One evening as the sun dipped behind the trio of white pines on the far side of Dollar Lake, a lone song sparrow let out a brief chortle before settling in for the evening at 9:30 pm. Just a few minutes before a gang of coyotes engaged in a spirited yipping session somewhere off in the distance beyond the lake. Their high barks echoed eerily through the trees. A single loon let out his plaintive call at exactly 10:05 pm before the crickets finally took over.
The bats began their flight at approximately 9:50 pm and concluded at 10:00 pm. Unlike the sparrow, loon, and frisky coyote pack, their part of the end-of-day show was basically a silent one– only an occasional click descended to human hearing level. In actuality, however, theirs was the loudest contribution of all. The decibel range of an echolocating bat generally exceeds that of a typical rock concert. Because the screams are delivered in the ultrasonic range they fall as sounds of silence upon our human ears (if only Alice Cooper’s voice was ultrasonic!). With the help of a bat detector I was able to convert the silent scene into 10 minutes of sonic mayhem. The creatures employed an incredible variety of chirps, buzzes, and pulses before moving on into the night.
As on this first night, the bats arrived and departed like clockwork every night. Over the course of four evenings at the lake there was a tight ten minute period when they flitted about the maple trees just off the front porch and out over the still waters of the lake. From our position sitting under the overhang, the ghostly forms were outlined against the pale blue sky only momentarily before they were swallowed back into the shadows.
There were 5 or 6 individuals -maybe more – but it was hard to keep track in the failing light. Unfortunately, I can’t tell you what kinds of bats were involved but I suspect they were Big Brown Bats (triple B’s for short). Their voice recordings were in the 39 kHz range which is smack dab in the BBB range. I did manage to capture one image after randomly firing off nearly forty shots into the night air (see below and detail here). As you can see, well………… it’s definitely a bat!
The resulting sound recordings were much clearer. Listen to these two tracts (No. 1) and (No. 2) and you’ll get a sense of the sound landscape surrounding a group of feeding bats. The first is a recording of the multiple beasts zipping about close to the porch and under the confined area of the trees. You’ll note that the clicks take on a crinkling or popcorn-like texture as they feel their way between the tree trunks. The second tract was recorded out over the open lake where there are no obstructions. The bursts are much louder and come much closer to the earlier rock concert analogy (the drum solo part). There are also fewer bats in this second recording – perhaps two at most.
It is hard to believe that all these sounds are made by simple larynx or tongue clicks. The resulting sonar waves are sent out to detect, locate, and eventually identify the potential prey around them. Returning sound waves, bouncing off of a flying insect, are picked up by the sensitive ears. Big Brown Bats are beetle specialists, but moths, midges and mosquitoes are also on their menu.
It is hard to tell exactly what is going on at any given time but there are moments within these “silent flights” in which the moment of prey capture is recorded. Listen again to the recordings and you’ll hear a series of ascending zipping sounds. As the bats close in on their quarry, their sonic pulses get closer together until finally ending at the source. The moth’s scream is silent. With a flip of the wing or tail membrane, the insect is gathered in and funneled to the open mouth. That zip is the sound of living bug zapper doing what it does best.