The Pandemic Shutdown in San Francisco Had Sparrows Singing Sexier Tunes

Science
جمعه 4 مهر 1399 ساعت 17:17
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Smith sonian mag
Elizabeth Derryberry has been studying the songs of white-crowned sparrows for over a decade. Her 2012 work recording and analyzing birdsongs helped demonstrate that San Francisco sparrows slowly shifted their songs to a higher register to be heard above the hustle-and-bustle of city life. In March of this year, when shutdown measures in response to the COVID-19 pandemic rendered the city’s once-busy streets nearly silent, Derryberry was struck with an idea.
“It wasn't until I was looking at some photos of actually the Golden Gate Bridge, I was like, ‘Oh my goodness. There really is a lot less traffic,’” says Derryberry. She wondered if the city’s sparrows—the same ones adapted to sing through the drone of city sounds—were shifting their songs. Derryberry hypothesized that without the onslaught of low-frequency sounds characteristic of urban life, the sparrows would drop their volume and pitch. In new study published this week in Science , she demonstrated just that.
“It's like a cocktail party,” says Derryberry. “When it gets louder and louder in the room, you get louder and louder. Then when the party ends, you don't keep shouting all night.”
To find out if and how the sparrows’ songs had changed, Derryberry and her colleagues looked at two sets of recordings: the first, from the spring of 2015; the second, from the spring of this year after shelter-at-home mandates. The recordings were taken in the same places, included locations in urban San Francisco and surrounding rural areas of Marin county.
When Derryberry’s team compared the recordings, they discovered that in spring 2020 city sparrows made a dramatic shift to lower, quieter song, while in 2015 the songs stayed high and loud. Rural birds in 2020 sang lower songs too, though their changes weren’t as dramatic as those of the city sparrows. As a result, between the different years surveyed, city birds underwent a big change, while rural birds only tweaked their tune slightly.
The finding is significant, says Derryberry, because singing ability and range really matter in the bird world. Songbirds like white-crowned sparrows use their tunes to lure mates and defend territories, which can make or break an individual’s survival. “The wider the bandwidth, basically, the sexier they are,” says Derryberry. “This gets at a fundamental tradeoff that these birds in noisy areas are facing where they can't be both sexy and transmit their song at a great distance.”
Clinton Francis , an ecologist at California Polytechnic State University who was not involved in this research, calls this study a rare glimpse into how wildlife might behave differently in quieter cities. Francis has spent his career studying the influence of anthropogenic noise on birds. “There have been all of these great studies that have experimentally introduced noise—made the landscape a lot louder—but there are so few of these opportunities where it's taken away,” he says.
Shelter-at-home measures had inadvertently reduced San Francisco’s traffic noise to 1950s levels, says Derryberry. Her team has recordings dating back decades, before the sparrows’ song became higher and louder. “They're singing aspects of their song that we haven't heard since the ’70s and that's really cool to me,” says Derryberry. “It's like we're listening to the Beatles again.”
One possible explanation for the birds’ lower tone is the Lombard effect : as animals sing more softly, their pitch naturally lowers. Derryberry found that while the lowest frequencies of the birds’ songs dropped, they still preserved their middle and upper tones, suggesting a change based on more than just a mechanical relationship between volume and pitch.
The birds also sang more softly than her team predicted based on decreased noise pollution. “The most exciting part of this study is that [the sparrows] went beyond what we expected, and that highlights that there are some real costs to singing loudly,” says Derryberry.
Derryberry says she was intrigued by reports of people hearing more birds during the shutdown. “We were shocked to find that, even though they're softer, their songs are transmitting at twice the distance—almost three times,” says Derryberry. “So, no wonder people are noticing them, even though they're singing more softly you can hear many more because it's so quiet.”
Jeffrey Podos , who studies birdsong at the University of Massachusetts Amherst and was not involved in the study, says he is curious to know if specific birds shifted their song, or if Derryberry’s team was hearing new birds. “It would be really surprising if individual birds were able to detect this and lower their frequencies,” he says.
Podos says sparrows learn songs in their first breeding season and the tune is crystalized for life, which makes him skeptical that individual sparrows would have the flexibility to make such a dramatic song shift. Instead, he thinks birds singing previously masked lower-frequency songs out competed those stuck singing the higher tune.
While sparrows learn one song for life, it is possible to tweak their tweets if they need to, says Francis. Though song types can shift naturally over time, Francis says these sparrows aren't resurrecting an ancient language. “They’re singing the same kind of song type,” he says. “But just moved down.”
What this change in song means for the white-crowned sparrow population, if anything, is unclear—it’s too soon to know if a sudden drop in noise pollution will have measurable impacts on the species as a whole. Derryberry’s team pinpointed this song change in San Francisco sparrows because of their extensive research from past decades, but Podos thinks this song shift is happening elsewhere, too.
In San Francisco, Derryberry is eager to see what happens as shutdown measures lift and city noise ramps up again. In the spring, she and her team plan to take new recordings to see if these sparrows continue to adapt their songs as noise patterns shift. “I'm really excited to figure this out,” says Derryberry. “To me, the story is only half told.”
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