This has been a silent spring for Bay Area residents, as COVID-19 trapped us indoors to wait, worry, and complain.
But outside, our birds have rediscovered their sexiest serenades. The stage was all theirs.
A first-ever acoustic comparison of avian songs before and during the spring closure reveals that birds respond quickly when humans are silent.
They sang more softly – and these songs were faster, with a wider, lower and more romantic tonal range, according to the study, published in the Thursday issue of the journal Science.
The sound levels of the birdsong decreased by more than four decibels during the shutdown; since decibels are measured on a logarithmic scale, the songs were about a third softer. No longer forced to compete with human pandemonium, even the birds have lowered the pitch by 1
“It highlights the effect humans have on wildlife behavior and how quickly wildlife can respond when human behavior changes,” said lead researcher Elizabeth Derryberry, animal communication expert at the University. of Tennessee.
“Nature takes over as soon as people get out of the way,” he said.
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In April and May, Derryberry was stuck in her Knoxville home with her family, as bored and frustrated as all of us.
Then he saw a photo of the nearly empty Golden Gate Bridge, with car traffic at levels not seen since 1954, reversing the over half-century increase in noise pollution.
“It’s so quiet in San Francisco!” she thought. And I asked myself, “Do birds sing differently?” Unable to travel, he sought help from former graduate student Jennifer Phillips, who was doing postdoctoral work at Cal Poly San Luis Obispo.
Phillips quickly collected the acoustic equipment and rushed to San Francisco, Marin and the East Bay in search of birds perched among the shrub 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 adjusts to hear itself in our urban soundscape. Previous work has shown that frogs, insects and whales change their behavior in an increasingly noisy world. Bird species that are highly adapted to city life, such as starlings, also woke up early in the day to defeat the rush hour cacophony.
Specifically, the team looked for white-crowned sparrows, common creatures loved by scientists for the flexibility of their songs. The simple chorus of the species – a clear whistle, then a series of buzzes or trills over different pitches – varies regionally and can change over time.
They even sing in dialects; 10 different models were counted in the Bay Area. By mistaking a buzz for a whistle or a trill, a San Francisco sparrow sounds noticeably different than a Marin sparrow, which sounds different than an East Bay sparrow.
Scientists have collected extensive archives of Bay Area sparrows songs, dating back decades and securely stored at the California Academy of Sciences. In the Golden Gate Park recordings of 1967’s Summer of Love, anti-war activists can be heard singing behind the bird 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 move to loud songs that can be heard above the din. And they sing louder than the low-frequency rumble 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’re having small talk, you don’t have in-depth conversations,” Derryberry said. “Or political demonstrations, where people shout banality, not politics”.
Every day Phillips walked with a Marantz digital recorder, a Larson Davis sound level meter, a rangefinder to calculate distances and a high-end microphone, mounted on a dish, like a receiver.
In the San Francisco Presidio, he 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 walkers, cyclists, dog-walkers and boaters. Then, in search of nature, she retreated to the shrubland of the wild Point Reyes National Seashore.
His first challenge was to distinguish the song of a sparrow among the symphony of other birds. Then he had to find the bird, noticing which way it was facing. And he had to sneak in front of it, placing his gear, ideally, right at the level of the beak.
It was an effort of patience, silent precision and concentration. Sometimes, just when things were acoustically perfect, the birds flew away.
His audio files were compared with recordings made in April and May 2015. They were also compared with historical recordings: old reel-to-reel tapes that had been preserved as digital audio cassettes.
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 minimum vocal frequencies.
Surprisingly, the birds at the Richmond site have been singing at unrecorded basses since the spring of 1971.
Silence also changed the way people perceived what they felt. Although the birds sang softer, their songs traveled twice the distance.
Scientists will continue their research to find out if bird behavior changes again, in response to our increasingly crowded soundscape – and whether next year’s chicks will learn their parent’s songs or adjust to their new, louder world. .
They also hope their discovery encourages human behavior to change as well.
“Animals can recover if we give them time to do it,” said Phillips. “If we reduce our noise pollution – by driving less, using electric cars, by changing the way we build our roads – hopefully they will be more successful in urban environments.”