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