By listening to these birds for decades, scientists track signs of a changing forest

Miranda Zammarelli hikes into the Hubbard Brook Experimental Forest in central New Hampshire, pausing when she sees a tree with a pink ribbon wrapped around its trunk.

Zammarelli, a behavioral ecologist and PhD candidate at Dartmouth College, knows that ribbon well. It marks the corner of a 25-acre field site first established in 1969 to map the territories of the songbirds inhabiting it.

Her task now is simply to wander, wait and listen.

On this early morning in June, it doesn’t take long before a soft melody wafts down from the treetops. “That’s a black-throated blue warbler,” murmurs Zammarelli with a kind of reverence.

She uses a mapping app on her phone to input the location of the bird based on where she’s just heard him. The warbler’s small enough to fit in the palm of her hand, but this is breeding season and he’s got something big to say.

“He is singing right now because this is his territory,” she says. “So one bird sings, another bird responds. They’re basically having what I call a territorial rap battle. ‘Hey, this is my space, stay away.’ And the other bird’s like, ‘No, this is my space, stay away.’ And they’re singing back and forth trying to claim their spot.”

A male’s territory is where he’ll attract his mate and raise his young, so there’s a lot riding on his song.

Zammarelli and others have been mapping these territories for the last month and a half, documenting where some two dozen species of songbirds are located. These birds are high up in the food web, and they help disperse seeds and eat insects. Many of them are migratory, traveling between here and the Caribbean or Central or South America.

It’s important to understand these territories, says Zamarelli, because they offer a window into “the quality of the forest habitat for these birds.” Territories mean the forest is a healthy place for birds. “If birds are not using the habitat or less likely to have a territory there,” she says, “we can begin asking why.”

The territory maps go back decades. Before the cell phone app, they were all sketched by hand.

Seeing the forest for the birds

Dick Holmes gridded out and launched this field site back when he was just starting out as a biologist at Dartmouth. Today, he’s almost 89, and a professor emeritus.

“We have maps for every species for each of the 57 years,” he says, unfolding one of them. “So all the observations, you add ’em up from day to day, and you end up with a seasonal pattern showing all the different territories on the plot.”

The maps are swarms of X’s, arrows, dotted lines, and circles, the latter of which delineate the territories.

In the beginning, Holmes had a fairly simple question. He wanted to know how energy flows through an ecosystem — that is, how songbirds fit into the food web of their forest.

“How much energy do they use?” he says. “Where do they get their food?”

As Holmes and his team gathered more information on the birds year after year, their questions evolved. “We got interested in what determines the number of birds and why are their numbers increasing or decreasing,” he says.

For instance, the researchers heard a bird called the least flycatcher on the plot for the last time in 1994. And the American redstart — a jetblack little fellow with orange splashes — pretty much stopped breeding here in the early 2000s.

The primary reason appears to be that the forest — which had been clearcut in the 1920s and then left to regrow, undisturbed — has been aging and changing ever since. “It’s naturally growing, and we’re seeing how the bird populations and bird communities change as a result of this natural growth,” says Zammarelli.

That is, the ups and downs of these little songbirds have revealed how the forest around them has grown up. It’s a result that matches work done elsewhere, including a classic 1950 study from the southeastern U.S.

“There’s some species that seem to do well in a young forest,” says Holmes. “And as that forest ages, those changes cause the birds to move elsewhere.”

This field site has hosted hundreds of students and yielded over 200 scientific publications, all of which depended on poring over those paper maps — until Zammarelli began her PhD a few years back and her advisor, Matthew Ayres, asked if she knew anyone who could digitize them.

“I was like, ‘I can do it,'” she recalls responding. “I had digitized collections before. That would be no problem at all.”

Holmes applauds Zammarelli’s efforts. “I had not even thought of the idea of digitizing, frankly, but I was glad to see it done,” he says. “Miranda’s done a bang-up job.”

A solid past, a flighty future

Getting the 50-plus years of maps into the computer enabled Zammarelli to bring the data to life in a new way and ask new questions. “That allowed us to look at how these birds are using the space, where are they using the space, and also how big of a territory they’re occupying in the habitat,” she says.

In a paper published in Ecology Letters late last year, Zammarelli and her colleagues considered seven species and found that birds, no matter the number, divide the habitat up equally. That means lots of chicks, which help keep songbird populations stable over time.

It’s a finding made possible by decades of mapmaking. “I think about all the hands these maps touched,” reflects Zammarelli. “This data’s like twice my lifetime. I feel very lucky and very grateful to be a part of a system that has been going on for over 50 years.”

To his knowledge, Holmes says this is the longest detailed record of breeding birds anywhere in the U.S., possibly the world.

“It’s important in terms of understanding what we’re doing to our environment,” he says. “Things are changing rapidly for various reasons … climate change being one. And trying to understand why these are happening, and how they’re happening is quite important.”

But he says the project now finds itself in a precarious place.

“Now, with the budgets being cut by the current administration, it’s hard to tell what’s gonna happen in the next few years,” says Holmes.

But at least for the next few minutes, Holmes and Zammarelli know exactly what will happen. The birds will continue to fill the air with melody, singing the collective song of their forest.