When the roads emptied of traffic in March and April because of the COVID-19 pandemic, Bay Area residents reported hearing more birds—and they were halfway right.

Jennifer Phillips, a researcher at Cal Poly, and Elizabeth Derryberry, a professor at the University of Tennessee in Knoxville, collaborated to evaluate whether and how songbirds responded to the quieter environment with much less traffic.

They compared the soundscapes and songs of the white-crowned sparrow recorded across the San Francisco area prior to and during the statewide shutdown.

"When I saw photos of an empty Golden Gate Bridge, it struck me just how little traffic there was," said Derryberry, lead author of the study. "I realized we were in a unique position to look at how changes in human behavior might affect wildlife and what the noise reduction might mean for the songbird we study."

The researchers found that the birds responded by producing softer songs that could travel over a larger distance, unimpeded by noise. The urban songs also became "sexier" in terms of vocal performance—meaning birds sang a wider range of notes in their song, in a wider bandwidth, during the shutdown.

As a graduate student from 2013-2017, Phillips studied the song of the white-crowned sparrow in the Bay Area. Now a postdoctoral researcher at Cal Poly, she returned to her former study sites in April and May 2020 to record noise and song samples from urban and rural sites surrounding San Francisco and Richmond, California. She measured how loud the sparrows were singing and how far away from her they were.

The team then compared those samples to the ones collected from 2012-2020 by Phillips, Derryberry and David Luther, a biologist at George Mason University. During the statewide shutdown, the team observed lower noise levels in the city, as a result of less traffic. In fact, traffic on the Golden Gate Bridge returned to levels not seen since 1954.

The dramatic reduction of vehicular traffic had erased a half-century of urban noise pollution.

"The rush hour roar was more like a gentle, sporadic purr," Phillips said. "Usually, the Presidio is full of visiting tourists coming to see the Golden Gate Bridge, often in large groups. This season, only a few local individuals or couples were out for morning walks or bike rides. It was quite peaceful."

According to Phillips, prior to the shutdown, bird territories in San Francisco had nearly three times more human noise than in rural western Marin County—but during the shutdown, people could hear effectively four times more birds than usual.

"When the birds don't have to compete with loud background noise, they can essentially sing more quietly but still communicate over greater distances — approximately twice as far," Phillips said.

This helps explain media reports suggesting that bird songs sounded louder during the shutdown and that people were more aware of the birds around them. Birds were actually singing more softly, but it was so quiet, they sounded louder.

"Our findings illustrate that behavioral traits can change rapidly in response to newly favorable conditions, indicating an inherent resilience to long-standing anthropogenic pressures like noise pollution," said Derryberry.

The study published in Science also found that during the shutdown male sparrows' songs covered a wider range of pitches and so might convey more information for females to use in selecting an effective mate and for males to assess a competitor.

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