On a summer night just before sunset on the summit of Mount Mansfield, biologist Nathaniel Sharp and a crew of volunteers are getting ready to try to catch Bicknell’s thrush.
“If you even give, like, a poor impression of a whistle, sometimes you’ll get them to come and check you out,” Sharp said, playing a recording of the bird’s call — a sharp “Vreet!” that ends on a down note.
If you’ve spent time on a high peak in the Adirondacks or the summit of Mount Mansfield at dawn or dusk, it might sound familiar to you. So too might its song.
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“It’s just this beautiful … sort of cascade of whistles and buzzes and flute-y notes,” Sharp said. “If we’re lucky, we’ll hear that going off on the mountain later this evening.”
These dainty looking songbirds that fit in the palm of your hand travel thousands of miles every year to nest and raise their young in some of the most rugged places in New England.
But their populations are declining, and scientists like Sharp are trying to learn more about them in hopes they can reverse the trend.
Tonight, the crew — led by the Vermont Center for Ecostudies, where Sharp works — is hoping to find Bicknell’s thrushes wearing tiny backpacks outfitted with GPS trackers.
To catch the birds, they string up long nets along the maze of hiking trails around Mount Mansfield’s ridge.
After the sun goes down, they split up to hike around and find out what birds they caught. A chorus of birdsong starts to swell as the sun drops.
“Bicknell’s thrush — yeah — there’s nothing like hearing that up on the mountain,” Sharp said. “So hopefully we get lucky and hear some late-singing males tonight.”
It will be a good sign if the scientists do hear them, because these birds appear to be in trouble.
Researcher Chris Rimmer has been studying Bicknell’s thrush and leading this project for Vermont Center for Ecostudies for about 30 years.
“Bicknell’s thrush [populations] are declining at about 4% per year, which is a pretty steep rate of decline,” he said, taking a break from setting nets.
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The songbirds spend their lives flying between two kinds of islands — they spend summers on tall alpine summits in southern Quebec and New England like Mount Mansfield, and spend winters in the cloud forests of the Dominican Republic.
“What I hope is that 50 or 100 years from now, you're going to be able to stand here on Mount Mansfield and still be able to hear the swirling, ethereal song of Bicknell’s,” Rimmer said. “Is that realistic? I don't know.”
Rimmer says for reasons scientists don’t yet understand, there are a lot more male birds than females in the adult population. The theory is that the females aren’t surviving the winters, and that this imbalance is causing their populations to dwindle.
The goal with attaching these tiny backpacks to the birds is to find out where they're overwintering in the Dominican Republic, so the Vermont scientists can work with colleagues in the Caribbean to protect those forests from being cut down.
The scientists are also experimenting with even lighter trackers that could let them learn exactly where birds are spending their time, without having to recapture them to collect the data when they return to New England.
They hope to publish their findings in the coming years.
The thrushes also face some longer-term challenges here in New England.
“These montane forests, as we call them, the dominant tree here is the balsam fir, which is right above me here,” Rimmer said, gesturing to a wall of stubby trees along the side of the trail.
Bicknell’s thrush nest in balsam fir trees, he says, which are very sensitive to hot summer weather. Research has shown that for every degree Celsius of warming New England sees, the trees’ range could move up in elevation by about 500 feet.
“There's already evidence of this, empirical evidence, that the hardwood forests are beginning to encroach upslope,” he said.
This is a concern because mountains here in the Northeast are only so tall. According to Rimmer, two degrees Celsius of warming would just about eliminate the birds’ habitat across the region. And this is the only place in the world where they raise their young.
Back on the trails, the sun is setting. And in the gloaming, a volunteer gently removes a juvenile Bicknell’s thrush from a net — the first of the night.
“This little guy hatched out on the mountain four weeks ago or so,” he says, gently holding the bird’s feet between his fingers as it flaps its wings and ruffles its feathers. “We’ll put a band on it, and then, in early September, it’ll look up one night and decide it’s time to go down to the Caribbean.”
Right now, the researchers can only hope this little bird survives the test of its first winter. In the meantime, they’re racing to figure out exactly where it’s going, so scientists there can protect it.
