  Some of these calls are directional, but most are designed to pin down the location flying moths, beetles, and (theoretically) mosquitoes. Relative velocity, range, size, elevation and azimuth are the kinds of things that can be determined using sonar technique. In this series of calls, you’ll notice that the blips start off at an even pace and then get faster before cutting off. As the bat approaches an insect, their call rate is increased just before making the grab and going temporarily silent while they munch their prize – this is what you are hearing.

  Listening to this extremely short segment recording the capture of at least three insects, it is easy to see how bats are able to eat their body weight in bugs every night. There is another side to this aerial battle that we can not hear or spend enough time discussing. The insects themselves have developed an impressive array of defense tactics since they too are equipped with bat detectors. Some moths are covered with sound deadening “fur,” but others send back false signals or drop out of the sky as soon as they hear an approaching bat.

  I’m looking forward to spending some more summer nights listening for the crack of the bat.

5 Comments »

  1. Are there any plans on resurrecting the bat walks? I thoroughly enjoyed the one you had organized at Oakwoods several years ago.

    Comment by Patrick — July 29, 2008 @ 7:59 pm

  2. Patrick:
    Yes, I’ve thought about doing more bat programs. For the past few years, however, I have invited Rob Mies from the Organization for Bat Conservation to come down to do a special bat presentation with live bats. I then take folks out on an evening walk after his program. This year’s program was last Saturday night (sold out crowd) and we had a nice walk afterward- no bats though!

    Comment by Gerry Wykes — July 30, 2008 @ 5:26 pm

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