When cities across the United States issued stay-at-home orders to fight the ongoing coronavirus pandemic, nature responded, too.
But beyond the rewilding seen in some cities around the world, new data from San Francisco's Bay Area shows just how much local animals were affected by human activity quieting down.
It turns out the decrease in human-produced noise, traffic in particular, is showing up in the songs of sparrows.
Songbirds in the Bay Area produced higher-quality songs during spring 2020 compared with previous years, a new study finds.
Because we are all staying home, ambient noise levels in 2020 are consistent with those of the mid-1950s. In turn, this quietening puts less pressure on birds and other animals that depend on their song for survival.
When birds have to compete with manmade noise, they sing at a higher frequency. Doing so carries their song further — but it also lowers its quality, which is measured by the song's amplitude.
It's a trade-off. Better song quality helps male birds on two important fronts: It improves their ability to call out to a mate, aiding reproduction, and helps them to protect their territory.
Traffic stalled — The primary reason for the decrease in human-produced noise this year was a drop in traffic. People working from home, and generally traveling less, means fewer automobiles drowning out the sounds of the natural world.
To measure the impact of this more tranquil world, researchers analyzed recordings of white-crowned sparrows in April to May 2020, and compared it with data from April to June 2015 taken from the same places. In 2020, the birds sang at lower amplitudes and frequencies, increasing their vocal performance.
The changes were especially noticeable for birds in urban areas, which have more opportunities to compete for space.
Researchers say the new findings, published Thursday in the journal Science, suggest birds adapt their behavior very quickly in response to human environmental factors.
This knowledge could help develop new interventions to improve the outlook for birds in human environments.
Accidental case studies — Songbirds' vocalizations are not the only surprise benefactors of the Covid-19 lockdown. The same may be true for other urban animals, according to a separate study on how traffic slowdowns affect wildlife, in the form of roadkill. That study determined that animal deaths declined by 21 to 56 percent during the early months of the pandemic, following a dramatic decrease in traffic.
One of the striking aspects of both studies is that the data are only possible by accidental means. Rather than relying on an experiment to test how human-produced noise affects bird songs, the researchers were able to gather real-life data. And such a dramatic reduction in real-world traffic is hardly a realistic study design.
Most previous studies on songbirds, meanwhile, have looked at how increasing urban noise levels have caused lower-performing bird songs.
Now, the opposite has been confirmed: When humans quiet down, life gets better for birds.
Abstract: Actions taken to control the coronavirus disease 2019 (COVID-19) pandemic have conspicuously reduced motor vehicle traffic, potentially alleviating auditory pressures on animals that rely on sound for survival and reproduction. Here we evaluate whether a common songbird responsively exploited newly emptied and acoustic space by comparing soundscapes and songs across the San Francisco Bay Area prior to and during the recent statewide shutdown. We show that noise levels in urban areas were dramatically lower during the shutdown, characteristic of traffic in the mid-1950s. We also show that birds responded by producing higher performance songs at lower amplitudes, effectively maximizing communication distance and salience. These findings illustrate that behavioral traits can change rapidly in response to newly favorable conditions, indicating an inherent resilience to long-standing anthropogenic pressures like noise pollution.