Birds Really Did Sound Louder During Lockdowns
When traffic noise disappeared from San Francisco, the city’s white-crowned sparrows took advantage of the silence, researchers found.
You can tell what part of the Bay Area a male white-crowned sparrow is from just by a few notes of its song. The buzzes and trills of the North American birds can vary dramatically over just a few miles. (Think Bronx versus Brooklyn accents, but for birds.) These distinctive dialects have made the species a focus of ornithological attention for decades; since the 1960s, researchers have mapped 10 birdsong dialects across San Francisco, their borders shifting and evolving over time.
But in recent years many of the urban sparrows’ melodies had been “masked” by noise pollution, and the birds began singing at a higher frequency to overcome the cacophony of cars and city life.
That changed in March, when Bay Area counties went into coronavirus lockdown. Traffic disappeared, coyotes began prowling the traffic-free streets, and nature, famously, began healing. Elizabeth Derryberry, an associate professor of behavioral evolution and phylogenetics at the University of Tennessee, Knoxville, wondered about the white-crowned sparrows in the San Francisco region that she’d been studying since 2012. What would they sound like, unmasked?
Using recordings collected from April through June 2015 as a comparison point, Derryberry and four other colleagues analyzed the vocal performance of male white-crowned sparrows in the period from April to May of this year. The sample area spanned breeding grounds from the rural forests and grasslands of Abbotts Lagoon and Commonweal in Marin County, north of the city, to the more urban East Bay city of Richmond, and Golden Gate Bridge-adjacent Lands End. There, co-author Jennifer Phillips, a postdoc at California Polytechnic State University, San Luis Obispo, recorded four distinct songs, with four distinct trill patterns.
What the researchers found is that as the city’s sounds dimmed, the urban sparrows’ songs changed. They didn’t get louder, though you may have noticed them more. Instead, they got “sexier.” The birds were able to say more, and say it better, because they didn’t have to shout so much.
Pre-pandemic, urban and rural soundscapes varied widely in these birds’ habitats: The white-crowned sparrow’s San Francisco breeding grounds are typically three times noisier than that of less-dense Marin County. In these normal conditions, urban males often have to sing more loudly than rural ones, at higher frequencies and at lower bandwidths, to compete with the high “sound energy” of a city. But they face a tradeoff between having their songs detected at a far distance and communicating information in the signals that they’re sending.
When lockdowns hit, the sound patterns of urban and rural areas converged into a blanket of quiet. Ambient noise — intermittent loud sounds like dogs barking or airplanes crossing the sky — dropped in both urban and rural locations, while the disappearance of background low-frequency noises like the hum of cars and buses was noticed more dramatically in cities. Derryberry’s team did their analysis before the city released comprehensive traffic data that quantified the slowdown, but they knew that vehicle crossings on the Golden Gate Bridge were down in April and May to “levels not seen since 1954.”
“[A] relatively brief but dramatic change in human behavior effectively erased more than a half-century of urban noise pollution and concomitant soundscape divergence between urban and nearby rural areas,” the authors wrote in their paper, which was recently published in Science. “In other words, the Covid-19 shutdown created a proverbial silent spring across the SF Bay Area.”
Urban white-crowned sparrows, whose breeding season started at the same time as the shutdowns, took advantage of this silence.
“When the noise disappeared, that tradeoff went away,” said Derryberry. “Suddenly, their signal could go a long distance and contain a lot of information.”
Male sparrows started singing about 30% more softly, but since the roar of the urban world had dropped by more than 50%, their signal carried twice as far. Features that hadn’t been heard since the 1970s reappeared, along with new trill patterns. “They weren’t actually louder, but they sounded louder,” she said.
To understand how that works, imagine you’re at a party, Derryberry suggested. (Bittersweet, I know.) As more people fill the room, voices creep louder and louder; vocal cords strain and get shrill, and still, you can barely hear the person in front of you. By the end of the night, though, when only a few stragglers remain, everyone speaks more softly and their words carry farther. You can’t help but perk up if you hear something juicy a couple of conversations away.
That emptying room is what the sparrows suddenly found themselves in this spring. Through the stillness, San Franciscans were able to hear four times the number of white-crowned sparrows as before.
For male birds, who use their birdsongs to repel rivals from their territory and to attract mates, being heard is essential to survival. Their tune is both a “keep out” and a “come hither” signal: The frequency exudes strength, and the trilling demonstrates stamina, says Derryberry. Better vocal performance can translate into more mates, and improved odds for successful breeding.
Because the life span of the sparrow is only about 13 months on average, Derryberry says it’s possible that male birds who were already suited to this emerging soundscape were favored for reproduction, and that what we’re hearing is the beginnings of an evolutionary adaptation. Even as traffic returns, Derryberry believes that the legacy of this “silent spring” will be long-lasting, and that songbirds in other cities may be experiencing similar effects.
“Whether it’s plasticity or selection, whatever it is, I think these birds are on a new trajectory,” she said. “Their songs have entered an acoustic space they haven’t been in over 30 years. I really doubt they’re just going to go right back to where they were before.”