“Cicadian” Rhythms

A suspended platform with a microphone (top of
photo) and an ARU (foreground).

We deploy an autonomous recording unit (ARU) for the purpose of recording nocturnal flight calls of

migrating birds during each migration season. As time allows, we review nightly recordings (from a half hour before sunset to a half hour after sunrise) by scanning them with software and creating visual sound spectrogram thumbnails of each sound that fits the parameters of a bird call. If only it were that simple…

Depending on the season, frogs and insects compete within the same frequency range of the birds we are trying parse from the hours of recordings.

How loud are the singing insects?

Friends and family that were visiting from the north last week noticed the cacophony of singing insects in Missouri. In fact we had raise our voices in order to speak over the cicadas, katydids, crickets, and grasshoppers. One of the guests in advanced age couldn’t hear them at all. We handed him a parabolic reflector and headphones to see if a mechanised boost in amplitude would give him is old ears back. “Ah… I think I hear it,” he said loudly. “Oh, wait… that’s Bill’s knee replacement clicking.”

After a round of laughs, he redirected the reflector to the insect source and could eventually hear them. “Man!”, he said. “Now I understand why people could find them annoying.”

A blessing or a curse?

It is impossible to find a place to record on our postage stamp property that doesn’t have what recordists call “noise.” Given time, we’ll try Bill Evan’s set-up. Over the years, he has mastered a way of limiting unwanted sounds. He has employed his system over the continent. The nearest listening station is at Riverlands Audubon at the confluence of the Missouri and Mississippi Rivers.

In the meantime, we’ve dealt with the curse of competing sounds and have learned from them. Singing insects are active throughout the day and night, depending on the species. The soundscape is dominated by them at times and a they exhibit patterns each night. Although our goals is to collect bird data, the timing and diversity of singing insects is a opportune medium for the study of daily and yearly patterns in the natural world. Climate change theorists have one more tool with which to track phenological shifts.

A dawn soundscape of mostly insects. You can see a couple birds in the 2-3 kHz range (Carolina Wren and White-breasted Nuthatch), but the main frequency bands for night flight calls of birds are dominated by the singing insects.

Soon the singing insects will be less competitive. The temperatures will drop and with it will come more migrating birds. Not the ideal situation, but until we have a Bill Evan’s set-up, the exploration will continue.

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