Until 1995 Bicknell's was considered a subspecies of the Gray-cheeked Thrush. It was first discovered by an amateur ornithologist in the 1880s on Slide Mountain in the Catskills (Rimmer et al. 2001).
Bicknell's Thrush is considered one of the most at-risk passerines in eastern North America and is the Partners in Flight top conservation priority among neotropical migrants in the northeast. Because this long distance migrant has a naturally fragmented breeding range and narrow habitat preferences, its overall population size is expected to be low. Thus, it is expected to be highly vulnerable to habitat loss and/or degradation of its high elevation spruce/fir forest habitat. Although the great majority of this habitat type is already protected in the Adirondacks and Catskills, it is currently under threat from climate change and contaminant deposition (Lambert et al. 2005).
Since 2001, 37 different peaks in the Adirondacks and another 12 in the Catskills have been monitored by the Mountain Birdwatch Program (Hart and Lambert 2007). While there are some limitations with the data (i.e., every peak not monitored all years, different observers), certain conclusions, especially regarding occupancy patterns, can be made at this time. First, this species has apparently not declined in abundance since in either mountain range, and in fact the number of Bicknell's Thrushes in the Adirondacks has doubled from 1.5 birds/site in 2001 to 3 birds/site in 2007. Abundance in the Catskills has remained nearly stable since 2000. Occupancy trends can be analyzed back to 1992 using data from Atwood et al's. (1996) seminal paper in conjunction with the Mountain Birdwatch data. Here again, no significant declines are evident. In the Catskills, occupancy has remained high since 1992 with over 85% of the monitored peaks harboring breeding populations year to year, while in the Adirondacks occupancy has declined slightly (-1%/yr.) since 1992. There have been some site extirpations, but these seem to be readily recolonized, and the overall colonization rate is above the extinction rate. For both distribution and abundance of Bicknell's Thrushes in New York State, the prevailing pattern seems to be one of population fluctuations, rather than negative (or positive) population trends. Factors promoting these fluctuations apparently occur on the breeding grounds, and appear to relate to predator abundance, which in turn is dictated by the Balsam fir cone crop. Significant declines have been noted in the White Mountains since 1993 (King et al. in press) using similar monitoring techniques as the Mountain Birdwatch Program.
The overall number of Gray-cheeked plus Bicknell's Thrushes declined significantly between 1969-1995 at one fall migratory site in Virginia (Wilson and Watts 1997). A historically-known population on Mt. Greylock (since 1888) in Massachusetts gradually declined between 1961 and 1973 and has been vacant ever since. Nevertheless, Atwood et al. (1996) found that 63 of 73 known historical peaks throughout the northeast were still occupied in 1992-1995, including every known surveyed site in New York. Ellison's (2001) rangewide genetic analysis suggested that the Bicknell's population in the entire northeast has been relatively stable over the past several thousand years.
At the inaugural meeting of the International Bicknell's Thrush Working Group in November 2007, habitat loss on the wintering grounds in Hispaniola was ranked by experts as the most critical threat to this species. On the breeding grounds in the northeast, climate change, inducing reduced coverage of spruce/fir forests also presents a potential threat (Rodenhouse et al. 2007). Other potential breeding habitat threats addressed by the working group included ski resort and wind tower development, acid precipitation and other toxic deposition (especially mercury), recreational hiking, and predation by red squirrels.
The majority of occupied peaks in New York are within the state-owned forest preserve in the Catskills and Adirondacks, so extensive site-specific management is precluded. Most key demographic parameters were found not to be significantly different between existing ski areas in the northeast and natural forests. Limited ski trail development (i.e., disturbance) may not be overly detrimental if implemented properly (Rimmer et al. 2004). A new Unit Management Plan for Whiteface Mountain Ski Area included recommendations to minimize the potential negative impacts of new trail development on Bicknell's Thrushes. These included reducing trail width, managing the edges of trails and the establishment of a Conservation Fund to help protect the species on its wintering rounds in Hispaniola (Rimmer et al. 2004). Degradation of both wintering and breeding ground habitat will require larger scale policy solutions that can't be implemented solely within New York State.
Preliminary data indicate that population size fluctuates with the biennial cycle of balsam fir cone crops, which correlates to elevated predator (red squirrels, chipmunks, voles) populations and depressed reproductive success in summers following high cone crops. There is also evidence for density-dependent population regulation on the breeding grounds (Rimmer et al. 2001). Thus, research is critically needed to disentangle the limiting factors regulating population distribution and abundance. Despite the fact that the most critical threats appear to be on the wintering grounds, if population regulation occurs primarily on the breeding grounds, mitigating wintering ground threats may be of lesser importance. Also, this species' naturally fragmented habitat suggests that metapopulation and source/sink dynamics may be relevant, and there is some evidence for this (Ellison 2001; Hobson et al. 2001). Further analyses of Mountain Birdwatch long-term data will be necessary to tease apart population dynamics.
This species is a habitat specialist restricted to naturally, and human-disturbed, montane spruce/fir forests with lesser amounts of other hardwoods such as Mountain Ash and Yellow Birch above about 3000 ft. Since there is a strong negative correlation between latitude and temperature, spruce/fir caps mountain tops at comparatively higher elevations in the Catskills than in the Adirondacks. Therefore, thrushes will nest down to about 2700 ft. in the Adirondacks, but generally only above 3500 ft. in the Catskills (Lambert et al. 2005). The highest nest densities are often associated with stunted Balsam fir areas along exposed ridgelines chronically disturbed by high winds and ice accumulation (Rimmer et al. 2001). There is some uncertainty about the importance of Bicknell's use of habitat ecotones at the lower elevational limits of occupancy (Hale 2006) and interspecific interactions with congeners, especially Swainson's Thrushes (Able and Noon 1976) at lower elevations. As the climate in the northeast warms, the spruce/fir ecotone presumably will migrate upslope (Rodenhouse et al. 2007), possibly favoring Swainson's, since Bicknell's is physiologically adapted to cooler temperatures (Holmes and Sawyer 1975). Such ecotones are well correlated with growing season temperatures; but in both the Catskills and Adirondacks only winter temperatures, not summer temperatures, have significantly risen since 1900 (Jenkins and Keal 2004). Further north in Canada, the species breeds in a wider array of habitats including low elevation coastal evergreen stands with similar forest structure to higher elevation sites, as well as dense second-growth forests (with a significant hardwood component) following large-scale disturbances such as clearcutting and/or fires (Nixon et al. 2001; Connolly et al. 2002; Campbell and Whittam 2006).
In New York, this species occupies mountain-top forests dominated by balsam fir and red spruce at elevations greater than about 3000 ft. in the Catskills and Adirondacks. In the Catskills, breeding is restricted to peaks generally above 3500 ft. within the forest preserve in southern Greene and central Ulster counties (Pierce-Berrin 2001). Occupied mountains in the Adirondacks are clustered around the High Peaks region in northwest Essex county, trending southwest (into central Hamilton county) to northeast into extreme southern Clinton County. The Adirondacks contain the largest area of suitable habitat in eastern North America, harboring perhaps 25% of the known breeding pairs (Atwood et al. 1996).
The breeding range in eastern North America is naturally fragmented into several "sky islands" of high elevation montane spruce/fir forests at the highest elevations in eastern New York (Catskill and Adirondack Mountains), formerly Massachusetts (Mount Greylock), the spine of the Green Mountains in central and northern Vermont, the White Mountains in northern New Hampshire and Maine, northern Cape Breton, Nova Scotia, higher elevations on the Gaspe Peninsula, and the interior highlands of New Brunswick (Rimmer et al. 2001). The wintering grounds are poorly documented; known from Hispaniola, Cuba, Puerto Rico, and St. Croix. The non-breeding stronghold almost certainly is the Dominican Republic (Rimmer 1996); not known from Central or South America (Ouellet 1993). Migration records encompass eastern coastal states of the U.S. and Bahamas (Ouellet 1993).
This is a medium-sized thrush (6-7"; 1 oz.), but smaller and more slender than most Catharus thrushes. The upper parts are a warm brownish color and the breast is heavily spotted with brown over a white belly. A reddish tinge is present on the wings and tail.
The Bicknell's Thrush song consists of two introductory plucking notes (only audible at 10-12 meters) hurriedly followed by 2-4 high-pitched vibrant, ringing phrases that slur downward: rendered as "chook-chook, wee-o, wee-o, wee-o-ti-t-ter-ee" (Rimmer et al. 2001). Nests (12x12 cm) are cup-shaped and bulky, built primarily of twigs and moss, with an interior layer of decayed vegetation. The small (22x17 mm) eggs are subelliptical and bluish-green with light brown speckling (Rimmer et al. 2001).
Subtle, but clear differences in song distinguish Bicknell's from other Catharus thrushes.
This thrush species has a male-skewed sex ratio and is considered to be polygynandrous. Single broods are sired by multiple males, and multiple males also bring food to a single nest (Goetz at al. 2003). This breeding system is assumed to be an adaptation to harsh montane conditions (Strong et al. 2004) where Bicknell's expend large amounts of energy for thermoregulation (Holmes and Sawyer 1975). Individuals are secretive and elusive, staying hidden among dense undergrowth and seldom coming out into the open. Males perform an evening flight song (Evans 1994).
Bicknell's Thrushes feed on insects and other arthropods, especially beetles and ants. The species is considered predominantly a ground forager, but sometimes fly-catches from an exposed perch (Rimmer et al. 2001).
This is one of the last migrants to return to the breeding grounds in the spring (Ouellet 1996). Courtship activities begin shortly after arrival in mid May, nest-building begins by early June, eggs are laid by mid June, eggs hatch in late June, and the young fledge during July (Rimmer et al. 2001).
The time of year you would expect to find Bicknell's Thrush active and reproducing in New York.
Catharus bicknelli (Ridgway, 1882)
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Information for this guide was last updated on: April 5, 2019
Please cite this page as:
New York Natural Heritage Program. 2020. Online Conservation Guide for Catharus bicknelli. Available from: https://guides.nynhp.org/bicknells-thrush/. Accessed September 26, 2020.