With less traffic and fewer people on the streets of San Francisco this spring, the songs of resident male white-crowned sparrows became more audible despite actually being quieter, according to a report in Science today (September 24). The birds also increased in the bandwidth of their songs to include lower notes, which, according to experts, makes them more appealing to the females of the species.
“It’s a very nice paper,” says Hans Slabbekoorn, who studies the effects of noise on animal behavior at Leiden University and was not involved in the research. “The data are very convincing that these birds adjusted immediately to the lower noise levels.”
In cities that are normally bustling, the sudden shutdown of schools, shops, restaurants, and places of work in mid-march this year virtually eliminated the buzz of traffic, construction, and people almost overnight. In the relative quiet, city dwellers started noticing how noisy the birds were.
“There has been a lot of armchair observation going on that birds and other animals are being more active and are singing louder,” says ornithologist Mike Webster of Cornell University who also was not part of the research team. “To be honest, I put that down to people being at home more and paying attention more, but what these guys have shown pretty clearly is that the birds they studied really are singing differently . . . and are responding very strongly and very quickly to all the COVID-19–induced shutdowns.”
I think there will be increased interest in minimizing our own acoustic footprint.—Mike Webster, Cornell University
The “guys” to whom Webster refers are animal communication researcher Elizabeth Derryberry of the University of Tennessee and colleagues. Derryberry has been studying and comparing the songs of male white-crowned sparrows (Zonotrichia leucophrys) in the noisy urban environments and quiet rural settings around San Francisco for a number of years. The female sparrows rarely sing.
“These authors were in a perfect position because they had all this pre-pandemic, highly quantitative data,” says Jeff Podos of the University of Massachusetts who studies songbird communication and was not part of the research team. For them, he says, “a really nice opportunity presented itself from amidst the grim news of the pandemic.”
A unique opportunity it may have been, but Derryberry was initially too preoccupied to notice, she confesses. “I was at home with a two-year-old and an eight-year-old and trying to keep working . . . and to [deal with] virtual school. We all were, I think, slightly bamboozled by this [shutdown].”
It wasn’t until she saw some pictures of San Francisco’s Golden Gate Bridge on social media that the penny dropped, she says. “I was like, ‘Oh, it’s really empty.’”
Although Derryberry was in Tennessee, unable to fly to California, Jennifer Phillips, who had studied the sparrows for her PhD with Derryberry, was nearby at California Polytechnic State University, a few hours south of San Francisco, working as a postdoc in the lab of Clinton Francis.
“Everything just lined up,” says Derryberry, and with Phillips being able to record the birds and noise levels in the same locations as the team had done years earlier, “we had this nice paired comparison.”
The team first confirmed that background sound had indeed dropped. Their recordings showed on average a 7-decibel reduction in background hum in the city, bringing the noise levels close to those of the surrounding rural area, which remained largely unchanged. “The soundscape . . . of the city was almost identical to that of the countryside during the shutdown,” Derryberry says.
The question was, what if anything had happened to the birdsong? In their earlier studies, the team had found that birds in the city sing louder and at higher frequencies than their rural counterparts, presumably to be heard over the sounds of traffic and human life. When that noise was gone, they found, the resident birds quieted too—by about 4 decibels, says Derryberry.
The wider the bandwidth, the sexier and more competitive the male.—Elizabeth Derryberry, University of Tennessee
Despite their softer calls, the songs were audibly clearer and thus traveled further. That’s because they hadn’t dropped in volume to the same degree as the background noise, explains Derryberry. The signal-to-noise ratio was essentially higher, which accounts for the anecdotal reports of louder birdsong during shutdown.
In addition to being quieter, the sparrows’ shutdown songs had increased in bandwidth, says Derryberry. Specifically, they produced lower frequency notes that would ordinarily be lost in the low rumbling of traffic. Prior studies have shown that female birds find males who sing songs with greater bandwidth more attractive. “The wider the bandwidth, the sexier and more competitive the male,” Derryberry says.
Whether the altered songs this spring will influence the reproductive success of the city sparrows is unclear. Nevertheless, the study’s findings show that “natural systems can respond quickly to reductions in anthropogenic interference, in this case, noise,” says Webster, “so that’s a very positive thing.”
Even though human life and its accompanying noise is returning to cities as the restrictions have eased, there will be “increasing awareness that humans are having this effect on natural populations,” Webster says, and that nature can bounce back if such human disturbances are removed. As a result “I think there will be increased interest in minimizing our own acoustic footprint,” he says.
People might decide to work from home more often, for example, or make their next car an electric one. “People are probably interested in buying electric cars for lots of reasons . . . this is just one more positive benefit of doing something like that,” Webster says. In short, the study increases awareness of noise pollution, he says, and “awareness can lead to action.”
E.P Derryberry et al., “Singing in a silent spring: Birds respond to a half-century soundscape reversion during the COVID-19 shutdown,” Science, doi:10.1126/science.abd5777, 2020.