At the Hubbard Brook Experimental Forest in northern New Hampshire, the pandemic broke a decades-long streak of field research. Now, scientists there are adapting with new technology — recording the sounds of the forest, which they hope will transform their long and influential record of a changing world.
In late fall, Dartmouth biologist Matt Ayres stands out in camouflage and cargo pants against the bright yellow of the woods. He’s got binoculars around his neck, and he’s loading gear from his truck into a backpack — two kinds of batteries, microphones, GPS devices and more.
Ayres’ specialty is caterpillars and other insects, but today’s he’s here on a bird-monitoring mission in this 12-square-mile research forest, created by the federal government on the southeast slopes of Mount Moosilauke in the 1950s.
Hubbard Brook is where scientists first discovered acid rain, which Ayres says helped transform environmental protection in America.
“So when they started collecting those water samples and analyzing the chemistry ... they didn’t know that that would inspire the Clean Air Act of 1967,” Ayres says. “And they didn’t know that those same samples now show that the Clean Air Act has been one of the best examples of environmental stewardship ever.”
Decades of research at Hubbard Brook have created a remarkably useful record of a single, changing forest. Visible signs of that work are everywhere — tags on trees, fluorescent flags demarcating plots of soil. But until recently, Ayres says, they’ve been missing one crucial feature of the landscape.
“No one knows what it sounded like in the Hubbard Brook Forest even five years ago,” he says. “Sounds leave no fossils, no isotopic signatures. For the most part, they’re here one millisecond and gone the next.”
Sound can be a key measure of an ecosystem’s health — revealing, for example, how changing weather patterns are affecting the behavior of migratory songbirds like the black-throated blue warbler, potentially causing them to nest or breed in different locations at different times.
Ayres stands in a strip of forest where scientists — led by Dartmouth’s Richard Holmes, a founding father of Hubbard Brook — have surveyed those birds every summer for 51 years, and recently began taking audio samples.
“It’s globally extraordinary data,” he says. “But there’ll always be an asterisk on 2020.”
The COVID-19 pandemic made it impossible for field biologists to live together at Hubbard Brook during survey season.
So Ayres says they had to find new ways to fill in the gaps. They turned to their sound recorders, and set them up in this part of the woods to run on their own.
“Up this ridge and off in both directions from us ... is a network of 30 passive acoustic recorders that all turn themselves on at the same time in the morning, record for three hours, turn themselves off, come back for another hour in the evening,” he says. “And it’s amazing — there’s a symphony.”
The success of remote recording last summer gave them the idea to do it again — during mud season, when the roads and trails of the forest are all but impassable and the migratory birds are just starting to come back from the tropics to breed in New Hampshire.
So today, Ayres is programming these recorders to turn on by themselves in the spring, weeks before a human can check on them.
Ayres follows his compass and GPS off a main road into the woods, down a steep slope, to the edge of the brook. Strapped to a tree is a metal equipment box. He opens it up and starts loading it with big new batteries, a fresh soundcard and a tiny microphone.
This system and all the others like it in this valley will start recording at 5 a.m. on April 15.
“I’ve done things like this before, long-term monitoring where I’m orienteering around to put sensors in, but they were temperature recorders, mainly,” Ayres says as he programs the recorder with a tablet, “nothing that produces the volumes of data that a sound recorder does.”
Ayres says a major goal this year is to record the birds’ spring return from the tropics for the first time.
Spring is coming earlier in New Hampshire due to climate change — Hubbard Brook’s own data shows its winters have shortened by three weeks in the last century.
How these migratory birds react to that is a case study for how that shift affects the whole ecosystem.
“We’ve never been more interested in phenology — the timing of events — because that’s one of the most dramatic and evident changes associated with climate warming,” Ayres says.
Scientists can already learn a lot about that by analyzing their sound recordings with human and digital eyes and ears.
Crowdsourced artificial intelligence, in development at Cornell University’s Center for Conservation Bioacoustics, is getting better at picking apart the songbird chorus and isolating different species’ calls.
In the coming years, it might be able to count the individual birds in the audio like scientists would in person.
As technology improves, Ayres says there’s no telling how these recordings might benefit future Hubbard Brook research — the same way they’re still learning from the early acid rain samples that made this forest famous.
“It’s inspiring to all the researchers that work on this, knowing that you’re contributing to this data stream, which is going to have value that goes beyond what we can imagine now,” he says, “like being able to go back and re-analyze those samples of bottled water from the Hubbard Brook and validate that the Clean Air Act worked. How powerful is that?”
As the world works on other tools to address climate change, Ayres says Hubbard Brook hopes to continue to be useful — in ways scientists may not have even considered yet.