In late June, the route to the 3,839-foot summit of Plateau Mountain remained closed following a late spring storm that dumped heavy snow and ice, leaving a trail of downed trees that devastated many trails in the Catskills. Steve Chorvas waited as long as he could, hoping for better conditions, but on a clear night after the summer solstice, he began his trek up the mountain. The 2-mile-long trail traversing the summit vanished under a mess of blown-down firs. Crawling by headlamp through the debris, under which half a foot of snow still lingered, Chorvas arrived at the eastern end of the ridge just in time for the predawn chorus.
Chorvas is one of roughly 80 volunteers who spend an early morning each June on mountains in eastern New York, Vermont, New Hampshire, and Maine, scouring the ridgelines and peaks of these spruce- and fir-covered “sky islands” at dawn, listening for the boreal birds that breed there. These community scientists are part of Mountain Birdwatch, a program started by the Vermont Institute of Natural Science (VINS) in 2000 and now directed by the Vermont Center for Ecostudies (VCE). For the past 25 years, Mountain Birdwatchers have recorded data for 11 focal species (including red squirrel, an important nest predator), information VCE uses to create its State of the Mountain Birds report each year.
Many of these monitored species are spruce-fir obligates: blackpoll warbler, Bicknell’s thrush, white-throated sparrow, boreal chickadee, yellow-bellied flycatcher, and fox sparrow. Outside of Maine’s expansive boreal forests, these species’ breeding ranges in the Northeast are largely confined to high-elevation spruce-fir stands in the Adirondacks, Green Mountains, and White Mountains – a patchwork of subalpine habitat that covers a small fraction of the region’s landscape yet supports a unique avian community distinct from lower elevations. Given their small, fragmented range, these bird species are considered to be of conservation concern in the Northeast. Most are boreal species at the southern periphery of their breeding ranges and may be indicative of larger population processes. Bicknell’s thrush is the most range-restricted Mountain Birdwatch species, with a breeding range confined to spruce-fir stands of the northeastern United States and southeastern Canada.
Given their remote, hard-to-access breeding habitat, these spruce-fir obligate species are inadequately monitored through traditional bird census programs that follow roadways, such as U.S. Geological Survey’s North American Breeding Bird Survey.
Other species included in Mountain Birdwatch surveys, such as Swainson’s thrush, also commonly inhabit forests occurring at lower elevations than the spruce-fir zone, as do hermit thrush and winter wren, which also breed in the boreal-hardwood transition zone and even down into the hardwoods. Roadside bird censuses wouldn’t capture the dynamics of how these populations may be shifting up or down the mountain. Last, the black-capped chickadee is an adaptable species that sometimes breeds as high as the spruce-fir, and may do so more often as the climate warms.
Mountain Birdwatch data are revealing a sobering picture for these high-elevation species. Black-capped chickadee is the only one of the 10 focal bird species monitored by Mountain Birdwatch that is clearly increasing in the spruce-fir zone. Alarmingly, the 2024 report revealed that most of the monitored bird species have declined since 2010, some of them sharply. Spruce-fir specialists account for most of the steepest declines, including white-throated sparrow (52 percent overall since 2010), Bicknell’s thrush (48.5 percent), and blackpoll warbler (48 percent). Yellow-bellied flycatcher declined more moderately (31 percent), except in the Catskills, the southernmost outpost of montane spruce-fir, where it dropped by 67 percent.
These trends are not wholly surprising. The spruce-fir ecosystem where these birds breed is largely protected from development, but quite vulnerable to the effects of climate change. Researchers expect New York and New England to lose more than half of their spruce-fir habitat during the next two centuries, with many plant and animal communities retreating poleward and upslope as the climate warms and becomes wetter in our region. Scientists predict that by the end of the century, the breeding ranges for most of these bird species will have shifted wholly into Canada and Alaska, where the vast majority of their populations already breed.
But climate shifts aren’t the whole story. Lower-elevation species one might expect to move upslope and become more common in the survey areas are declining, too. Swainson’s thrush numbers reported through Mountain Birdwatch have dropped by 27 percent since 2010, and hermit thrush showed the steepest decline of any focal species: more than 60 percent, and as much as 77 percent in Vermont. Scientists don’t know the reasons, but because most hermit thrushes breed at elevations below the spruce-fir zone in the hardwoods, the report could reflect a shift in distribution, although other data sources also suggest a shrinking population.
The global populations of most songbirds, after all, are declining. Various aspects of climate change, from extreme weather events to changes in the timing of insect emergence, can diminish bird populations, but so can habitat degradation on their wintering grounds. “All of these species are subjected to all the pressures that other migratory birds are encountering,” said VCE conservation biologist Jason Hill, who has coordinated Mountain Birdwatch since 2017. “But they have the added burden of occupying a relatively rare habitat on the planet.”
Deciphering these population trends and the various factors contributing to them would be impossible without Mountain Birdwatch data. Even eBird data from high-elevation areas are sparse, and when birders do contribute data, they often submit traveling checklists that start at the trailhead and include all birds encountered as hikers climb and descend the mountain. Because montane birds are extremely sensitive to elevation and forest type, those data are minimally useful from a scientific perspective except for broadscale presence-absence analyses.
Mountain Birdwatch lists 131 montane routes and last year relied on 78 volunteers to collect data, which, combined with data from Breeding Bird Survey, eBird, and other sources, gives a fuller picture of population health. These community scientists choose one fair-weather morning in June to rise in the dark and complete their counts.
“I see these folks [Mountain Birdwatchers] as ambassadors for our high-elevation ecosystems,” said Hill. Indeed, some Mountain Birdwatchers have participated since the project’s inception. They range from casual naturalists to scientists and serious birders, from college students to retirees. The only requirements are a love of hiking and willingness to learn to identify the 10 species of birds and red squirrel by sound.
In a former career, Mountain Birdwatcher Sean Lawson studied another issue alpine birds face: high-elevation mercury deposition from cloud water. VCE scientists documented that Bicknell’s thrush and other alpine songbirds carry relatively high methylmercury loads. Lawson, now at the helm of Vermont brewery Lawson’s Finest Liquids, picks fresh balsam tips each spring to brew a beer in honor of the rare thrush, with proceeds going to VCE.
Although Hill encourages surveyors to bring a companion, Lawson and many others choose to hike and complete their counts alone. The night before his survey, Lawson camps near a little cabin and pond in Vermont’s Breadloaf Wilderness, where he listens to the birds at sunset and plays the flute. “For me, it’s kind of an annual ritual around solstice,” he said. “My heart is attached to that area.”
Many Mountain Birdwatchers became involved not solely because of their enthusiasm for birds, but because they enjoy spending time in the mountains and value the montane ecosystems. Nancy Eaton was an alpine steward on Franconia Ridge in New Hampshire’s White Mountains when she heard about Mountain Birdwatch. Her birding knowledge at that time was mainly limited to robins and blue jays. Now, she records all the birds she hears or sees, not just the focal species.
Another surveyor, Ellen Cronon, joined the Mountain Birdwatch effort when she saw an ad in 2001 for “hikers who wanted to bird.” She learned the songs of the target species from cassette training tapes from VINS. Ahead of the count, volunteers review the protocol, attend a Q&A Zoom session with Hill, and listen to audio tracks to learn vocalizations and compare songs that could be confused. “Most, I found, were distinctive enough that I didn’t have much trouble with it,” Cronon said.
Many Mountain Birdwatchers remember their earliest encounters with Bicknell’s thrush, a species that appears to be in steady decline throughout the region – and especially so in the Catskills, where Mountain Birdwatch data shows the species vanishing faster than elsewhere: by 65 percent since 2010. Chorvas, who has hiked there for decades, recalls that in the higher elevations, Bicknell’s outnumbered Swainson’s thrushes in the early years. “Then I could see, with each passing year, there were more Swainson’s and fewer Bicknell’s,” he said, “to the point where now Swainson’s is by far the most common species of thrush at the summits of these peaks.”
Bicknell’s thrush has been retracting toward the center of its range, vacating more marginal areas, since the species disappeared from Mount Greylock, Massachusetts’ highest peak (elevation 3,491 feet), in the 1970s. Just in the past decade, some coastal populations in Canadian Maritime forests have disappeared.
“That’s an indication of a species on the decline,” said Hill. Competition with Swainson’s thrush could be one factor, but that evidence is circumspect and the reasons why likely have more to do with challenges and human pressures on the forests of their wintering grounds – predominantly the Dominican Republic – than their breeding grounds in the Northeast. These boreal species, after all, spend barely a third of the year on their northern breeding grounds. Most of them migrate to wintering grounds in the tropics, which are far less protected. Hill determined that about 77 percent of Bicknell’s thrush breeding grounds in the United States, harboring 85 percent of the population, is already nominally protected as national forest or state conservation lands. Still, ski resort expansion and energy developments occasionally threaten the habitat of Bicknell’s thrush and other forest birds, so Mountain Birdwatch data is crucial for informing management actions. It has revealed the forest structure that Bicknell’s needs and led to the conservation of prime habitat around Camel’s Hump in Vermont and Crocker Mountain in Maine.
Bicknell’s thrushes are attracted to disturbed areas, including along the edges of fir waves and ski slopes, as well as in actively logged, lower-elevation forests in Maine and Quebec. Despite favorable habitat in Canada, however, Hill said the species doesn’t seem to be shifting north to fill it. In 2016, when the U.S. Fish & Wildlife Service was considering listing the thrush as endangered, Hill said VCE estimated the species’ U.S. population at just 70,000. Ultimately, the species was not added to the Endangered Species List because it is not in immediate danger of extinction, despite the longer-term pattern Hill predicts: “At the end of the century, if Bicknell’s is still here, they might be remnant populations on the tops of only our tallest mountains.”
The same goes for the other declining boreal species – yellowbellied flycatcher, blackpoll warbler, and white-throated sparrow – which already show signs of vacating the Northeast. Like Bicknell’s thrush, they are experiencing the steepest declines at the southern periphery of their range in the Catskills. White-throated sparrows, whose melancholy whistles (“Oh, sweet Canada-Canada-Canada”) are often associated with northern forests, have declined by 52 percent since 2010, but by a whopping 93 percent in the Catskills. They have also gradually moved upslope. Hill said that while these species’ populations will still be globally secure if they disappear from the Northeast, the overall northward shift means their populations will probably be smaller, which carries risks such as diminished genetic diversity.
The trend most noticeable to veteran Mountain Birdwatchers is that, over the years, lower-elevation birds have moved upslope, increasing competition for limited food and breeding territory. Chorvas and Lawson have both noted that warblers – black-throated blue, black-throated green, and Blackburnian – which they typically find in lower hardwood and mixed forests, are now common around the spruce-fir summits. Both surveyors have been shocked to see robins and blue jays on high mountain summits, which even a few years ago seemed unlikely. And Hill said VCE bird-banders now “routinely capture robins on top of Mansfield during the breeding season.”
Many scientists think climate change will bring about a homogenization of avian communities, with common birds becoming more common, and rare birds even more rare, Hill said. Generalists, including black-capped chickadees, one of the few species monitored that has steadily increased in spruce-fir zones, have become regular around the parking lots and roads at the top of Mount Mansfield (elevation 4,395 feet). Within these general trends, however, Hill expects there will be surprises. He found that blackpoll warblers have moved upslope over the years, but not uniformly, suggesting that local factors mediate the movement of birds in response to climate change. The patterns might be visible to us in 30 years, Hill said, but right now, “We’re at the infancy of understanding how species are capable of responding to climate change.” Some, he said, may even move downslope to avoid competition with other range-expanding species, or because a new food source might become more abundant at a lower elevation. “So, it’s definitely not a uniform response,” Hill said. “Species will evolve and come up with novel mechanisms to survive. The risk is that change might happen so quickly that they don’t have time to adapt before disappearing.”
Mountain Birdwatch has also evolved over the years to best gather this crucial data, which is also shared with eBird and is freely available from VCE’s website for anyone to use. When Hill took over management of the program in 2017, he simplified the protocol in response to feedback from participants. Some routes have changed, too. Prior to 2010, survey locations were limited to prime spruce-fir habitat, said Hill, but the best places are where you’d last expect to see declines. Since 2010, Mountain Birdwatch also samples at the ecotone between hardwoods and spruce-fir forest types as well; these shoulder areas might be where you’d expect change in the avian community to happen most rapidly.
Twenty-five years into the Mountain Birdwatch program, volunteers continue trudging into the balsam-infused air in the predawn hours, and they remain dedicated to the work – and to the special experience of sitting on a mountaintop at daybreak, listening for the birds. That experience includes unexpected encounters. One morning, Mountain Birdwatcher Jessica Talbot Halm was surveying on New Hampshire’s South Twin Mountain. It was still a bit dark when a bird flew in and landed about 8 feet away from where she sat – a songbird, she assumed. Seeing a bird is always exciting because in dense forest, surveyors identify birds almost entirely by ear. But Halm soon realized her bird companion was a tiny saw-whet owl, close enough that she could look in its eyes.
“It’s special to be on the trail when nobody else is there,” said Halm, who has covered two Mountain Birdwatch routes in the White Mountains for several years and sees the program as a happy excuse to spend time in the mountains. “I need to be in good enough shape that I can hike up this mountain either really early, or spend the night in the wilderness. It’s really lovely,” she said. “There’s something special about returning to the same place. I can picture every little spot. I know those spots by heart.”
Learn more about Mountain Birdwatch here.
The Hazards of Migration
Most of the 10 focal species monitored by Mountain Birdwatch spend only a few months on their northern breeding grounds. Winter wren, fox sparrow, and white-throated sparrow are short-distance migrants that winter in the continental United States. The sparrows, especially the white-throated, commonly visit winter birdfeeders in the Northeast. Hermit thrush has a similar wintering range in the United States, but it extends down through Mexico.
The remaining species are neotropical migrants, meaning they spend much of the year on wintering grounds in Mexico and Central and South America. Most Bicknell’s thrushes overwinter in the mountain forests of the Dominican Republic, with smaller numbers in Haiti, Cuba, Jamaica, and Puerto Rico – a wintering range even more restricted than the species’ breeding range. Yellow-bellied flycatchers migrate to wet tropical forests across southeastern Mexico and Central America. Most adults stay on their boreal breeding grounds for barely 70 days.
Blackpoll warblers breed the farthest north of any New World warbler, but also fly the farthest south, a migration that, round-trip, may span 12,000 miles. Most fly east before launching from the coast of New England, traveling nearly 2,000 miles straight over the Atlantic Ocean to tropical forests in northern South America. Swainson’s thrushes also overwinter in South America, down along the spine of the Andes. To get there, they migrate south along the East Coast before crossing the Gulf of Mexico. Along the way, they seem to be frequent victims of collisions with structures.
During the long-distance migrations from breeding ground to wintering areas, these birds face many perils, including:
- Degradation or loss of their stopover sites, where migrants pause to refuel along their journey, often lingering for a week or more to replenish their fat stores. A 2020 study by scientists at Carleton University found that nearly half of these sites occur within human-dominated landscapes, and only a fraction of these areas are protected.
- Storms, droughts, or heat waves that may cause mortality directly, disorient birds, or diminish their food supplies at stopover sites. Climate change is causing these weather events to be more extreme.
- Outdoor cats, which researchers estimate kill 2.6 billion birds per year in North America, the largest direct source of human-caused mortality.
- Light pollution, a growing problem that attracts and disorients migrating birds, often causing them to collide with structures such as skyscrapers. Cities are sometimes the site of collision events with appalling numbers of avian casualties. Wind turbines, communications towers, and vehicles all pose collision threats, but none so much as glass windows, which researchers estimate kill 624 million birds each year in North America.
- Deforestation or degradation of their wintering habitat.