Human behavior changed drastically with the COVID-19 pandemic. So too did animal behavior, researchers have now shown: Birds altered their songs as anthropogenic noise plummeted thanks to stay-at-home orders and elective quarantining. These findings highlight the malleability of behavioral traits and birds’ resilience to noise pollution, researchers have suggested.
Throwback to the 1950s
The Golden Gate Bridge, built in 1937, is a major transportation artery in the San Francisco Bay Area: In recent decades, more than 100,000 vehicles have traversed its span each day. But in April of this year, during California’s COVID-19-induced shutdown, vehicle crossings fell to fewer than 35,000 per day, a level not seen since the 1950s. That pronounced decrease in traffic contributed to significantly lower noise levels in urban areas, Elizabeth P. Derryberry, a behavioral ecologist at the University of Tennessee, Knoxville, and her colleagues found.
By comparing sound recordings made in April and May 2020 with prepandemic data collected in 2015 and 2016, Derryberry and her collaborators found that background noise in urban San Francisco and Richmond decreased by roughly 7 decibels, or about 55%, during California’s shutdown. Intermittent noise, such as planes flying overhead and dogs barking, also decreased by about the same amount, the researchers showed.
Birds took advantage of that new soundscape, Derryberry and her colleagues demonstrated: Male white-crowned sparrows (Zonotrichia leucophrys) sang differently—and more effectively—during the shutdown than they did prior to the pandemic.
The team’s results were published in Science in September.
A Softer and Different Song
To begin with, the team found that the birds sang more softly. That makes sense, Derryberry and her collaborators proposed, because of the Lombard effect. This phenomenon, observed across species, refers to an animal making louder vocalizations in the presence of higher levels of noise.
“They’re not shouting anymore,” said Derryberry.
But the researchers were surprised to discover that the sparrows were overcompensating: They were singing about 35% more softly, whereas the Lombard effect predicted a decrease of only about 3%. “They’re singing even more softly than we thought they would only due to the Lombard effect,” said Derryberry.
The scientists also discovered that the birds sang at different frequencies. In prepandemic times, the lowest frequencies of the sparrows’ songs tended to be masked by traffic noise. But as that background noise faded in spring 2020, the birds adjusted their vocalizations, said Derryberry. “As the traffic noise dropped out of those lower frequencies, they widened the bandwidth of their song.”
That change boosted the males’ appeal to potential mates, the researchers proposed. “Females are particularly interested in song that has a wide bandwidth,” said Derryberry.
Below are two audio recordings, the first taken before the pandemic and the second, during the pandemic.
A Bumper Crop of Birds?
White-crowned sparrows were able to communicate over longer distances in 2020, Derryberry and her colleagues showed. That’s because the drop in background noise outpaced the drop in the birds’ volume. The sparrows roughly doubled the distance at which they could be heard by another bird, the researchers calculated.
That change, combined with the males’ more enticing songs, might have led to more mating, a hypothesis Derryberry and her collaborators plan to investigate next spring by doing more fieldwork. “We can go in and see if there’s ‘a bumper crop’ of young birds,” said Derryberry.
It’s remarkable that these birds changed their songs so quickly, said Sue Anne Zollinger, a behavioral physiologist at Manchester Metropolitan University in the United Kingdom not involved in the research. “It’d be interesting to know if many species are as plastic as the white-crowned sparrow.”
These nearly immediate changes are heartening because they underscore the birds’ resilience to noise pollution, said Derryberry. “It takes a while to see the positive outcomes to things like recycling or reducing your emissions.”
—Katherine Kornei (@KatherineKornei), Science Writer