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For birdwatchers, even professional ones, catching sight of a Bicknell’s Thrush doesn’t come without torment. During migration it’s difficult to decipher from the Gray-cheeked Thrush, and during the breeding season one must hike—or drive—up into the remote, windswept mountaintop forests of the Northeast, preferably at dawn or dusk when the reclusive birds are most active. Battling past swarms of black flies adds to the struggle. “This is an incredibly challenging bird to study,” says Chris Rimmer, executive director of the Vermont Center for Ecostudies and one of the few experts on the Bicknell’s Thrush. “We jokingly refer to ourselves as brute-force biologists.”

Rimmer muses that he should have devoted his life to a more attainable bird, such as the House Finch or the American Robin. But he’s quick to add that he finds Bicknell’s Thrushes fascinating. The bird, which has a grayish-brown back, black spots on its breast, a white belly, and a melodic, flute-like song, isn’t much of a leaf peeper: It forgoes deciduous trees for the dense stands of balsam fir and spruce that emerge at roughly 3,000 feet above sea level in New York, Vermont, New Hampshire, Maine, and a small portion of Canada. These boreal “sky islands” are the thrush’s kingdom; no other bird breeds there exclusively.

I stepped foot on these “sky islands” in the summer of 2011, while hiking the northern third of the 2,185-mile-long . I made it my personal quest to see the thrush, keeping an eye out for it every time I found myself surrounded by sweet-smelling balsam firs. Over a month into my journey, however, having failed to locate a single Bicknell’s Thrush, I was feeling Rimmer’s pain.

Named after an amateur ornithologist who discovered it in the Catskills, the Bicknell’s Thrush was considered a subspecies of the closely related Gray-cheeked Thrush until 1995, a few years after Rimmer began studying it. “We had this bird essentially in our backyard,” Rimmer says, “and we knew almost nothing about its ecology, its population, or its conservation status.” It quickly became apparent that the Bicknell’s Thrush was exceedingly vulnerable to climate change. In fact, a recent study predicted that a rise in temperatures would wipe out more than half of the bird’s spruce-fir habitat in the United States, and maybe even eliminate it from all but the Northeast’s most massive peaks. Additional habitat might open up in Canada, Rimmer says, “but I wouldn’t just blithely assume that they’re going to move north.”

Like all neotropical migrants, Bicknell’s Thrushes are threatened by habitat destruction, as well as by invasive species; rats have been observed eating them on their wintering grounds in the Caribbean. To make matters worse, Rimmer has found elevated levels of mercury in every single Bicknell’s Thrush he has tested. He and his team also uncovered a highly unbalanced sex ratio in the species, with more than two adult males for every adult female. Rimmer’s theory is that bigger and more aggressive males are commanding the best habitat in the tropics and pushing females into the outskirts where they are more likely to perish. This dearth of females, Rimmer says, may have led to the thrush’s unusual system of breeding, in which both sexes take on multiple long-term partners. Among North American songbirds, only the Smith’s Longspur shares this equal-opportunity approach to promiscuity.

Already, the Bicknell’s Thrush has disappeared from Mount Greylock—the highest mountain in Massachusetts—and from portions of the Canadian Maritimes. “That’s often an early warning sign,” Rimmer explains, “when a species starts contracting at the edge of its range.” Yet he calls the Bicknell’s Thrush “resilient” and points out that, for the moment, its population “is not spiraling out of control into extinction.”

On Day 36 of my Appalachian trek, I broke camp a few miles away from Mount Washington in New Hampshire and passed through Pinkham Notch, before ascending what felt like an enormous helical staircase to the top of Wildcat Mountain. I reached the summit of the 4,832-foot Carter Dome in the late afternoon—my favorite time to hike since it felt like I had the mountains to myself. Then, a mile or so later, just below the tree line, I briefly trained my binoculars on a Bicknell’s Thrush as it flitted about from one stunted fir to another…before vanishing. I waited 10 minutes to see if it would return, but it never did.