Decades-Long Songbird Mapping Reveals Changes in Hubbard Brook Forest Habitat

In the heart of central New Hampshire, behavioral ecologist Miranda Zammarelli navigates through the Hubbard Brook Experimental Forest. A tree adorned with a pink ribbon serves as a landmark for her, marking the corner of a 25-acre field site initially established in 1969 to chart the territories of songbirds inhabiting it.

Her task now is simply to roam, wait, and listen. On this June morning, the soft serenade of a black-throated blue warbler echoes from above, eliciting a murmur of appreciation from Zammarelli. She uses a mobile app to pinpoint the bird’s location based on its song.

“He is singing right now because this is his territory,” she explains. “So one bird sings, another responds. They’re essentially engaging in a territorial dispute – ‘This is my space, stay away.’ The other bird counters with, ‘No, this is my space, stay away.'”

A male bird’s territory is where he seeks a mate and raises his offspring, making the quality of his song paramount. Over the past month and a half, Zammarelli, along with others, has been meticulously mapping these territories for over two dozen songbird species. These birds play vital roles in their ecosystem, residing high up the food chain and dispersing seeds while consuming insects.

Many of them are migratory, traveling between New Hampshire and tropical destinations like the Caribbean or Central or South America. Understanding these territories, Zammarelli notes, offers insight into “the quality of the forest habitat for these birds.” The existence of territories suggests a healthy environment – one where questions can be raised if the opposite is observed.

The territory maps span decades, with earlier ones sketched by hand rather than digitalized. Dick Holmes, a biologist at Dartmouth College and the founder of this field site, unfolds a map from 1969. Now nearly 89 years old and a professor emeritus, Holmes started this project with a simple question: “How does energy flow through an ecosystem – in other words, how do songbirds fit into the food web of their forest?”

As data accumulated over the years, the questions evolved. “We became interested in what determines the number of birds and why are their numbers increasing or decreasing,” Holmes explains. One example is the disappearance of the least flycatcher from the plot in 1994 and the decline of the American redstart in the early 2000s.

The primary culprit appears to be forest aging and change, a process that has been ongoing since the forest was clearcut in the 1920s and left undisturbed ever since. “We’re observing how bird populations and bird communities change as a result of this natural growth,” Zammarelli adds.

This field site has hosted hundreds of students and produced over 200 scientific publications, with most relying on the paper maps until Zammarelli began her doctorate program and her advisor requested digitization. Holmes applauds Zammarelli’s efforts: “I had not even considered the idea of digitizing, frankly, but I was glad to see it done.”

The digitalized data has allowed for new insights and questions, such as “how these birds are using the space, where they are using the space, and how large their territories are in the habitat.” In a paper published in Ecology Letters last year, Zammarelli and her colleagues found that birds occupy space evenly, regardless of their number. This finding contributes to maintaining songbird populations over time.

“I think about all the hands these maps have passed through,” reflects Zammarelli. “This data is older than my lifetime, and I feel incredibly lucky and grateful to be part of a system that has been going on for over 50 years.” To Holmes’ knowledge, this long-term study is more important than ever in the face of current environmental change; however, he voices concern about potential future cuts to science funding.