SCOTT SIMON, HOST:
Deer in the northeastern United States have been munching through forests, eating up bushes and small trees - no doubt looking for BJ Leiderman, who writes our theme music. The overpopulation of deer has changed the way that forests look. And as NPR's Nell Greenfieldboyce reports, the deer have also changed the way that forests sound.
NELL GREENFIELDBOYCE, BYLINE: Megan Gall is an ecologist at Vassar College. She's studied how birds react to human-created noise from nearby cars or overhead planes. After talking with a couple of colleagues who study how deer can consume the entire understory of a forest, she started wondering how that might change the soundscape.
MEGAN GALL: So if you think about going into a gymnasium, there's lots of really hard surfaces. And sound will bounce all around. And that'll change how it hits your ear compared to going outside after a fresh snow; it'll be very quiet. And the same thing happens in forests versus other kinds of habitats.
GREENFIELDBOYCE: Her colleagues already had two experimental areas set up inside a forest. One area was fenced off. The other was marked off but left open so that deer could enter and gobble up plants. Gall's team set up audio equipment in each place to test how the different environments affected the transmission of some basic sounds.
GALL: All of these things are found in different kinds of animal sounds. And so we wanted to have sort of a very broad palette of sounds to look at.
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GALL: One was just pure tone. So pure tones are like single notes.
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GALL: We're also looking at white noise, which covers a large range of frequency. And then we had trills. So trills are things you'll hear in a lot of bird songs.
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GREENFIELDBOYCE: What they found is that sounds changed less when they travelled through areas grazed by deer.
GALL: When deer were browsing, we actually found that the sound was clearer. And that's probably because there was less vegetation in the way. And so you don't have as many sounds bouncing off of leaves and sticks and things like that.
GREENFIELDBOYCE: The results are published in a journal called PLOS One. And they raise questions about how this is affecting birds as they sing songs to defend territories or find mates. Bernie Lohr is a bioacoustics researcher at the University of Maryland, Baltimore County. He says birds are known to change their songs in response to ambient noise near, say, airports. The deer's activity might have a similar effect.
BERNIE LOHR: Something like this should change the acoustic structure of the signals, especially learned signals, which evolve culturally. And so that can happen quite quickly.
GREENFIELDBOYCE: He says forest birds might start using higher frequencies that are more typical of birds living in open habitats, like grassy fields. Nell Greenfieldboyce, NPR News.
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