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

"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."

""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."

A behavioral ecologist maps songbird territories within a 25-acre field site in Hubbard Brook Experimental Forest using ribbon markers and a phone mapping app. Observers wander, wait and listen to record about two dozen species, locating individuals by their songs. Male warblers sing during breeding season to defend territories, exchanging songs in back-and-forth interactions that establish ownership. Territories indicate places suitable for attracting mates and raising young. Songbirds occupy high trophic positions, disperse seeds, consume insects, and many migrate long distances, so territory use reflects forest habitat quality.

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