As the sun set over the Great Meadows National Wildlife Refuge in Sudbury, Joan Walsh spotted one arrival after another from the annual spring migration of birds.
Tree swallows over the marsh. A phoebe. A red-winged blackbird. On the horizon, an osprey diving for fish.
But these seasonal migrations are a shadow of what they used to be.
A landmark study in the journal Science estimates North America has lost about 30% of its bird life since 1970 because of habitat degradation, urbanization and the use of toxic pesticides.
That's about 3 billion birds, a drop that has scientists sounding the alarm — in part by highlighting birdsong that's no longer sung.
Walsh, chair of field ornithology for Mass Audubon, remembers watching migrating flocks of red knots over Delaware Bay in the 1980s.
"It could have rivaled any nature documentary that you watch about the wildebeest going across the Serengeti," she said. "And now it's down to just a fraction. And it's a tragedy."
Three billion birds is a massive loss, but in some ways, it's almost invisible. Each year the flocks are a little thinner, and the birdsong a little quieter.
So how do you make people aware that populations are collapsing before they’re gone?
It's a question that's haunted Simon Butler, an associate professor of ecology 3,000 miles away at the University of East Anglia in the U.K.
Butler said he was out walking his dog one morning, listening to the birds, when he had a thought.
He realized he could use the same historic bird count data from the Science study to reconstruct how different places would have sounded, five, 10, or 20 years ago.
He and his research team pulled population data from more than 200,000 sites across North America and Europe. Then they started matching records of individual birds with audio recordings of their songs.
"So if there were five American robins recorded, for example, we'd insert five clips of American robin song," he said.
The team layered the songs of each individual bird to build a "composite soundscape" that broadly represented what it might have sounded like at the survey site in that moment of time.
Butler and his team analyzed patterns between all 200,000 sites in North America and across Europe. They found that overall, these soundscapes are now quieter and more homogeneous than they were two decades ago.
"What we're finding over time is that nature's orchestra — we're losing players from that, but we're also losing instruments," he said. "And although we won't necessarily detect that those particular things have gone, the quality of the overall sound is reducing."
Mass Audubon's Walsh said she was moved when she heard Butler's soundscapes. Butler reconstructed how the Great Meadows site in Sudbury would have sounded in 1997 — the sound file teems with songs of yellowthroats, eastern kingbirds and meadowlarks.
Walsh said the soundscape gave her new perspective on just how much we've lost.
"It's like taking the Crayola crayons and just leaving you with the palette of grays instead of letting you have all the colors," she said. "It's taking the orchestra and removing the piccolo and the flute and the bass. And we deserve just so much more than that."
Walsh thinks it's not too late to reverse the loss. We've even done it before.
The federal government's ban on the toxic pesticide DDT, following Rachel Carson's book "Silent Spring," helped save bald eagles from the brink of extinction. The book also spurred the nascent environmental movement.
But Walsh said we're running out of such "silver bullets."
"A lot of the challenges that animals face now is a general degradation. It's the inching away, the chipping away of habitat," she said.
Walsh said habitat preservation is key. But there are other things people can do.
Lights in skyscrapers and office buildings can be turned off during peak migration periods. Many birds migrate at night and building lights can confuse them and throw them off track.
Boston had a "lights out" program in the early 2000s, but it fizzled. Walsh thinks the city should bring it back. Advocates also argue that pet owners should keep their cats indoors. Studies find that outdoor cats kill billions of birds in the U.S. every year.
Walsh said we need to restore these lost soundscapes before it's too late.
"The baby who's just being born today in Boston, in Methuen, in Deerfield, anywhere in this state, deserves to have that in their future," she said.
This segment aired on June 10, 2022.