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A Talk on the Wild Side.

Vermont: Climate Change Poses Challenges for the Bicknell’s Thrush

Bicknell's thrush perched in an evergreen

Bicknell’s thrush nest in mountain-top evergreen forests in Vermont, areas that are shrinking due to global warming. Photo: © T.B. Ryder

Watch a video of a Bicknell’s thrush feeding her young.

Bicknell’s thrush has one of the most restricted breeding ranges of any North American bird, nesting primarily in stunted spruce-fir forests found at or near the highest elevations of mountains in Vermont and other New England states. These mountain tops are like a chain of islands separated by a sea of habitat that is unsuitable for this species.

As the climate warms and precipitation patterns change, deciduous trees – those that shed their leaves in the fall – are likely to become more prevalent in higher elevations, shrinking the size of the mountain-top evergreen conifer forests that are home to the Bicknell's thrush.

This is just one of many challenges that climate change poses for the rare bird.

Another potential threat is a mismatch between the arrival time in spring of Bicknell’s thrush and other birds, which is regulated by day length, and the abundance of insect prey, linked to temperature. If the peak food supply for birds comes earlier due to warmer spring temperatures, late arriving birds may lay fewer eggs and produce offspring that have less chance of reaching adulthood.

The red squirrel, which also lives in the mountain-top forests, also presents a danger. The squirrels feed mainly on spruce and fir cones, but will also raid the nests of Bicknell’s thrush to feed on eggs and young birds.

The Bicknell's thrush is a medium-sized thrush with a brownish-gray upper body and pointed beak
The Bicknell's thrush is among the rarest of eastern North America's songbirds. Climate change threatens the viability of both its wintering and breeding areas. Photo: Steve Faccio, courtesy of the Vermont Center for Ecostudies 

The red squirrel population is cyclical and tied to the availability of cones. Conifer trees typically produce robust cone crops every other year, and the red squirrel population rises and falls accordingly. Some predict that climate change will result in more frequent cone crops in balsam fir, even as conifer forests diminish due to climate change. That could mean an increase in the red squirrel population and a subsequent increase in the threat to Bicknell’s thrush. The U.S. Fish and Wildlife Service’s Northeast Region Division of Migratory Birds is working with the Vermont Center for Ecostudies (VCE) to study the Bicknell’s thrush and assess the effect of climate change on the birds. One effort is the Mountain Birdwatch monitoring program, which tracks population trends of Bicknell’s thrush and other species and their responses to climate change and other threats. The program coordinates 150 volunteers who conduct dawn bird surveys along foot trails that pass through some of the region’s most rugged and remote forests.

Bicknell's thrush perched in an evergreen

Biologists are studying the effect of climate change on the Bicknell’s thrush. Photo: Steve Faccio, courtesy of the Vermont Center for Ecostudies

In addition to the monitoring program, the VCE and the Service are working with other partners in the International Bicknell’s Thrush Conservation Group to develop a research program that would test the adaptability of Bicknell’s thrush to predicted ecological changes. How resilient is the species to climate change and other threats? The results will help guide future efforts to protect the Bicknell’s thrush.

Climate Change Focus:Engagement

Authors: Debra Reyonlds and Frank Wolff


  • Randy Dettmers, USFWS, 413-253-8567, Randy_Dettmers@fws.gov
  • Terri Edwards, USFWS, 413-253-8324 Terri_Edwards@fws.gov

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