When the cat’s away the mice will play, or so the saying goes. But what happens when humanity’s away, locked inside to slow the Covid-19 pandemic? It turns out that birds will play—a sexier song, that is.
Writing today in the journal Science, researchers report that male white-crowned sparrows around the San Francisco Bay Area exploited the sudden drop in anthropogenic noise when the region went on strict lockdown in April and May. From their field observations during previous years, the researchers had lots of data to show that urban birds sacrifice song quality for higher amplitudes—basically, they’re yelling to be heard in a noisy environment. When that din suddenly died down, the birds switched to songs that more closely resemble the softer, higher-quality calls of their nearby rural counterparts. With less of a racket around them, they could afford to focus on a more complex sound.
“The pandemic has been terrible in a lot of ways,” says University of Tennessee behavioral ecologist Elizabeth Derryberry, co-lead author on the paper. “But as a scientist, there’s sort of this exciting opportunity for a natural experiment there: We remove noise from a whole soundscape and see what happens.”
This isn’t the first time scientists have capitalized on the lockdown’s quiet to better spy on the natural world. For example, Covid-19 has provided rare opportunities for seismologists to collect high-quality data without interference from traffic, heavy industries, and even the crowd noise from rock shows and sporting events. But it’s significant for ornithology because previously, scientists could only add noise to see how birds responded, for instance setting up speakers in bird territories to play extra traffic noise. “And we do see them sing more loudly,” Derryberry says. “And so we sort of said, ‘OK, when we take sound away, they’ll drop by that same amount.’ But what we found is they went beyond that—they didn’t just drop how much we thought they would drop, they dropped even further.”
Why is this happening? Well, let’s imagine ourselves at a bygone gathering called a “cocktail party.” In a crowded room, your brain does a remarkable job of prioritizing the speech of the person you’re talking to, so you’re not just hearing a roar of white noise. But when it comes to verbal communication, the quality of information you can transmit depends on the distance. “In other words, at a cocktail party, you’re not having deep philosophical discussions,” says Derryberry. “It becomes small talk as it gets louder through the night. Political rallies, when you’re yelling on the bullhorn, you’re saying platitudes, right? It’s very hard to contain a lot of information and transmit a long distance in noise.”
Male white-crowned sparrows have the same problem in a soundscape dominated by traffic, and they simplify their song to compete with the uproar. So when you eliminate that noise, can the birds then transmit their songs long-distance while still including lots of info? They sure can. “They double their communication distance,” says Derryberry. “And they also have these really wide-bandwidth songs, which means they contain a lot of information.”
Higher-performance songs are sexier to female birds. (Males also sing to defend their territories from other males.) I doubt you’re a white-crowned sparrow, so let’s define “sexier” here as more intricate songs. These songbirds sound so sweet because they create pure tones by filtering out the harmonics that makes a jay’s call so raspy. It’s the complex beauty of a well-trained opera soloist versus the growling of a metal singer.
“When they’re singing, they’re actually modifying the length of their vocal tract by opening and closing their beak,” says Derryberry. “When their beak’s more open, their vocal tract is short, like a piccolo. When it’s more closed, it’s longer, like a flute, so it’s lower.”
In a bravura performance, a male wants to demonstrate that he can do both, producing a range of sounds. But a male can only open and close his beak so quickly and so wide. “Females like it, they like how hard that is,” says Derryberry. “A high-quality male is one that can perform that trade-off well. So this suggests that when it’s noisy, males aren’t really reaching their full potential, right? They can’t signal just how sexy they can be. That’s what we saw when we took noise out of the soundscape: We got some really sexy males. It really increased that performance, and it was at a level that you normally hear in the countryside, in more rural areas.”
You may have noticed that during lockdown you’ve been hearing more birds. But what Derryberry found, with white-crowned sparrows at least, is that they’re actually singing much softer, even softer than the researchers had predicted. So what gives? “Even though they sing more softly, you can hear birds at a greater distance now,” Derryberry says. “If you think about sort of a circle around you, if you can hear birds at twice the distance, you take that radius and draw a big circle around you—that means you can hear four times more birds. So that’s why they sound louder, because you could just hear more of them.”
But here’s where things get a bit trickier. Songbirds like white-crowned sparrows have a sort of cultural transmission of songs, in which individuals copy the songs of others. Yet there’s also real-time plasticity in the way they sing; in this case, they’ve been tweaking the bandwidth of their songs during the quiet of a pandemic.
Urban and rural white-crowned sparrows have their own styles—Derryberry’s previous data shows that—but she can’t yet nail down the mechanism for how the shift during the pandemic came about. “I don’t think we know whether that’s individual males changing, or males that happen to have copied wider-bandwidth songs—maybe in a quieter part of the city—are now holding more territory during that one breeding season,” Derryberry says. She adds that she and her colleagues will be exploring this question next spring.
Regardless, the dramatic song shift among white-crowned sparrows shows just how badly we humans have mangled the soundscape, and how wildlife suffers for it. “I think this is a great reminder that noise pollution is real, and it really does affect the lives of a lot of different animals,” says Jack Dumbacher, curator of ornithology and mammalogy at the California Academy of Sciences, who wasn’t involved in this research.
The loss of human life has been staggering, as has the toll on our economies. But in a way, the pandemic has given us a glimpse of a slower, quieter world, and given ecologists an unprecedented opportunity to watch what happens when human activity grinds to a halt—what one group of researchers has termed the “anthropause.” They’ve been able to track animal movements and foraging behavior in urban areas, and how well endangered species are faring in the wild without human protection.
“That means we get to hear more birds and see more wildlife,” says Derryberry. “And that I think has been a real silver lining for a lot of people—not just from a scientific standpoint, but just a quality-of-life standpoint. And that, to me, has been uplifting, because that is rare.”
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