As Covid-19 spread through the United States in the spring of 2020, previously bustling cities fell silent. A new study shows that the dip in noise in the early months of the pandemic led to an increased abundance of birds, like hummingbirds, warblers and raptors, in cities. The study published today in Science Advances is the latest to suggest that even a moderate drop in noise pollution could allow some animals to better thrive in urban areas.
The flock to cities included "everything from birds like hawks and eagles all the way down to small songbirds and even hummingbirds," says Michael Schrimpf, study co-author and postdoctoral fellow at the University of Manitoba's Natural Resources Institute, to NPR’s Scott Neuman. "The actual physical environment didn't change," Schrimpf says. "What did change was the activity of people in those spaces."
Though there were false and sensationalized reports of wild animals infiltrating cities early in the pandemic, the drop in noise pollution has provided scientists with a rare opportunity to see how animals behave when they don't have to compete with anthropogenic noise. Recent research revealed that white-crowned sparrows adapted their tune when San Francisco streets fell quiet, and humpback whales in Alaska sang softer songs in the absence of cruise ships.
“The pandemic created a unique—hopefully—opportunity to understand the effects of traffic separated from the effects of the human-altered landscape at a scale that would be impossible under any other circumstances,” says study co-author Nicola Koper, a biologist at the University of Manitoba, to National Geographic’s Elizabeth Anne Brown.
To see if and how birds were using now-quiet cities differently, the team of researchers analyzed more than 4.3 million bird observations of more than 80 bird species recorded on the community science app eBird. Their data included every county in the United States and every census division in Canada that met three requirements: an international airport, a municipality of at least 50,000 residents, and at least 200 eBird checklists between March and May 2020.
They looked at more than three years of observations taken by experienced birders leading up to and through the first few months of the pandemic. Scientists found significant changes in birds’ migratory patterns and use of urban habitats, like venturing closer to usually-noisy areas like roads and airports. Bird abundance increased in cities overall, and especially during spring and fall migration.
“That suggests that if birds were migrating through at the same time as the lockdown was occurring, then they would look around them and be like, ‘Wow…it feels really safe here, I’m going to hang out here awhile before I move on,’” Koper tells Kate Baggaley for Popular Science.
Around 80 percent of the species Koper, Schrimpf, and their colleagues studied showed changes in their use of urban areas during the pandemic, with most species increasing on the order of 10 to 20 percent. Many birds shifted their behavior within weeks of lockdowns starting, though not all species responsed similarly. Ruby-throated hummingbirds were more likely to be spotted near airports during the pandemic, for example, and American robins moved closer to major roads.
“We assume that robins are well adapted to our human landscape,” Koper tells Popular Science. “Even the common species that are around us are actually much more sensitive to human disturbance than we have really appreciated before.”
Koper notes warblers and native sparrows seemed to benefit most from the reduction in noise, while it was a more complicated picture for other birds. Red-tailed hawks were spotted in urban areas overall, but sightings decreased near major roads, potentially because of reduced roadkill. Osprey and bald eagles were two species that appear to seek out the quietest areas.
“They actually moved from counties where they would have been historically more abundant to the counties that had stronger lockdowns,” Koper says to National Geographic. “They sort of traveled up the whole U.S. through these routes, these potentially safer counties…hundreds or thousands of kilometers away from where they maybe normally were.”
North America has lost almost a third of its birds in recent decades due to a combination of factors, including climate change, predation by feral cats, habitat loss, and building strikes, so any information scientists can glean about how to make urban environments more bird-friendly is critical. The study’s authors note that one of the most remarkable findings is that so many different birds benefitted from a reduction in noise, which doesn't have to be limited to pandemic lockdowns.
"There is an opportunity to adjust how we live, to slow down," says Schrimpf to NPR. "We hope that it might be a lesson for us that we can take away in a post-pandemic world.”