Bicknell's Thrush

Catharus bicknelli

Image by Gregory Gough, US Geological Survey
  • TURDINAE
  • Thrushes
  • Passeriformes
  • Zorzal de Bicknell
  • Grive de Bicknell
Introduction

Ranked as the Neotropical migrant of highest conservation priority in the Northeast U.S., the Bicknell's Thrush is among the least-known breeding birds in North America. Long considered a subspecies of the Gray-cheeked Thrush, it was afforded its own species distinction in 1995. Restricted to high elevation forests, this cousin of the American Robin shuttles between Caribbean and northeastern mountains to breed.

Appearance Description
The Bicknell's Thrush is a medium-sized songbird measuring about 6.75 inches long with an 11.5 inch wingspan and weighing 1.1 ounces. Its breast is spotted, and its olive-brown back contrasts with the hint of chestnut on its tail and wings. The lower mandible is bright yellowish orange and the legs are flesh-colored. Males and females look alike. Bicknell's and Gray-cheeked Thrushes are difficult to separate in the field. The song of the Bicknell's distinguishes it best: four phrases having a constant or slightly rising inflection at the end. In contrast, Gray-cheeked Thrush song drops in frequency at the end. Bicknell's differs from other spotted eastern thrushes as follows: Hermit has an all-rufous tail which it raises slowly when alarmed; Swainson's has cold olive-brown upperparts and a buffy face with buff-colored "spectacles"; Veery is uniformly reddish brown above and less spotted below.
Range Map
Courtsey Kenn Kaufman
Range Distribution
Bicknell's Thrush breeds in northeastern North America from the northern Gulf of the St. Lawrence and easternmost Nova Scotia southwest to the Catskills of New York, from about 3,000 to 4,300 feet. Within this range, it occupies only small, scattered patches in Vermont's Green Mountains, New Hampshire's White Mountains, New York's Catskills and Adirondacks, and Maine's western and central mountains. Most U.S. breeding sites are within publicly owned areas. The wintering grounds are largely confined to the mountains of Hispaniola (Dominican Republic and Haiti), Jamaica, Puerto Rico, and Cuba. Its close relative, the Gray-cheeked Thrush, breeds farther north and winters farther south.
Habitat
This thrush breeds most often in dense stands of balsam fir regrowing from damage by wind, ice and snow, fire, and insect outbreaks. Known as the krummholz, or "crooked woods," this forest usually occurs on steep slopes above 3,300 feet in the Northeast and is moderately wet to wet. Some spruce, white birch, mountain ash, and other hardwoods may be present. More recently, Bicknell's Thrush has taken advantage of areas disturbed by timber harvesting, ski trail and road construction, and other human activities. The migration routes appear to cross a variety of wooded habitats, and its wintering grounds are moist broadleaf forests in Caribbean mountains.
Feeding
The Bicknell's Thrush captures insects with a variety of strategies: snatching them from the air, scratching the ground or plant litter to find them, hovering near plant surfaces to pluck them, and hopping about dense foliage and inspecting all surfaces. Only the diet during the breeding period has been studied well. Young are often fed larvae or small insects. Many beetles are consumed: leaf, click or snapping, false longhorn, and soldier beetles. Ants, bees, aphids, flies, crane flies, snails, spiders, daddy longlegs, lace-winged flies, and caddisflies are also taken. Migrating thrushes eat many fruits, which may also be important on the wintering grounds.
Reproduction
The mating system of the Bicknell's Thrush is extraordinary. Pair bonds are poorly understood, and both sexes are likely to have more than one mate, so that single nests are very likely to have eggs of different paternity. In mid-May, males arrive for breeding but do not defend traditional territories. Females arrive about a week later. Courtship includes a chase flight, in which the male raises his crest and holds open his bill, while singing behind the female. Apparently, the female picks a nest site. At the base of a horizontal branch next to a tree trunk, she constructs a cup-shaped nest of twigs and moss, padded with rotting vegetation and lined with fine plant materials like horsehair fungus.

For 12-14 days, the female incubates 3-4 blue-green eggs, spotted with brown. The hatchlings are naked, blind, and helpless. The female broods them, but at least one of the males, usually two or more, helps bring food. A male will bring food to nests with or without "his" eggs. After 9-13 days, the young leave the nest, and a parent commonly takes one part of the brood to raise by itself, for up to another 2 weeks.
Migration
Migrating at night, it leaves the breeding grounds in September and appears to move quickly to the Northeast Coast of the U.S. (Massachusetts to Virginia), from where it probably flies to the Greater Antilles in October. Spring migrants have never been observed before May and appear to follow a coastal route east of the Appalachian Mountains. Although a few spring records come from the southeast, most range between Maryland and Massachusetts. Migrants do not appear to stop and rest for long.
  • 40,000
  • 40,000
Population Status Trends
This songbird is a challenge to count because its summer and winter habitats are difficult for people to access and because of its complex reproductive system. Recent, local population studies indicate declines in some regions and stable numbers in a few regions. Between 2001 and 2004, a statistically significant decline of -9% average per year was recorded for 47 Breeding Bird Survey routes in Vermont. An analysis of several surveys in Vermont and New Hampshire between 1991 and 2003 indicated a range wide decline of -7% . Nowhere does the Bicknell's Thrush appear to be increasing. It has probably disappeared from western Massachusetts and the southern coasts of New Brunswick and Nova Scotia.
Conservation Issues
Degradation of habitat throughout its range has been the biggest factor in the decline of Bicknell's Thrush. Since the 1960's, balsam fir and red spruce forests have been dying, apparently due to acid rainfall and, perhaps increasingly, because of global climate change. Passage of the Clean Air Act Amendments in 1990 helped decrease the amount of sulfate deposition, but it has not resulted in less acidic deposition. With the anticipated 1.6-6.3% increase in surface temperature associated with global warming, a study by the USDA shows that the area covered by balsam fir forests in the eastern United States may be reduced by 96%. Atmospheric deposition of heavy metals, released from coal-burning power plants, is also implicated in forest losses. Substantial levels of mercury have been found in the blood and feathers of Bicknell's Thrush. On the wintering grounds, montane forests have been cut by over 90% in the Dominican Republic and 98.5% in Haiti, where human settlements are expected to double over the next 10-20 years.

Within its breeding range, all North American states, provinces, and federal governments consider it a Species of Concern. This thrush is considered Vulnerable by Nova Scotia and Birdlife International/IUCN. As a species of highest continental concern and as a stewardship species, it was placed on the Partners in Flight 2004 Watchlist.

Since 2003, several Conservation Strategies and Plans have been developed and implemented for Bicknell's Thrush in North America. Conservation efforts and research are difficult to conduct for this species under natural conditions, but power line and radio tower cuts, commercial ski activities, and wind power stations may provide access and significant opportunities for habitat management. Following the suggestions of the Bicknell's Thrush Conservation Strategy for the Green Mountain National Forest, a partnership of public and private conservation groups, ski companies, and concerned individuals founded the Bicknell's Thrush Habitat Protection Fund, which supports conservation in the Dominican Republic and Haiti. At the same time, the Adirondack Park Agency adopted management plans for the Whiteface Mountain Ski Area, which respect Bicknell's Thrush, support economic development, and allow for more research.
What You Can Do
Beat the summer heat and look for the Bicknell's Thrush in the balsam fir forests of our great northeastern mountains. These birds can be very difficult to find and their habitat is almost always treacherous, so a local guide or bird club trip is your best and safest bet.

Several Important Bird Areas support this thrush, including the Adirondack High Peaks and Catskill Peaks in New York. Audubon's Important Bird Area program is an essential tool for the conservation of Bicknell's Thrush as well as other species. Explore IBA's in New York, Vermont, New Hampshire, and Maine, and learn how you can support them.

Become a volunteer for Mountain Birdwatch!  The Vermont Institute of Natural Science launched a long-term monitoring program for Bicknell's Thrush and other northeastern montane forest birds in 2000. Volunteers adopt a mountain to survey in June and are trained to identify common montane forest birds. 

Find out what you can do including Audubon programs and activities.
More Information
Learn more about the Bicknell's Thrush Habitat Protection Fund  through the Adirondack Council. Audubon was a seminal contributor to this fund and participates in the management of the Bicknell's Thrush in New York.

Learn more about this species and other birds through these resources.
Natural History References
Rimmer, C. C., K. P. McFarland, W. G. Ellison, and J. E. Goetz. 2001. Bicknell's Thrush (Catharus bicknelli). In The Birds of North America, No. 592 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.

Rimmer, Christopher C., J. Daniel Lambert, and Kent P. McFarland. Bicknell's Thrush (Catharus bicknelli) Conservation Strategy for the Green Mountain National Forest. December 2005. VINS Technical Report 05-5. Vermont Institute of Natural Science. 2723 Church Hill Road, Woodstock, VT 05091. Accessed 19 August 2007.

Sibley, David Allen. 2000. The Sibley Guide to Birds. Alfred A. Knopf, New York.
Conservation Status References
Rimmer, C. C., K. P. McFarland, W. G. Ellison, and J. E. Goetz. 2001. Bicknell's Thrush (Catharus bicknelli). In The Birds of North America, No. 592 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.

Rimmer, Christopher C., J. Daniel Lambert, and Kent P. McFarland. Bicknell's Thrush (Catharus bicknelli) Conservation Strategy for the Green Mountain National Forest. December 2005. VINS Technical Report 05-5. Vermont Institute of Natural Science. 2723 Church Hill Road, Woodstock, VT 05091. Accessed 19 August 2007.