Cal Poly Study Finds Recording of Whitewater River Noise Drives Bats and Birds Away
SAN LUIS OBISPO — A new Cal Poly study finds that birds and bats often avoid habitat swamped with loud whitewater river noise.
“Naturally loud environments have been largely neglected in ecological research,” said Dylan Gomes, a Cuesta College alumnus who led the study as part of his doctoral research at Boise State University in Boise, Idaho. “We aimed to test the hypothesis that intense natural noise can shape animal distributions and behavior by experimentally broadcasting whitewater river noise at a massive scale.”
The study, published today in the journal Nature Communications, was a collaboration between Cal Poly and Boise State. In Idaho’s Pioneer Mountains, located about 400 miles northeast of Boise, researchers placed speakers in 60 riparian areas. The speakers played recorded river sounds that made the streams sound like whitewater rivers. The broadcast included realistic reproductions of river noise and river sounds that had been shifted up in frequency.
“The prevailing hypothesis for why many animals avoid noise is called masking,” said Cal Poly biology Professor Clinton Francis, one of the senior authors. “When noise overlaps in frequency — what we perceive as pitch — with a biological signal such as birdsong, it masks, or hides, that signal. By broadcasting noise of different frequencies, we hoped to assess the role that masking of important sounds plays in animals choosing to avoid noisy places.”
The research team found that the overlap between recorded river noise and song frequency predicted bird population declines until acoustic environments became about as loud as a highway. At that point other forces, such as an inability to hear predators and prey, likely become more important.
The number of bats in an area also decreased as sound levels rose
Birds that remained in areas with the louder soundscapes were less efficient visual foragers, suggesting that the auditory distraction affected their ability to visually concentrate. Bats in the same noisy areas switched from listening for prey sounds to using echolocation, ultrasound waves the mammals produce, above what a human can hear, that bounce off objects in their environment and return to the bats’ ears.
“The bats’ behavioral switch is likely driven by prey calls and footsteps being masked by river noise, and this type of problem solving likely explains why some bats can remain near the ruckus of a raging whitewater river,” Gomes said.
The authors conclude that by studying how animals respond to noise sources that they have faced throughout their evolutionary history, researchers can better understand how animals will respond to human noise.
“Our work showing that natural noise can structure where animals live and how they behave only increases the call to manage human-caused noise,” Francis said. “The spatial and temporal footprint of anthropogenic noise is far greater than loud natural environments.”
The research team is still analyzing data from a similar array of speakers that broadcast ocean noises in Central California.
This work was funded by the National Science Foundation.
Photo information: Cuesta College alumnus Dylan Gomes, left, led the research team that deployed speaker arrays early in the spring in Idaho’s Pine Mountains to begin playback of whitewater river noise before most birds and bats were using this 5,000- to 7,000-foot elevation area. The study was a collaboration between Cal Poly and Boise State, where Gomes was doing his doctoral research.
Contact: Clint Francis
May 24, 2021
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