This was a silent spring for Bay Area residents, as COVID-19 trapped us indoors to wait, worry and whine.
But outside, our birds re-discovered 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 Thursday’s issue of the journal Science.
No longer forced to compete with human pandemonium, bird songs were more than four decibels quieter during the shutdown; because decibels are measured on a logarithmic scale, this means songs were one-third softer. Remarkably, 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.
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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 graduate student Jennifer Phillips, who was doing postdoctoral work at Cal Poly – San Luis Obispo.
Phillips quickly packed up her acoustic equipment and rushed out to San Francisco, Marin and East Bay in search of birds among the shrubby habitats of lupine, poison oak, sage, and coyote bush.
The research is part of the new and growing field of “acoustic ecology,” which explores how nature struggles and adapts to hear itself amid our urban soundscape. Previous research has shown that frogs, insects and whales change their behavior in an increasingly noisy world. Even bird species highly adapted to city life, like starlings, have been found to awake earlier in the day, to beat rush hour cacophony.
Specifically, the team sought out white-crowned sparrows, a common creature that is beloved by scientists for the flexibility of their songs. Remarkably, its simple chorus — a clear whistle, then a series of buzzes or trills on different pitches — varies regionally, shifting over space and time.
They even sing in dialects; ten 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, who is 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, the chanting of peace activists can be heard among the bird chirps.
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 loud places shift to loud songs that can be heard above the din. And they sing at a pitch that’s higher than the low-frequency rumbling of traffic.
But this comes at a cost: They can’t produce the rapid trills at wide frequency bandwidths — the music that is so sweet to suitors.
“It’s like shouting at a cocktail party. You’ve making small talk, not in depth conversations,” said Derryberry. “Or political rallies, with shouts of platitudes, not policy.”
The team wondered: How would the sparrows respond to the great silence that spread over the natural world?
For days, Phillips hiked with a Marantz digital recorder, a Larsen 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 Highway 580, popular with chatty walkers, bicyclists, dog-walkers and boaters. Then, in search of nature, she retreated to the shrub land of southern Point Reyes National Seashore.
Her first challenge was to detect a sparrow song among the orchestra of other birdsongs. Then she had to find it, noting which way it was facing. And then she had to sneak around in front of it – positioning herself, ideally, right at beak level.
Just when things were acoustically perfect, it often flew away.
The sound files were compared the recordings made during April and May 2015. They were also compared to historic recordings — old reel-to-reel tapes that had been preserved as digital audiotapes.
They found that this year’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 team hopes to continue its research to learn if bird behavior changes again — and if next year’s babies will learn their parents’ songs, or adapt to their new and more noisy world.
They also hope that human behavior can 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 ew build our roadways — hopefully they can breed better.”
“Humans are still here,” she said. “We’re just quieter.”
