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Before, urban white-crowned sparrow's breeding territories were almost three times as loud as rural territories, the study found.
But during the pandemic, researchers noted that noise levels in urban areas were drastically lower. In fact, they were consistent with traffic flow in the mid-1950s.
"In other words, the Covid-19 shutdown created a silent spring across the SF Bay Area," researchers noted.
By analyzing traffic flow data from the Golden Gate Bridge, researchers found that vehicle crossings from April to May 2020 returned to levels not seen since 1954. While noise recordings are not available from the 1950s, researchers said this indicates that a brief but dramatic change in human behavior effectively erased more than a half-century of urban noise pollution.
Birds responded by producing higher performance songs at lower amplitudes, maximizing communication distance and salience.
"We found that birds sang more softly when noise levels were lower," researchers said in the study.
Even though the birds were singing more softly, the study found that communication distance nearly doubled, elevating species fitness and increasing mating potential.
"In addition, the signal-to-noise ratio doubled, which helps explain media reports suggesting that bird songs sounded louder during the shutdown," the researchers said.