Bicknell's thrush. Credit: Steve Faccio, Vermont Center for Ecostudies
Bicknell's thrush. Credit: Steve Faccio, Vermont Center for Ecostudies

Bicknell's Thrush

Shrinking Islands and Red Squirrel Population Explosion: Impact to Bicknell’s Thrush

Climate change is likely to alter population size and distribution patterns of migratory birds and other wildlife species, as they respond to environmental and related changes to their habitats. While the primary climate-related threat to species may be habitat loss or decreasing habitat quality and availability, just as serious may be increases in populations of nest predators or a disconnect from their primary food sources, which could have rapid and direct negative effects on many species.

Bicknell’s thrush has one of the most restricted breeding and wintering ranges of any North American bird, nesting primarily in stunted montane spruce-fir forests, found at or near the highest elevations of mountains in New England and New York. These mountain tops are like a series of “islands” separated by a “sea” of habitat that is unsuitable for this species. As the climate warms, deciduous tree species will “migrate” upwards in elevation, shrinking the size of the mountain-top “islands” of conifer forests in much the same way that sea level rise would shrink the size of oceanic islands. However, this is not the only threat that climate change may bring to this rare species.

Red squirrels are common residents of montane conifer forests. Feeding mainly on spruce and fir cones, their populations rise and fall in synch with the availability of cones. Across much of New England, and especially at high elevations, red squirrel populations are cyclical, soaring upwards and slumping to very low levels on a two-year cycle, because conifer trees typically have robust cone crops every other year. In years with a good cone crop, red squirrels successfully and rapidly reproduce, and their populations can explode. However, trees typically produce few cones in the following year. So as red squirrel populations grow, they face a rapidly decreasing food supply.

This up-and-down fluctuation in red squirrel populations directly affects the breeding success of Bicknell’s thrush during their short nesting season, because red squirrels – more than any other species – raid their nests to prey on eggs and young birds. Bicknell’s thrush populations are intimately linked to red squirrel population fluctuations and would face a grave threat if climate change results in more frequent cone crops in balsam fir (as some have predicted). Such changes could mean more abundant red squirrels in more years, and fewer chances for thrushes and other birds to experience years with high reproduction and population growth.

Bicknell's thrush with chicks. Credit: Steve Faccio, Vermont Center for Ecostudies
Bicknell's thrush with chicks. Credit: Steve Faccio, Vermont Center for Ecostudies

Another potential threat posed by climate change to Bicknell’s thrush and other birds is a mismatch between their arrival time, which is regulated by day length, and the abundance of insect prey, which is linked to temperature. If the peak food supply for birds occurs earlier, due to warmer springs, late-arriving birds may experience lower reproductive success, laying fewer eggs, and having babies that grow more slowly or are smaller and less likely to survive to adulthood.

Migratory bird biologist, Mitch Hartley, of the Atlantic Coast Joint Venture, has been studying the link between red squirrel and bird populations for over ten years. He is working with scientists at the Vermont Center for Ecostudies (VCE) to investigate the relationship between Bicknell’s thrush, cone crops, and red squirrels.

Migratory bird biologist, Randy Dettmers, with the U.S. Fish and Wildlife Service’s Division of Migratory Birds, has also been collaborating with biologists at the VCE to develop and implement the Mountain Birdwatch monitoring program. This program tracks population trends of Bicknell’s thrush (and other species) and their responses to various threats, especially climate change. The Mountain Birdwatch program coordinates 150 volunteers to conduct dawn bird surveys along foot trails that pass through some of the region’s most rugged and remote forests. In addition to collaborating on the development and implementation of this program, both Randy and Mitch have actively participated in these surveys since the inception of Mountain Birdwatch in 2000, often conducting surveys on two or three mountains each summer.

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 an associated research program that would test the adaptability of Bicknell’s thrush to predicted ecological changes due to climate change.

Understanding the effects of climate change on a given species will require ecologists to consider all aspects of the species’ life history, including its interactions with other species that may be affected by climate change. The importance of monitoring will be greater than ever, and is crucial for scientists to understand the reasons behind medium- and long-term population changes, given natural variation and typical fluctuations in the short-term.

Photo in slideshow credit to: Steve Faccio, Vermont Center for Ecostudies


Check out this video of a Bicknell's thrush feeding its chicks!

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bicknell's thrush

Bicknell's thrush. Credit: T.B. Ryder

Global Warming Changes Habitats, Threatens Survival of Rare Songbird

The Bicknell’s thrush is among the rarest of eastern North America’s songbirds. With a total population estimated at fewer than 100,000 birds, the species is considered “globally vulnerable.” Climate change threatens the viability of both its wintering and breeding areas.

First collected by Eugene Bicknell in 1881 in the spruce-fir forests of the Catskill Mountains in New York, the bird was originally thought to be a subspecies of the gray-cheeked thrush. After more than a century, the American Ornithological Society officially recognized the species in 1995 with the scientific name Catharus bicknelli. This designation helped spur the momentum of research and monitoring that have led to our current understanding of the species’ fascinating life history and complex conservation status.

At this time of year, an estimated 90 percent of the global Bicknell’s thrush population is concentrated on Hispaniola, a Caribbean island made up of the nations of the Dominican Republic and Haiti. The birds’ wintering habits were largely a mystery until 1994, when Chris Rimmer of the Vermont Center for Ecostudies (VCE) confirmed the presence of Bicknell’s thrush in high elevation broadleaf forests of Sierra de Bahoruco, during an exploratory trip to the Dominican Republic. A subsequent three-year study and other surveys of the species’ distribution and habitat use confirmed that Hispaniola is the core wintering area for the Bicknell’s thrush.

Climate change models project habitat changes in the thrush’s breeding and wintering areas that pose a significant threat to the species’ already precarious existence. The loss of forests, a shift in the seasonal timing of food availability, and increased extreme weather events resulting from global climate change may adversely affect the Bicknell’s thrush’s prospects for survival.

The Bicknell’s thrush relies on balsam fir-dominated forests in the northeastern U.S. and southeastern Canada to reproduce. The species is at risk from a variety of threats to its breeding habitats, including recreational development, telecommunication construction, wind power development; acidic precipitation, mercury deposition, and climate change. 

In addition to these habitat-related threats, direct mortality may limit Bicknell’s thrush populations. Catastrophic weather events, introduced predators on the wintering grounds, migration hazards, disease and parasites, and incidental take during forestry operations may contribute to the species’ mortality. However, knowledge is fragmentary, and further investigation is required to understand the magnitude and possible interaction of these threats.

While these and other potential threats need further study, VCE has come up with an innovative approach to conserve habitat for Bicknell’s thrush on its wintering grounds. VCE and other organizations will implement on-the-ground actions to both improve local economies and enhance or restore forested habitats through a payment for ecosystems services conservation approach (learn more about this innovative project in the VCE Field Notes). This strategy provides performance-based incentives to financially compensate landowners for achieving specific environmental outcomes. In areas most important to the Bicknell’s thrush, VCE and its partners will establish a landowner incentive program through sustainable agro-forestry production, forestry carbon credits (offsets), and water markets.

VCE and its network of partners are forging new ground with this project. There are no guarantees of success, especially against a backdrop of chronic ecological and socio-economic issues, but the energy and optimism of local collaborators will be a crucial ingredient to achieve lasting conservation.

For further information on this topic, please contact...

Randy Dettmers, Biologist, Division of Migratory Birds, 413/253-8567


To learn more about Bicknell's thrush, click on any of the links below.

Vermont Center for Ecostudies

Hispanolia Conservation

Keeping an eye on the Bicknell’s thrush in Sierra de Bahoruco (blog)

Bicknell's Thrush Images

Bicknell's Thrush Hispanola Images

Bicknell's Thrush Video

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