海角大神

When the pandemic began, Elizabeth Derryberry wasn鈥檛 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, 鈥淥h my goodness, there really are no cars.鈥� And as she stared at that image, Dr. Derryberry thought about how quiet it must be nearby without the normal hubbub of traffic 鈥� 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鈥檇 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 鈥� and of higher quality 鈥� from those recorded during previous springs. Their findings were published Thursday in the journal Science.聽

鈥淭hese birds were filling the soundscape that was newly emptied of human noise,鈥� Dr. Derryberry says. 鈥淭hat was really exciting to see that kind of resilience.鈥�

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.聽

鈥淲hen we鈥檙e going about our daily lives, we get used to the patterns of the animals that we see,鈥� says Allison Injaian, a lecturer in the Odum School of Ecology at the University of Georgia, who was not involved in the study. 鈥淚t鈥檚 pretty hard to know what we鈥檙e missing out on if that never is visible or audible.鈥�

鈥淏ut when this really unprecedented shift in human behavior occurred,鈥� she says, it presented 鈥渁 great opportunity for all of us to realize the impact that we ourselves are having on the wildlife around us.鈥�

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.聽

White-crowned sparrows, for example, listen to each other鈥檚 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鈥檙e 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. 鈥淚t鈥檚 like, 鈥業 can either make my song sound really awesome, but nobody can hear it or I can make it audible but make it sound [inferior],鈥欌�� 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鈥檛 have to be louder than the anthropogenic noise, and even still, their songs could travel twice the distance as before the shutdown.聽

The bandwidth of the trill at the end of the sparrows鈥� 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鈥檛 have to compete with the low hum of traffic. But during the shutdown, the team found that the city sparrows utilized their full range 鈥� 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.聽

Whether this has a long-term effect remains to be seen, says Dr. Derryberry. But she plans to study the San Francisco sparrows鈥� sounds during the breeding season once again next year. 鈥淚鈥檓 really excited to see what happened with the nestlings that learned their songs this year,鈥� she says. 鈥淲hat 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?聽聽

Adapting on the fly

Birds don鈥檛 just adjust their songs鈥� volume and range. Research has found that some urban birds will adjust the time of day that they sing to avoid rush hour.

鈥淭here are multiple strategies that birds can use to still be able to acoustically communicate,鈥� Dr. Injaian says.聽

鈥淎nd maybe that behavioral plasticity, that ability to change their behavior in this disturbed environment, allows them to evade any negative consequences. But we don鈥檛 necessarily know that yet. There might be negative consequences in the long term that haven鈥檛 shown up yet,鈥� or perhaps it鈥檚 species-specific and other bird species are unable to cope with such noise at all.

Still, she adds, 鈥淥ne 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.鈥�

Studies like this one, Dr. Otter says, can also help give us direction. 鈥淚t鈥檚 really important for understanding how we can move forward with planning,鈥� he says, 鈥渟o that we can create spaces that not only attract the birds, but allow them to be successful.鈥�

鈥淲e do have a sort of finite amount of space on the planet,鈥� he adds, 鈥渁nd as we increase the amount of it that鈥檚 urbanized, what we really need to be conscious of is also making allowance for space for other organisms other than just people.鈥�