On a mid-September day, scientists Chris Rimmer and Bill DeLuca drive to the top of the highest mountain in Vermont. They sling bags over their shoulders, lower themselves down a steep rocky path and hike through a balsam fir forest until they find a good spot. Then they pull out nets and old ski racing poles.

They untangle the nets and string them from pole to pole, stopping to point out a falcon gliding overhead. They work on the east slope of the mountain. The wind blows from the west.

As kids, DeLuca and Rimmer loved being outside. They grew up to become wildlife biologists. Rimmer is the executive director of the Vermont Center for Ecostudies, and DeLuca is an ecologist at the University of Massachusetts. They have dedicated themselves to saving a bird — a special songbird called the blackpoll warbler.

The blackpoll warbler is a fist-size bird with an unusually high-pitched song. The male has white cheeks and a black mark on its head like a snow hat; the female is mostly gray with darker cheeks and no head marking. But what makes the bird special is the remarkable way it flies south for the winter.

In the fall, millions of these songbirds travel from their mountaintop summer homes in Canada, Alaska and New England to the northeast coast of the United States to prepare for a dangerous journey. For weeks, they eat. They fill their bellies with caterpillars, spiders and beetles until their bodies can hold no more.

And one fall day, when the sky is clear and the wind is calm, they begin to fly. They fly east to catch the tail winds, then swoop south. The most athletic of the blackpolls fly for three days without stopping.

As they fly, their fat converts to energy. They follow stars in the night sky and light patterns in the day sky. Most migrating birds land often to eat and rest; the blackpoll keeps flying.

Recently, DeLuca and a team of scientists strapped dime-size backpacks called geolocators onto some of the birds to track their flight path. The birds, they learned, had flown from the U.S. East Coast all the way to Venezuela or Colombia. They had traveled as far as 2,100 miles without stopping — more than 13,000 miles round trip. It was one of the most incredible migrations on the planet.

Sadly, the blackpolls are in danger. Each year, there are fewer birds, and the scientists want to know why. It could be worsening storms, or tropical forests shrinking as trees are cut down, or other animals competing for the blackpolls’ food as the climate warms.

DeLuca and Rimmer work until dark placing nets and then sleep on the floor of a nearby hut. They return before sunrise and find birds in the nets: a Lincoln’s sparrow, a Tennessee warbler and dozens of blackpolls.

In a small forest clearing, Rimmer holds one of the blackpolls, cradling the bird’s head between his pointer and middle fingers. He gently blows the feathers aside, measures its wingspan and tail and weighs it. Then delicately, using pliers, he closes a tiny aluminum band around its leg. Each band has a nine-digit number for tracking. They need to understand what’s hurting them to protect them. Maybe they’ll meet the bird in Vermont next fall.

When Rimmer is done, he opens his hand, and the songbird flies farther up the mountain and lands on a branch, facing south. Clouds are gathering; rain is coming. This blackpoll’s long journey will begin soon but not today.