New tweets, squawks, trills — birds change their tunes
As COVID-19 forces humans indoors, feathery friends sing softer, lower, sexier songs
This was a silent spring for Bay Area residents, as COVID-19 trapped us indoors to wait, worry and whine.
But outside, our birds rediscovered their most sexy serenades. The stage was all theirs.
A first- ever acoustic comparison of avian songs before and during the springtime shutdown reveals that birds respond quickly when humans hush.
They sang more softly — and these songs were faster, with a wider, lower and more romantic range of pitch, according to the study, published in the Sept. 24 issue of the journal Science.
The sound levels of bird songs fell by more than four decibels during the shutdown; because decibels are measured on a logarithmic scale, songs were about one-third softer. No longer forced to compete with human pandemonium, birds also dropped their pitch by 160 vibrations per second.
“It highlights how much of an effect that humans have on wildlife behavior — and how quickly wildlife can respond when human behavior changes,” said lead researcher Elizabeth Derryberry, an animal communication expert at the University of Tennessee.
“Nature takes over as soon as people get out of the way,” she said.
In April and May, Derryberry was stuck at her Knoxville home with her family, as bored and frustrated as the rest of us.
Then she saw a photo of the near-empty Golden Gate Bridge, with vehicle traffic at levels not seen since 1954 — reversing more than a half-century rise in noise pollution.
“It’s so quiet in San Francisco,” she thought. And wondered: “Are the birds singing differently?” Unable to travel, she enlisted the help of former graduate student Jennifer Phillips, who was doing postdoctoral work at Cal Poly San Luis Obispo.
Phillips quickly packed up acoustic equipment and rushed out to San Francisco, Marin and the East Bay in search of birds perched among the shrubby habitats of lupine, poison oak, sage and coyote bush.
The research is part of the new field of “acoustic ecology,” which explores how nature struggles and adapts to hear itself amid our urban soundscape. Previous work has shown that frogs, insects and whales change their behaviors in an increasingly noisy world. Even bird species that are highly adapted to city life, such as starlings, have been found to awaken earlier in the day to beat rush hour’s cacophony.
The great silence that suddenly spread over the natural world offered a chance for new insights, said Pam Young, director of Berkeley’s Golden Gate Audubon Society.
“What is a rare opportunity for we humans is a quiet moment when we can finally hear the spectacular and elaborate sounds that birds are capable of producing,” she said. “The emerging science of avian bio accoustics — the study of bird song — reveals complex and varied ranges of sounds that just one bird from one species may project.”
The team sought out white- crowned sparrows, common creatures that are beloved by scientists for the flexibility of their songs. The species’ simple chorus — a clear whistle, then a series of buzzes or trills on different pitches — varies regionally, and can change over time.
They even sing in dialects; 10 different patterns have been counted in the Bay Area. Swapping out a buzz with a whistle or a trill, a San Francisco sparrow sounds markedly different from a Marin sparrow, which sounds different from an East Bay sparrow.
Scientists have collected vast archives of Bay Area sparrow songs, dating back decades and safely stored at the California Academy of Sciences. In Golden Gate Park recordings from 1967’s
Summer of Love, anti-war activists can be heard chanting behind the birds’ chorus.
Singing is critical to a bird’s survival, helping it find a mate and defend territory.
Derryberry and fellow researcher David Luther of Virginia’s George Mason University have documented that sparrows in noisy places shift to loud songs that can be heard above the din.
And they sing at a pitch that’s higher than the lowfrequency rumbling of traffic.
But this comes at a cost: They can’t produce the rapid trills at wide- frequency bandwidths — the information-rich music that is so sweet to suitors.
“It’s like shouting at a cocktail party. You are making small talk, not having in-depth conversations,” said Derryberry. “Or political rallies, where people shout platitudes, not policy.”
Every day, Phillips hiked with a Marantz digital recorder, a Larson Davis sound level meter, a rangefinder to calculate distances and a high-end microphone, mounted on a parabola, like a receiver.
In San Francisco’s Presidio, she recorded above Fort Point, a spot usually filled with the roar of traffic from the Golden Gate Bridge. She also went to Richmond’s Point Isabel near busy Interstate 580, popular with chatty walkers, bicyclists, dog-walkers and boaters. Then, in search of nature, she retreated to the shrubland of wild Point Reyes National Seashore.
Her first challenge was to distinguish a sparrow’s song among the symphony of other birds. Then she had to find the bird, noting which way it was facing. And she had to sneak around in front of it — positioning her equipment, ideally, right at beak level.
It was an effort of patience, quiet precision and focus. Sometimes, just when things were acoustically perfect, birds flew away.
Her sound files were compared to recordings made during April and May 2015. They also were compared to historical recordings — old reel-to-reel tapes that had been preserved as digital audiotapes.
The team found that this spring’s sparrows, exposed to less background noise, exhibited greater vocal “performance,” with drops in vocal amplitudes and reductions in vocal minimum frequencies. Remarkably, the birds at the Richmond site sang at lows not recorded since the spring of 1971.
The silence also changed how people perceived what they heard. Even though birds sang more softly, their songs traveled twice the distance.
The scientists will continue their research to learn if bird behavior changes again, in response to our evermore crowded soundscape — and if next year’s chicks will learn their parents’ songs, or adapt to their new and more noisy world.
They also hope their discovery will encourage human behavior to change, too.
“Animals can recover if we give them the time to do it,” said Phillips. “If we reduce our noise pollution — driving less, using electric cars, changing how we build our roadways — hopefully they will be more successful in urban environments.”