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When the lockdown arrived in San Francisco, the city became muted. Traffic dwindled, crowds dispersed, fewer planes soared overhead, and the daily roar of industrial civilization receded to a murmur.
In its place came the birdsongs. The chirping and twittering were always there, of course, muffled by the sounds of the city. But now the songs were different. A study published yesterday found that white-crowned sparrows in San Francisco sang with a broader range, hitting the low notes once drowned out by cars and trucks. When the researchers compared the songs to archival recordings, they found that some of the birds were singing tunes not heard since the 1970s.
“These birds were filling the soundscape that was newly emptied of human noise,” says Elizabeth Derryberry, a professor of ecology and evolutionary biology at the University of Tennessee, Knoxville, and the study’s lead author.
The findings not only reveal how quickly certain birds can adapt to changing environmental conditions, but also illustrate the outsize influence that humans have on other animals.
To Allison Injaian, a University of Georgia biologist who was not involved in the research, the study presents “a great opportunity for all of us to realize the impact that we ourselves are having on the wildlife around us.”
When the pandemic began, Elizabeth Derryberry wasn’t thinking about her research. Her focus was on the basics: how to teach remotely as an associate professor at the University of Tennessee, Knoxville; how to manage the lockdown with her young family; and how to keep everyone safe and healthy.
But as she scrolled through social media one evening, she saw a picture of a coyote at the empty Golden Gate Bridge in San Francisco. She recalls thinking, “Oh my goodness, there really are no cars.” And as she stared at that image, Dr. Derryberry thought about how quiet it must be nearby without the normal hubbub of traffic – and about the birds she had been studying there.
Along with her colleague David Luther of George Mason University, Dr. Derryberry had been recording the songs of white-crowned sparrows in both the urban setting of San Francisco and the more rural Marin County to study how the birds responded to the hum of human-made noise. They’d found that the city sparrows sang more loudly, but with a much more limited range, than their country cousins. And the shutdown presented an unprecedented opportunity for the researchers to see if those urban birds changed their tune.
Indeed, the urban sparrows took full advantage of the relative silence. When the research team recorded birdsongs near the Golden Gate Bridge in April and May of this year, they sounded notably different – and of higher quality – from those recorded during previous springs. Their findings were published Thursday in the journal Science.
“These birds were filling the soundscape that was newly emptied of human noise,” Dr. Derryberry says. “That was really exciting to see that kind of resilience.”
As people stayed home this spring, many noticed more wildlife around them. Some pondered whether there were actually more birds, for example, or if the quieter cities just made their songs (and presence) more obvious. Regardless, the shutdown yielded a renewed awareness that, even in the most densely populated cities, humans share the world with other creatures.
“When we’re going about our daily lives, we get used to the patterns of the animals that we see,” says Allison Injaian, a lecturer in the Odum School of Ecology at the University of Georgia, who was not involved in the study. “It’s pretty hard to know what we’re missing out on if that never is visible or audible.”
“But when this really unprecedented shift in human behavior occurred,” she says, it presented “a great opportunity for all of us to realize the impact that we ourselves are having on the wildlife around us.”
Softer songs in silent cities
Birdsongs are more than just friendly twittering in treetops. For the birds themselves, their songs encode information crucial to their existence.
White-crowned sparrows, for example, listen to each other’s songs to pick potential mates in spring, and as a way to assess the fitness of another male from afar when deciding whether or not to fight him to try to take over his territory.
But in cities, they’re typically making a trade-off between the quality of their songs and simply being heard, says Ken Otter, a biologist at University of Northern British Columbia who was not involved in the new study. “It’s like, ‘I can either make my song sound really awesome, but nobody can hear it or I can make it audible but make it sound [inferior],’” he says.
Before the shutdown, Dr. Derryberry and Dr. Luther found that birds in San Francisco were competing with nearly three times as much noise as those in Marin County. But when the pandemic closed everything down, there was no difference in noise levels. They attribute that to less traffic, as the amount of cars passing through had reverted to levels not seen since the 1950s.
As a result, birdsongs could travel much farther. The researchers found that the birds sang more softly because they didn’t have to be louder than the anthropogenic noise, and even still, their songs could travel twice the distance as before the shutdown.
The bandwidth of the trill at the end of the sparrows’ song is also key to communicating physical fitness to potential mates or rivals. Researchers found previously that the urban birds limit their trills to higher frequencies so they don’t have to compete with the low hum of traffic. But during the shutdown, the team found that the city sparrows utilized their full range – and when they compared the 2020 songs to historical recordings in the area, they found that some of the sparrows were singing in ways not heard in the city since the 1970s.
Whether this has a long-term effect remains to be seen, says Dr. Derryberry. But she plans to study the San Francisco sparrows’ sounds during the breeding season once again next year. “I’m really excited to see what happened with the nestlings that learned their songs this year,” she says. “What do they do? Do they keep those sexy songs? Do they modify them to transmit better as those noise levels go back up? Can they do that?
Adapting on the fly
Birds don’t just adjust their songs’ volume and range. Research has found that some urban birds will adjust the time of day that they sing to avoid rush hour.
“There are multiple strategies that birds can use to still be able to acoustically communicate,” Dr. Injaian says.
“And maybe that behavioral plasticity, that ability to change their behavior in this disturbed environment, allows them to evade any negative consequences. But we don’t necessarily know that yet. There might be negative consequences in the long term that haven’t shown up yet,” or perhaps it’s species-specific and other bird species are unable to cope with such noise at all.
Still, she adds, “One of the fascinating things about this study is that it shows that these birds are highly capable of acclimating to a new environment. We as humans changed our behavior, and we almost immediately saw a response in the behavior of these wild animals. And that, I think, can be a source of hope.”
Studies like this one, Dr. Otter says, can also help give us direction. “It’s really important for understanding how we can move forward with planning,” he says, “so that we can create spaces that not only attract the birds, but allow them to be successful.”
“We do have a sort of finite amount of space on the planet,” he adds, “and as we increase the amount of it that’s urbanized, what we really need to be conscious of is also making allowance for space for other organisms other than just people.”