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Bicknell's Thrush (Catharus bicknelli) Conservation Strategy for the Green Mountain National Forest

Bicknell’s Thrush (Catharus bicknelli)
Conservation Strategy
for the Green Mountain National Forest
VINS Technical Report 05-5
Prepared By:
Christopher C. Rimmer, J. Daniel Lambert, and Kent P. McFarland
Vermont Institute of Natural Science
2723 Church Hill Road
Woodstock, VT 05091
Submitted To:
Green Mountain National Forest
231 North Main Street
Rutland, VT 05701
December, 2005
Dan Busb
Executive Summary
Bicknell’s Thrush is one of eastern North America’s most rare, at-risk migratory songbirds,
believed to number no more than 50,000 individuals across its restricted and highly fragmented
breeding range. The species is a habitat specialist, nesting in the U.S. only in fir-dominated
montane forests generally above 900 m in elevation. These forests face numerous threats that
include global climate change, atmospheric deposition of acidic ions and mercury, recreational
ski area development, telecommunications tower construction, and wind turbine development.
On its Greater Antillean wintering grounds, Bicknell’s Thrush is largely restricted to moist,
primary broadleaf forests, many of which have been severely reduced in extent and are poorly
protected. The species is inadequately monitored by traditional sampling methods such as the
Breeding Bird Survey, and estimates of breeding or wintering densities are unreliable at best. An
elevation-based model of Bicknell’s Thrush habitat in the U.S. indicates 136,250 ha of
potentially suitable conifer-dominated montane forest habitat. Of this, a total of 3,151 ha of
potential Bicknell’s Thrush habitat occurs in the Green Mountain National Forest (GMNF),
representing 27.2% of all potential habitat within Vermont (11,580 ha) and 2.3% of the species’
U.S. habitat. Analysis of 47 mountaintop survey routes censused annually from 2001 to 2004
shows Bicknell’s Thrush in a four-year decline, averaging -9.0% per year.
Because few timber harvesting operations occur in the montane fir-dominated forests
preferred by Bicknell’s Thrush, the most promising opportunities on the GMNF for active
management exist in areas of habitat that are currently developed for recreation,
telecommunications, or wind power facilities. Measures to minimize or mitigate possible
impacts of activities that may alter Bicknell’s Thrush habitat include: 1) restricting timing of
activities to before 15 May or after 1 August; 2) avoiding areas where natural disturbance, either
chronic or random, could maintain suitable habitat; 3) striving for a “no net loss” habitat
mitigation process; 4) consolidating small, adjacent habitat fragments into single, larger blocks
on developed peaks; and 5) conducting vegetation management on ski areas or other managed
sites to maintain and enhance appropriate habitat for Bicknell’s Thrush.
A coordinated Conservation Strategy for Bicknell’s Thrush should include 1) continuation of
standardized, long-term population monitoring and demographic research; 2) development of a
spatially explicit Population Viability Assessment; 3) development of spatially explicit density
estimates from the GMNF and the species’ entire breeding range, to enable accurate calculations
of total population size; 4) small-scale experimental manipulations of forest stands above 2500’
in the GMNF for the purpose of developing habitat improvement techniques for Bicknell’s
Thrush; 5) formation of a species management team to oversee and guide conservation planning
for Bicknell’s Thrush on the GMNF, and throughout its breeding range; 6) formalization of an
international consortium that includes GMNF, VINS, and other U.S. and Hispaniolan partners, to
plan Hispaniolan conservation projects that target Bicknell’s Thrush; and 7) collaborative
development of informational materials on the GMNF and elsewhere that showcase Bicknell’s
Thrush habitat management efforts and educate the public about montane forest ecology and
Bicknell’s Thrush (Catharus bicknelli), once considered a subspecies of Gray-cheeked Thrush
(C. minimus), gained full species status in 1995 (American Ornithologists’ Union 1995). It has
since been considered one of the passerines most at risk of extinction in eastern North America.
The North American Bird Conservation Initiative identifies Bicknell’s Thrush among the
Highest Priority Landbirds in the Northern Atlantic Forest (Dettmers 2003), while Partners in
Flight has placed the species on its Continental Watch List, citing multiple causes for concern
(Pashley et al. 2000, Rich et al. 2004). The International Union for the Conservation of Nature
(BirdLife International 2000) classifies Bicknell’s Thrush as “vulnerable” on its list of globally
threatened species. Although there is no conclusive evidence of rangewide population declines,
reports of regional declines (Rompré et al. 1997; Rimmer et al. 2001a, Lambert 2005, Lambert et
al. 2005a) and local extinctions (Christie 1993, Atwood et al. 1996, Nixon 1999) have elevated
concern for this rare species. In Vermont, Bicknell’s Thrush is listed as a Species of Special
Concern, with a Natural Heritage rank of S3B (Vermont Nongame and Natural Heritage Program
2005). On the Green Mountain National Forest (GMNF), Bicknell’s Thrush is designated as a
Regional Forester Sensitive Species (USDA Forest Service 2000).
A 2001 Conservation Assessment for the GMNF and White Mountain National Forest
(WMNF) (Rimmer et al. 2001b) provided an overview of the current state of knowledge on
Bicknell’s Thrush, including primary threats facing the species and priorities for future research
and monitoring. This Conservation Strategy synthesizes and updates that information to (1)
develop explicit objectives for the management of Bicknell’s Thrush populations and habitats on
the GMNF, (2) recommend specific actions by which to accomplish those objectives, and (3)
refine research and monitoring needs for Bicknell’s Thrush. The Conservation Strategy, while
focused on the GMNF, has broad applications that extend throughout the species’ northeastern
U.S. breeding range.
Distribution and Status
Breeding range. Bicknell’s Thrush occupies a restricted and highly fragmented breeding
range (Figure 1 and 2). Breeding is documented north to southwestern Quebec in Réserve La
Verendrye, southeastern Quebec along the north shore of the St. Lawrence River and Gaspé
Peninsula (Ouellet 1993, 1996), northwestern and north-central New Brunswick (Erskine 1992,
Nixon 1996), and Cape Breton Island, Nova Scotia, including the small, outlying St. Paul and
Scaterie islands (Erskine 1992, D. Busby pers. comm.). Southern breeding limits are reached in
the Catskill Mountains of New York (Atwood et al. 1996, Peterson 1988), the Green Mountains
of southern Vermont (Atwood et al. 1996, Kibbe 1985), the White Mountains of central New
Hampshire (Atwood et al. 1996, Richards 1994), the mountains of western and central Maine
(Adamus 1987, Atwood et al. 1996), south-coastal New Brunswick (possibly extirpated; Erskine
1992, Christie 1993), and southwest-coastal Nova Scotia (probably extirpated; Erskine 1992, D.
Busby pers. comm.). Possible but unconfirmed local and sporadic breeding has been
documented in north-coastal Maine (Atwood et al. 1996, Rimmer and McFarland 1996).
Bicknell’s Thrush is widespread at high elevations in the GMNF, where surveys conducted
between 1992-2003 confirmed the species’ presence on 42 mountains (Table 1, Figure 2 ).
Occupied peaks were distributed from Searsburg Ridge in the south to Mt. Ellen in the north. A
recently-constructed model of Bicknell’s Thrush distribution in the northeastern U.S., using
elevation, latitude, and forest type as co-variables, predicts the species to be present in conifer-
dominated forests above an elevation threshold that descends with an increase in latitude
(Lambert et al. 2005b). The slope of the threshold (-81.63 m/1° latitude) reflects climatic effects
on forest composition and structure. At the southern edge of GMNF, the model predicts
Bicknell’s Thrush occurrence above 985 m elevation, while the lower limit of the species’
occurrence at GMNF’s northern extent is 870 m. The model further predicts that a total of 3,151
ha of potential Bicknell’s Thrush habitat occurs in the GMNF (Figure 3); this represents 27.2%
of all potential habitat within Vermont (11,580 ha) and 2.3% of the species’ U.S. habitat
(136,250 ha; Lambert et al. 2005b).
Winter range. The known wintering distribution of Bicknell’s Thrush is confined to the
Greater Antilles (Figure 1). Specimen and field survey data indicate that the majority of
wintering birds occur in the Dominican Republic (Wetmore and Swales 1931, Ouellet 1993,
Rimmer et al. 1997, 1999), where the species is widely distributed and locally common from sea
level to 2220 m (Rimmer et al. 1999, 2001a). Few records exist from Haiti, where it is restricted
to higher elevations, mainly in the southwest (Massif de la Hotte) and east (Massif La Visite;
Wetmore and Swales 1931, Woods and Ottenwalder 1983, 1986). Surveys conducted by VINS
in Massif de la Hotte during February of 2004 confirmed Bicknell’s Thrush to be fairly common
in mesic karst broadleaf forest at 1175-1250 m elevation and in the wet montane broadleaf forest
at 1825-1915 m elevation (Rimmer et al. 2004). In the Massif de la Selle, VINS found the
species in small patches of remnant, highly threatened wet broadleaf forest at 1175-2060 m
elevation during January of 2005 (Rimmer et al. 2005). Bicknell’s Thrush is uncommon and
local in Jamaica, mainly in the Blue Mts. from 1200–2225 m elevation (R. and A. Sutton,; VINS, unpubl. data). The species is a rare winter resident in eastern and
southeastern Puerto Rico, known only from the Luquillo Mts. at 450-720 m elevation and Sierra
de Cayey at 720 m (Arendt 1992; J. Wunderle unpubl.). Bicknell’s Thrush has recently been
recorded in eastern Cuba at 1600-1960 m in Sierra Maestra (Rompré et al. 2000, Y Aubry and G.
Rompré pers. comm.); two October specimens from western Cuba (Havana) in the 1960s
(Garrido and Garcia Montaña 1975) probably represent transients. There are no confirmed
winter records elsewhere.
Breeding range. In the United States, Bicknell’s Thrush is a habitat specialist restricted to
montane forests dominated by balsam fir (Abies balsamea), with lesser amounts of spruce (Picea
rubra and P. mariana), white birch (Betula papyrifera var. cordifolia), mountain ash (Sorbus
sp.), and other hardwood species. At the southern extent of its range in the Catskill Mountains,
Bicknell’s Thrush generally breeds above 1100 m elevation, with birds recorded as low as 750 m
on several Maine peaks (Lambert et al. 2005b). The lowest nest in Vermont was documented at
1006 m, although the species is regularly found at elevations below 900 m (Rimmer et al.
2001a). Bicknell’s Thrush is often associated with recently-disturbed areas undergoing vigorous
succession, characterized by standing dead conifers and dense regrowth of balsam fir (Wallace
1939, Rimmer et al. 2001a). Highest densities are typically found in chronically-disturbed (high
winds, heavy winter ice accumulation) stands of dense, stunted fir on exposed ridgelines or along
edges of human-created openings (e.g. ski trails), or in regenerating “fir waves” (cf. Sprugel
1976, Marchand 1984; Rimmer et al. 2001a, 2004). In the White Mountains of New Hampshire,
Sabo (1980) found Bicknell’s Thrush at a mean elevation of 1290 m in exposed mid to upper
slopes dominated by conifers (75% of foliage volume) with a mean canopy height of 4.8 m.
In Canada, Bicknell’s Thrush occupies montane fir forests in southern Quebec and New
Brunswick up to 1178 m elevation (Ouellet 1993, Rompré et al. 1997, Connolly 2000, Nixon et
al. 2001; D. Busby pers. comm.), coastal maritime spruce-fir forests in New Brunswick and
Nova Scotia (Erskine 1992, D. Busby pers. comm.), and regenerating stands of mixed forest
following forest fires or clear cutting in Quebec and New Brunswick, generally above 450 m
(Ouellet 1993, Nixon 1996, Nixon et al. 2001).
Winter range. The current preferred winter habitat of Bicknell’s Thrush is mesic to wet
broadleaf montane forests in the Dominican Republic (Rimmer et al. 1999), Haiti (Wetmore and
Swales 1931, Woods and Ottenwalder 1983, 1986; Rimmer et al. 2004), Cuba (Rompré et al.
2000, Aubry and Rompré pers. comm.), Jamaica (R. and A. Sutton, pers. comm., VINS), and
Puerto Rico (J. Wunderle unpubl.). In the Dominican Republic, the species is found at all
elevations from sea level to 2200 m, although 62% of occupied sites were in forests > 1000 m
elevation (Rimmer et al. 1999). The majority (75%) of occupied sites (n = 24) were in
broadleaf-dominated forests (“cloud/montane broadleaf forest” and “submontane broadleaf
rainforest”; Tolentino and Peña 1998) at all elevations, 19% were in mixed broadleaf-pine
forests, and 6% occurred in pine-dominated forests. Primary, wet and/or mesic forests
constituted 78% of all occupied sites; only 6% of occupied sites were in predominantly dry
forests (Rimmer et al. 1999). The use of regenerating secondary forests (22% of occupied sites)
in the Dominican Republic may indicate winter habitat flexibility or a recent shift from preferred
primary broadleaf forest habitat, much of which has been lost or degraded.
In the Dominican Republic, some evidence exists for sexual macro- or micro-habitat
segregation, or segregation of sexes by geographic area (Rimmer and Goetz 2001, Rimmer et al.
2001a; Rimmer and Townsend unpubl. data). In Sierra de Bahoruco on the Haitian border, in
predominantly undisturbed broadleaf montane forests, 70 of 86 (81%) of known-sex birds mist-
netted from 1998-2003 were male. At a smaller, mid-elevation, moderately disturbed wet forest
site in the Cordillera Septentrional, 27 of 52 (52%) of known-sex birds were female. These
results, while still preliminary, suggest that females may be limited by availability of high quality
winter habitat; they warrant more intensive investigation.
Life history
The life history, demography, and ecology of Bicknell’s Thrush are described in detail in
Rimmer et al. (2001a). Only those aspects most relevant to the species’ conservation status and
management on the GMNF are presented here.
Mating system and sex ratio. The mating system of Bicknell’s Thrush is unusual and not
easily categorized; it may be most similar to that of Smith’s Longspur (Calcarius pictus), which
has been termed “female-defense polygynandry” (Briskie 1993), in that both males and females
mate with multiple partners, multiple paternity is common, and > 1 male often feeds nestlings
(Goetz et al. 2003). In Vermont, > 75% of broods are sired by multiple males; some males have
offspring in two nests in the same breeding season. Of 13 broods in 1998 and 1999, 10 had 2 or
more sires, 3 a single father (Goetz et al. 2003). This unusual mating system, combined with the
species’ unpredictable spacing patterns, make estimation of breeding densities unreliable at best.
The overall 4-yr mean male:female ratio on 3 Vermont study plots was 1.8:1.0 (annual range
= 1.4-2.8:1.0; Rimmer et al. 2001a). The cause of a male-biased sex ratio is not known, but may
relate to the ratio at hatching, differential natal dispersal patterns, differential survival of first-
year birds, or events on the wintering grounds (e.g., differential male and female survival due to
winter habitat segregation).
Breeding phenology. The earliest known arrival date of a breeding male in Vermont is 15
May, of a female 23 May. Males arrive significantly earlier than females (mean difference = 1.7
days, 95% CI = 3.2 – 0.3). Mating activities probably begin shortly after female arrival, as
evidenced by frequent singing and calling throughout the day in late May and early June
(Rimmer et al. 1996). Mating associations are dynamic and probably tied to the stage of
individual females’ fertile periods, and are likely influenced by the availability of other mating
opportunities and chick feeding by males.
The earliest confirmed nest construction date in Vermont is 1 June, with other extrapolated
nest initiation dates of 2-4 June (Wallace 1939, Rimmer et al. 2001a). In Vermont, 71% (n = 89)
of clutches are initiated in the first 3 weeks of June. Later clutches probably represent renesting
attempts. Clutch initiation dates in Vermont range from 7 June-14 July (n = 89), and in New
Hampshire 21 June-14 July (n = 5; Wallace 1939, Richards 1994). Known hatching dates range
from 23 June-29 July (70% by 6 July) in Vermont (n = 68; Wallace 1939, Rimmer et al. 2001a).
Known Vermont fledging dates range from 3 July-3 August (70% by 14 July, n = 53; Wallace
1939; Rimmer et al. 2001a). Young stay in the nest 9-13 days (average 11.4 ± 1.3, n = 17;
Wallace 1939, Rimmer et al. 2001a).
Second broods are rare, with only one confirmed instance in Vermont. A female that fledged 2
chicks on 2 July initiated a second clutch 5 days later, building her nest while feeding fledglings
and continuing to feed her first brood during egg laying (Rimmer et al. 2001a). Renesting
attempts after early-season failures are common. The mean interval between loss of a first nest
and initiation of a second clutch in Vermont is 6.8 days (range 5-12, n = 5). One female renested
successfully on her third attempt, requiring only 2 days from loss of her second clutch to
initiation of her third (Rimmer et al. 2001a).
Nest characteristics and microhabitat. Nests are usually located in dense stands of young to
mid-successional fir or “krummholz”, uncommonly in more mature, open forests (Wallace 1939,
Rimmer et al. 2001a), and are often found in dense regrowth along natural or artificially created
edges. On 2 ski areas (Mt. Mansfield and Stratton Mountain) in the Green Mountains, 57% of all
nests were within 10 m of a ski trail edge (Table 2), and 45% of these were < 2 m from the edge
(Rimmer et al. 2004). Of 118 Vermont nests, the mean height above ground was 2.05 m + 1.18
SD (range = 0.46-10 m; Wallace 1939, Rimmer et al. 2001a). Most nests (103 of 118; 87%) are
built in balsam fir, but also in red spruce (n = 10), paper birch (n = 3), and dead standing fir (n =
2; Wallace 1939, Rimmer et al. 2001a). Average nest tree height was 3.2 m ±1.55 SD (range 0 –
11m, n = 102) and mean DBH was 5.7 cm ± 5.24 SD (range 1-31.5 cm, n = 102).
Incubation and care of nestling and fledglings. In Vermont, the incubation period is 9-14
days (mean 12 ± 1.6, n = 8; Wallace 1939, Rimmer et al. 2001a). Chicks generally hatch within
24 hours of each other. Only females brood, but both sexes feed chicks. Nestlings fledge 9-13
days after hatching (mean 11.4 ±1.3, n = 17 known to the exact day; Wallace 1939, Rimmer et
al. 2001a). Fledglings may remain with adults up to 14 days after leaving the nest. Adults often
split broods, although one known case exists of 2 males splitting a brood, apparently
emancipating the female. In another case, a female and one of 2 male feeders split the brood,
while the second male continued to feed nestlings in another nest. Movements of family groups
are not well documented, but adults with dependent fledglings have been found up to 280 m
away from known nest sites. (Rimmer et al. 2001a)
Movements and habitat use of both adults and fledglings during the post-fledging period are
poorly known. Of 11 Mansfield fledglings radio-tagged in 2000, 7 were known to have been
depredated (mean survival 8.1 + 6.6 days after fledging, range 1-19), 2 disappeared after 8 and
19 days, respectively, and 2 survived until their transmitter batteries expired (40 and 31 days,
respectively). Of these latter 2 birds, one remained within 275 m of its natal nest site in montane
fir forest, while the other moved nearly 1 km downslope after about 10 days to hardwood-
dominated forest at elevations of 700-900 m and remained there. One free-flying juvenile
banded on 25 July stayed within a 100-m radius of its banding location in stunted fir forest at
1150-1175 m elevation until 22 August, then disappeared. (Rimmer et al. 2001a).
Population Biology and Viability
Age at first breeding; intervals between breeding. Bicknell’s Thrushes breed at
approximately 1 year and annually thereafter. Of known-age female breeders at 85 Vermont
nests in 1994-1999, ASY (after second-year) females outnumbered SY (second-year) females 73
to 12 (85.9% to 14.1%). Of 25 Vermont males with known paternity at 1998 and 1999 nests,
only 2 (8%) were SY birds, while the SY age class comprised about 25% of entire male study
population (Goetz et al. 2003). Highly irregular settlement patterns further suggest that some SY
males may fail to achieve fertilizations. (Rimmer et al. 2001a).
Annual and lifetime reproductive success. In Vermont, annual reproductive success among
males is skewed but generally low. Paternity data from 12 broods monitored in 1998 and 1999
suggest considerable reproductive skew. Of 19 males with known paternity, 12 (63%) males
sired 1 young, 2 (11%) sired 2 young, 4 (21%) sired 3 young, and 1 (5%) sired 4 young; these
are minimum estimates (Goetz et al. 2003).
The annual Mayfield daily survival rate of nests (probability of nest surviving 1 day without
failure) on Stratton Mtn. was 0.98 + 0.014 SE (n = 39 nests), and on Mt. Mansfield 0.96 + 0.007
SE (n = 56 nests). Daily survival rates of Vermont nests are strikingly biennial in response to
balsam fir cone production and red squirrel population cycles. From 1994-2000, fall cone crops
were very high in even-numbered years, resulting in high red squirrel populations during the
following springs and summers, with consequent low productivity for Bicknell's Thrush due to
nest depredation. In odd-numbered years, fall cone production was invariably lower, spring and
summer squirrel populations reduced, and thrush nesting success markedly higher (Rimmer et al.
2001a). The average number of young fledged/nest in Vermont was 2.1 ± 1.37 SD (range = 0-4,
n = 30) on Stratton Mtn. and 1.5 ± 1.59 SD (range = 0-4, n = 46) on Mt. Mansfield (Rimmer et
al. 2001a).
Life span and survivorship. The longevity record for both male and female Bicknell’s
Thrush, based on band returns, is 9 years. The annual survival rate of ASY birds captured on
Vermont breeding grounds, based on a Cormack-Jolly-Seber model (Cooch and White 1998,
White and Burnham 1999), was not dependent on time or sex on four study plots. To account for
uncertainty in model selection, the range of mean parameter estimates was averaged over all 16
models in the candidate set for each study plot, weighted by Akaike model weights, and the most
parsimonious model was used (Bertram et al. 2000, Burnham and Anderson 2002). Survivorship
on the Mt. Mansfield ridgeline in 1992-1999 was 54.7% ± 6.5% SE with mean parameter
estimates for all models ranging from 54% - 55.8%; on Mt. Mansfield’s east slope in 1995-1999,
74.8% ± 8.6% SE, mean estimates 71.9% – 79.1%; on VINS’ Stratton Mtn. ski area plot in 1997-
1999, 73.9% ± 10.1% SE, mean estimates 75.6% - 88.3%; and on VINS’ Stratton Mtn. natural
plot in 1997-1999, 94.6% ± 28.4% SE, mean estimates 86.1% - 94%. A more detailed analysis
of 1994-2003 adult survivorship data from Mansfield and Stratton showed no significant
difference in survivorship between birds on ski area and natural area plots (Rimmer et al. 2004).
Survival rates of juveniles are poorly known due to apparent natal dispersal; only 9 of 157
(5.7 %) fledglings and 3 of 16 (18.8 %) independent juveniles banded in Vermont from 1992-
2004 were documented to return to their natal mountain. Two nestlings that returned to Stratton
Mountain were females from the same nest. On Mt. Mansfield in 2000, only 2 of 11 (18.2%)
radio tagged fledglings were known to have survived beyond 30 days. The annual survival rate
of wintering individuals captured at a montane broadleaf forest site in Sierra de Bahoruco,
Dominican Republic, based on Cormack-Jolly-Seber model estimates, was not time dependent in
1994-1999: 72.9% ±14.3% SE, with mean parameter estimates for all models ranging from
68.4% - 79.7% (Rimmer et al. 2001a). Return rates differ significantly for thrushes banded in
Vermont (64.7%: 225 of 348 birds) and the Dominican Republic (28.3%: 25 of 99 birds).
However, return rates of age classes are similar within each area: 61.5% (112 of 182 birds) for
yearlings and 68% (113 of 166 birds) for older individuals in Vermont; 22% (12 of 55 birds) for
yearlings and 29.6% (16 of 54 birds) for older individuals in the Dominican Republic.
Relationship to Land Management and Human Activity
Of the 3,151 ha of potential BITH habitat in the Green Mountain National Forest, 75% occurs
in areas where timber harvesting is restricted. Of the remaining 25%, (1) 11.8% occurs in areas
where “Wildlife and timber management activities are selected, scheduled, and located to ensure
that backcountry recreation is protected.”; (2) 7.8% occurs on newly acquired lands where
“management options will be kept open until inventories can be done.”; and (3) 5.7% occurs on
lands where recreation benefits are emphasized, including 46 ha on Mount Snow designated as
potential ski area expansion.
Adjacent lands contain hundreds of additional unconserved hectares, including 313 ha on
Stark Mountain, 178 ha on Mount Equinox and Little Equinox, and 79 ha on the northeast slope
of Stratton Mountain.
Population Viability Goals
Because a comprehensive Population Viability Analysis (PVA) for Bicknell’s Thrush is
lacking, and because population estimates remain imprecise, it is not currently feasible to
establish quantifiable goals to maintain or enhance Bicknell’s Thrush populations. The species is
believed to number fewer than 50,000 breeding individuals rangewide (Rimmer et al, 2001a,
Rich et al. 2004), and recent estimates suggest a global population of fewer than 20,000 birds
(Hale 2006, Lambert et al. 2006a). These figures represent coarse approximations at best, based
on a number of assumptions (Rimmer et al. 2001b). Few reliable population trend data are
available from any part of the breeding range. Bicknell’s Thrush is inadequately monitored by
the North American Breeding Bird Survey of the USGS Patuxent Wildlife Research Center.
Since 2000, a regionwide (New York to Maine), long-term population monitoring program,
Mountain Birdwatch, has used trained volunteers to conduct standardized point counts on >100
mountains annually (Lambert 2005). Analysis of 47 routes surveyed annually from 2001 to 2004
shows Bicknell’s Thrush in a four-year decline, averaging -9.0% per year (P = 0.07; Figure 4 and
5; Lambert 2005).
Lambert et al. (2005a) recently completed an analysis of existing Bicknell’s Thrush census
data from New Hampshire and Vermont. The study integrated and analyzed survey data
gathered from the GMNF by the University of Vermont (1991-2000), from elsewhere in
Vermont by VINS (1992-2000), and from the WMNF by Forest Service personnel and the
Audubon Society of New Hampshire (1993-2003. Results provided the first evidence of a
sustained decline in a major population of Bicknell’s Thrush. Counts on the WMNF
significantly declined by an average of 7.0% per year (P < 0.1), while Vermont counts did not
significantly change, declining annually by an average of 1% (P = 0.71; Lambert et al. 2005a).
On GMNF survey routes (n = 4), Bicknell’s Thrush exhibited a nonsignificant, annual decline of
6.2% (P = 0.27). The apparent disparity in the magnitude of population declines between New
Hampshire and Vermont may be due to sampling error, as Vermont surveys were conducted on
only six routes, while 39 routes contributed data from New Hampshire. However, data from the
North American Breeding Bird Survey indicate that population trends from New Hampshire and
Vermont differ in direction for nearly half of the bird species that typically occur in montane
forests (King et al. 2005), suggesting that the difference in Bicknell’s Thrush trends between the
two states may be real. Factors that might account for such a difference include potentially
higher levels of atmospheric pollutants in the White Mountains, or geographic separation and
differential mortality of populations on the species’ wintering grounds. There is no solid
evidence for either possibility, however, and expanded, long-term monitoring will be necessary
to elucidate regional trends.
In the absence of robust data on population size or trends, only general population viability
targets can be proposed. Partners in Flight goals advocate the “…protection of 100% of sites
that support Bicknell’s Thrush populations large enough to be considered source populations for
other sites, and as many additional high-elevation habitat patches with smaller populations as
possible” (Hodgman and Rosenberg 2000). Although statewide Partners in Flight population
objectives for Vermont and New Hampshire are unavailable (Rosenberg 2004), due to the
paucity of data on Bicknell’s Thrush, a minimum goal should be maintenance of current
numbers. Given the extremely high priority of this species in the Partners in Flight conservation
ranking scheme, eventual objectives will almost certainly recommend increasing statewide
populations of Bicknell’s Thrush. If the current population decline indicated by Mountain
Birdwatch data continues, population increases of 50-100% over 30 years will likely be proposed
as targets (Rosenberg 2004).
Management Recommendations/Standards and Guides
Because most Bicknell’s Thrush habitat in the U.S. lies above the elevational limit of
conventional land management activities, the species affords relatively few opportunities for
active management. Few timber harvesting operations occur in the montane fir-dominated
forests preferred by Bicknell’s Thrush due to logistical constraints of accessing or operating in
high elevation terrain and relatively low commercial value of the lumber. On the GMNF, very
little timber harvesting has occurred in forested stands that extend above the lower elevation
threshold of Bicknell’s Thrush occupancy (R. Hoelscher, pers. comm.), such that current or
future harvesting activities pose a negligible risk to the species’ conservation status on the
The most promising opportunities for active management exist in areas of habitat that are
currently developed for recreation, telecommunications, or wind power facilities. These
typically afford accessibility via work roads, and they are often characterized by disturbance
regimes that can be maintained to varying degrees by targeted management. Undeveloped
habitats, on the other hand, are typically difficult to access and are characterized by natural
disturbance regimes that may be problematic to maintain or enhance. Bicknell’s Thrushes
occupy habitats that are extremely dynamic and often ephemeral in nature (Reiners and Lang
1979). Habitat suitability for the species tends to be patchy within a given area of montane
forest, and often changes over time, as birds shift their local distribution in accordance with
changes in forest composition and structure. Whether natural disturbance processes (e.g.,
weather events, fir waves) can be effectively mimicked in currently undisturbed areas is
unknown, but raises complex issues of ecological effects, practicality, cost effectiveness,
aesthetics, public relations, and ethics. A small number of carefully controlled, experimental
manipulations might yield valuable information.
A. Minimization and Mitigation of Management Impacts. In general, we recommend several
measures to minimize or mitigate possible impacts of activities that may alter Bicknell’s Thrush
habitat. These include:
1) Timing of activities. Field observations suggest that the most critical time for Bicknell’s
Thrush in regards to noise and human disturbance is during female nest site selection,
nest building, mating, egg laying, and through at least the first half of the incubation
period. Thrushes generally do not return to breeding sites before 15 May. Spring
management activities can therefore safely be conducted as late as this date. Actual
habitat removal, however, may adversely impact thrushes during any point in the active
nesting cycle. Of 142 nests with known termination dates on Stratton and Mansfield, 25
(17.6%) were terminated on or after 20 July, while 89 (62.7%) had termination dates
between 29 June and 19 July. Only 7% of all nests were still active after 26 July.
Among the 65 nests that fledged at least one chick, only 6% were active after 26 July.
All late nests were those of individual females that renested after failing to complete
earlier nesting attempts (mostly due to natural depredation events). Overall fledging
dates on Mansfield and Stratton ranged from 2 July to 2 August. Fledglings remain with
adults for up to 2 weeks after leaving the nest (Rimmer et al. 2001a) and show relatively
little mobility during the first 4-5 days after fledging (personal observation). To
minimize impacts to late-season nests and family groups with recent fledglings,
management activities (e.g., cutting, thinning, mowing, construction) should be delayed
until 1 August. This date can be earlier in a given area, provided that monitoring has
been sufficient to determine that all known active nests are terminated. We believe that a
1 August cut-off date, which poses a risk to less than 5% of thrush nests in an average
year, is acceptable.
2) Habitat alterations should be avoided in areas where natural disturbance, either chronic or
random, could maintain suitable habitat for Bicknell’s Thrushes. Such areas include
west-facing slopes, ridgelines, fir waves, and areas adjacent to fir waves. Management
activities or other habitat alterations should be limited to sheltered areas, where natural
disturbances tend to be minimal, and especially where mature hardwood species comprise
greater than 1/3 of the forest canopy. These are areas least likely to provide suitable
habitat for Bicknell’s Thrushes.
3) In instances of habitat removal or alteration (i.e., ski trail establishment or expansion) in
fir-dominated montane forests, a “no net loss” mitigation process is warranted, such that
an area of currently developed habitat equal to (or greater than) that to be altered will be
actively restored or passively allowed to recover to conditions suitable for Bicknell’s
Thrush occupancy. Any impacted areas selected for mitigation (e.g., packed gravel work
roads) that are not currently favorable for plant growth may require careful site
preparation to promote regeneration (see below). A “no net loss” policy should be a
primary habitat mitigation tool at montane forest sites where the Forest Service or other
landowners have both the necessary control and capacity to achieve mitigation.
4) Measures to accomplish habitat mitigation primarily involve restoration of montane
forest areas that currently provide unsuitable habitat for Bicknell’s Thrush. These may
include existing ski trails dominated by herbaceous or low shrubby vegetation, gravel or
other packed surfaces, ski lift openings that may be closed for skiing but maintained in an
early successional state, or any other human-altered habitats that do not provide the dense
fir-dominated cover favored by Bicknell’s Thrush. Areas that are selected for restoration
should be near or adjacent to currently occupied habitat and should, to the extent
possible, be subject to natural disturbance from wind and ice (e.g., west-facing slopes and
forested ridgelines). Habitat restoration should generally take one of two forms: a) active
revegetation of bare or herbaceous ground, via planting of native vegetation, or b) passive
revegetation through natural succession. In either case, it is important to realize that
forest regeneration occurs slowly at high elevations. A timeframe of 8-10 years is a
likely minimum before regular occupancy by Bicknell’s Thrushes can be expected.
5) For restoration efforts involving active revegetation, balsam fir seedlings or saplings
should be planted. If possible, these should be transplanted from nearby forest stands to
ensure genetic compatibility, provided that minimal alteration of these stock habitats will
occur. If transplanting can not be accomplished without unacceptable loss from, or
damage to, adjacent habitats, balsam fir seeds may be planted. In either case, soil may
need to be prepared in advance, and possibly augmented in thin areas overlying bedrock,
to accelerate the growth of fir seedlings or germination of seeds. Prior to and during any
active revegetation efforts, activities should be assessed by a certified forester on private
lands, and by an agency forester in the case of public land. Restoration via natural
succession may be especially warranted where succession by balsam fir and/or red spruce
has already begun. In such cases, advance site preparation may not be needed. Active
revegetation of montane forest sites has not previously been attempted, and this method
should be considered experimental, possibly requiring a “trial and error” approach.
6) Consolidation of habitat islands in mitigation plans, especially on ski areas, should be
given high priority. Consolidation of small, adjacent habitat fragments (< 0.1 ha) into
single, larger blocks will enhance overall habitat suitability for Bicknell’s Thrush in a
given area. Most islands smaller than 0.1 ha are not regularly used by thrushes, other
than as a waypoint during trail crossings. Decreasing the amount of overall edge through
creation of larger, contiguous habitat blocks promotes microclimate and microhabitat
features that enhance the likelihood of breeding by thrushes and other montane forest bird
7) For any mitigation sites, especially on ski areas, that are planned for forest regeneration,
some type of barrier should be erected to block these sites from human traffic, in order to
minimize disturbance to young trees that will eventually protrude above the winter
snowpack, exposing new growth to potential damage. Barriers should include
conspicuous signage to inform potential users about the closure and to educate them
about its benefits.
8) Any habitat restoration efforts will need to be guided by standards to evaluate their
success. Development of specific objectives for restoration, including a timeline, and
measures to objectively determine success are recommended. Field surveys to monitor
progress should be conducted over a timeframe of at least 15-20 years, by an agency or
certified forester or by other individual(s) knowledgeable about montane forest ecology.
There should be a specific contingency plan in place to compensate for the possible
failure of initial restoration efforts. Careful, sustained monitoring is necessary not only to
gauge the success of site-specific restoration attempts, and to apply corrective measures
as warranted, but to document the feasibility of montane forest restoration. There are
currently no standards or specific protocols to guide restoration of montane forest habitat,
and these are needed.
9) Conservation research and monitoring since 1994 on the Hispaniolan wintering grounds
of Bicknell’s Thrush leave few doubts that winter habitat loss and degradation pose the
greatest single threat to the species’ long-term survival (e.g., Rimmer et al. 2001a, 2003,
2005). Remaining forest cover in the Dominican Republic is estimated at <10%, while
fewer than 1.5% of Haiti’s original forests are intact (Stattersfield et al. 1998). Despite
chronic deforestation and a poor protected lands system in both countries, tangible
opportunities exist to conserve mesic and wet broadleaf forests on Hispaniola. We
propose the formalization of an international consortium that includes GMNF, VINS, and
other U.S. and Hispaniolan conservation partners, to plan and implement Hispaniolan
conservation projects that target Bicknell’s Thrush. One viable and far-reaching
mitigation measure that this consortium might promote is a designated fund for the
protection, restoration and management of Bicknell’s Thrush habitat in the Dominican
Republic and Haiti. A source of revenue for such a fund, which would be administered
by a Steering Committee, would be contributions from ski areas and other entities that
alter or remove montane forest vegetation in North America. For every hectare of habitat
converted from its natural state to one used for skiing or other development, a
contribution would be set aside to enable the purchase or restoration of thrush wintering
habitat. Specific conversion metrics would need to be resolved for such an innovative
program, but the concept has met with enthusiasm from habitat conservationists at both
ends of the species’ range.
B. Post-management Habitat Maintenance. – Bicknell’s Thrush nest mainly in low, dense fir-
spruce on high exposed ridges, in blow-downs, or in fir-wave areas. Small numbers have also
been found in mixed forests, particularly in the presence of a well developed conifer sapling
layer. Areas along ski trails or other human-created openings often mimic these naturally
disturbed forest types, and development of suitable habitat structure often is greatly accelerated
because of increased exposure. In the Adirondack Mountains and New England, Bicknell’s
Thrush nest mainly above 900 m in elevation and occasionally lower if the habitat is appropriate.
Furthermore, it appears that birds regularly descend below 900 m for foraging, especially early in
the breeding season (Rimmer et al. 2001a). It should be noted that there are few data on
fledgling or post-breeding dispersal in fall, but that both juvenile and adult thrushes have been
documented to use lower elevation forests at this time.
VINS’ recent research has determined that leaving fir-spruce cover along ski trail edges can
maintain habitat for Bicknell’s Thrush by providing suitable structure and a buffer (Rimmer et al.
2004). Bicknell’s Thrush will use these areas for foraging, perching, and for cover when moving
along and crossing trails. VINS has also documented regular nesting in narrow buffers covered
with low, dense fir-spruce along ski trails.
The following specific measures could maintain and enhance appropriate habitat on ski areas
or other managed sites for Bicknell’s Thrush.
1) Vegetation management is warranted mainly in areas where the adjacent forest is fir-
spruce dominated and characterized by a high stem density in the understory, often
forming a dense thicket. Taller (>5 m) trees may be present, but these are often damaged
by wind and/or insects and do not form a complete canopy, thus promoting understory
growth. In these areas, which may include only one (usually the wind-exposed) side of a
ski trail, low fir-spruce should be allowed to extend along the edge outward for 3-6 m (or
wider) at heights of 0.5-2.0 m (or higher). An attempt should be made to “feather” such
vegetation at the edge of ski trails or other permanent openings, i.e., gradually decreasing
tree height from the forest to the grassy trail edge. When these areas are cut back, there
should be an attempt to maintain woody vegetation at heights of 0.5 m or more. Also,
regeneration cuts should be made as infrequently as possible to maximize habitat
availability and continuity.
2) Management of gladed skiing trails for Bicknell’s Thrush is important to maintain habitat
integrity within ski areas. Although the effects of glading have not been studied, any
removal of understory montane forest vegetation is likely to reduce habitat suitability for
Bicknell’s Thrush. To minimize adverse impacts, existing gladed trails in suitable habitat
should be kept as narrow as possible, and remaining patches of low, dense fir-spruce
should be left intact or minimally altered. In general, new glading should be restricted to
hardwood forest stands. In any habitat type, new glading should stringently minimize
understory removal. Annual maintenance should ensure that some tree saplings are
retained, so there is continual recruitment to older age classes. This will help to prevent
tree mortality events that could cause the longer-term conversion of gladed trails to
completely open trails. Altering the spatial arrangement of maintenance cutting in
established glades, to allow for areas that are continually regenerating, might help
promote habitat suitability for Bicknell’s Thrush.
3) Concerted efforts should be made to prohibit any unauthorized gladed trail establishment
or maintenance, or unauthorized habitat alteration (i.e., cutting) of any kind. The
proliferation of trails illicitly cut by recreational, off-trail skiers has been recently
documented by VINS on some Vermont ski areas and should be actively discouraged.
4) Another potential habitat enhancement for Bicknell’s Thrush involves islands of trees in
ski trails. Islands often have a low, dense fir-spruce component and provide crossing
points for Bicknell’s Thrushes, which tend to avoid wide crossings of open ski trails.
Maximizing the size of islands between ski trails will benefit movements of Bicknell’s
Thrush between patches of suitable habitat and may provide increased nesting
opportunities. In situations where one or more small (generally < 0.5 ha) islands can be
combined into a single, larger island, Bicknell’s Thrush habitat will be improved.
5) The timing of vegetation management in areas of Bicknell’s Thrush breeding habitat is
important and should be delayed until August 1, when the majority of nesting activities
are complete (see above).
6) Trail areas that are appropriate for thrush habitat management should be explicitly
identified in individual ski area management plans, subject to input and regular review by
GMNF staff and regional experts in Bicknell’s Thrush ecology. Each management plan
and its accompanying map should be reviewed annually by the GMNF, appropriate ski
area staff, and other concerned agencies. Specific management actions should be
identified, progress on previously-identified actions should be assessed, and measures
should be taken to ensure compliance/accountability by each ski area.
Collaborative Potential
Increased public awareness of the uniqueness and vulnerability of montane forest habitats in
the Northeast will be necessary for full and successful implementation of any conservation plan.
This can be best accomplished by the synergistic efforts of public agencies (National Forests,
state agencies), non-governmental organizations (e.g., VINS, Audubon Vermont), private
landowners (e.g., ski areas, timber companies, wind power companies, telecommunications
companies), and citizens’ groups. These groups have unique individual and collective
opportunities to showcase Bicknell’s Thrush habitat management efforts and to educate the
public about montane forest ecology and conservation. Educational efforts could take many
forms, including, but not limited to: (1) development of informational displays at ski areas,
National Forest or State-owned facilities, and private lands on or near montane forest habitats;
(2) offering public programs on various topics relating to montane forest ecology, including
Bicknell’s Thrush; (3) sponsorship of summer field trips at ski areas or other sites to learn about
Bicknell’s Thrush and other aspects of montane forest ecology; (4) development of booklets or
brochures for the GMNF, ski areas, and other publicly-visited facilities summarizing montane
forest ecology, with a focus on Bicknell’s Thrush; and (5) development of simple or annotated
checklists to the birds of the GMNF, to individual ski areas, and to other montane forest sites
visited by the public. Efforts should be made to educate visitors at all times of year.
Efforts should be made to increase collaborative support for and participation in monitoring
and research programs that focus on Bicknell’s Thrush on the GMNF, such as VINS’ Mountain
Birdwatch program and long-term research program on Stratton Mountain. To ensure the
perpetuation of these and other important programs, ongoing commitments of financial, logistic,
and human resources will be necessary, both on the GMNF and throughout the breeding range of
Bicknell’s Thrush.
Expanded collaboration between GMNF, the USDA Forest Service International Program
(USFSIP), VINS, other U.S. partners, and conservation partners on Hispaniola should be
promoted. The beginnings of a working partnership have been established among USFSIP,
VINS, GMNF, and The Nature Conservancy, following a successful February, 2002 field trip to
the Dominican Republic. This international consortium should be diversified and strengthened
to assist in planning and implementing Hispaniolan conservation programs that target Bicknell’s
A management team should be formed to oversee and guide conservation planning for
Bicknell’s Thrush on the GMNF, and throughout its breeding range. This should include
representatives from groups most closely involved in research, monitoring, management, and
regulatory issues related to Bicknell’s Thrush. The team would identify research and monitoring
needs, review and revise as necessary adaptive management practices, craft conservation and
regulatory recommendations, develop educational approaches, and critically assess the species’
conservation status.
Research and Monitoring Needs
Many aspects of the breeding and wintering ecology, demography, and behavior of
Bicknell’s Thrush remain poorly known. Lack of baseline population data and logistical
difficulties hinder attempts to clarify the species’ conservation status. The regionwide Mountain
Birdwatch program has been established to determine breeding population trends and
distributional changes. Similar efforts are warranted on the wintering grounds, where limiting
factors may be most severe. Until the interactions of limiting factors throughout the annual cycle
of Bicknell’s Thrush are better understood, it will not be possible to prescribe fully meaningful
management and conservation measures. While the Forest Service is not expected to play a
major role in research and monitoring efforts, collaboration by GMNF staff via a regional
management team or international consortium, could prove valuable in achieving the following
1. Development of accurate methods to census populations and estimate densities are
needed in both areas. Accurate calculations of total population size, based on GIS
projections of occupied habitats and spatially-explicit density estimates, are needed
throughout the breeding range.
2. Development of a Habitat Suitability Index and its incorporation in a spatially explicit
Population Viabilty Analysis (PVA) would be a critical step in developing ecological risk
assessments and sound conservation planning for Bicknell’s Thrush. A spatially explicit
PVA is necessary to better understand how local (development projects at single sites),
regional (atmospheric depositions, forest disturbance), and continental (global climate
change) perturbations will interact to limit the species’ population. Parameterizing a
spatially explicit model with long-term data on demographic parameters (density, birth
rates, survival rates, and dispersal rates) would enable a better assessment of the severity
of anthropogenic threats, and would guide development of management practices. This
would be of particular importance in planning how best to use limited resources of time
and funds for conservation and management. A spatially explicit PVA model could be
used to assess risks to other high elevation species such as Blackpoll Warbler (Dendroica
striata) and might be applicable to mammals (e.g., northern bog lemming; Synaptomys
borealis), insects such as carabid beetles (Strong et al. 2002), and plants.
3. Many landscape-level questions about the ecology and population dynamics of Bicknell’s
Thrush require focused research. Information is needed on reproductive success,
demographics, and site persistence in habitat patches of different size and isolation; on
the possible existence of source/sink population dynamics; on levels of population
interchange among habitat patches. The apparent male-biased breeding sex ratio requires
rangewide investigation; its causes and demographic/ecological correlates must be
determined. Accurate estimates of breeding population density in different habitat types
across the species’ range are needed. The species’ breeding status and ecology in
regenerating clearcuts in both montane and low elevation forests of Maine and Maritime
Canada should be more closely investigated.
4. Natal dispersal and migratory connectivity should be investigated through (a)
determination of biogeochemical signatures in natal feathers of Bicknell’s Thrush from
each major breeding region and subsequent matching with feathers of yearling birds
captured at breeding sites; and (b) determination of biogeochemical signatures in adult
thrush feathers from each major breeding region, and subsequent determination of the
breeding origin of individuals captured on their Greater Antillean wintering grounds.
Documenting dispersal movements and geographic linkages at local, regional and
hemispheric levels will provide crucial life history data and should promote collaborative,
international commitments for the conservation of Bicknell’s Thrush.
5. Expanded research is warranted on the demographic effects of mercury body burdens in
Bicknell’s Thrush and possible interactions with calcium depletion and other potential
stressors. Recent studies have shown that thrushes throughout the breeding range exhibit
elevated mercury concentrations in their blood and feathers (Rimmer et al. 2005).
Additional studies are needed to examine whether atmospheric deposition of acid ions
and mercury interact synergistically to negatively affect thrush populations. Long-term
monitoring of both mercury burdens and thrush demography stands to contribute ground-
breaking scientific and conservation data. Studies of mercury burdens are also needed on
the species’ wintering grounds, where preliminary data show blood concentrations to be
up to three times higher than in the Northeast (Rimmer et al. 2005).
6. Research is needed on potential effects of food availability and its temporal-spatial
variability on breeding system structure and reproductive success; diets of adults,
nestlings and fledglings; post-fledging dispersal and habitat use; post-breeding
movements and habitat use of adults;
7. Site-specific research is needed on effects of human activities (e.g. recreational
development, telecommunications towers, wind turbines) on behavior, spacing patterns,
reproductive success, and population persistence over time. Such studies should include
both pre- and post-construction phases, and ideally should be replicated in space and
8. The influence of hikers on breeding Bicknell’s Thrush activity needs more focused study.
Use of foot paths by day hikers and backpackers has steadily increased on both the
GMNF and WMNF (Stokowski et al. 2005, USDA Forest Service 2005). Yet, the impact
of hikers on Bicknell’s Thrush behavior or breeding success has received little attention.
Anecdotal evidence from Stratton Mountain and Mt. Mansfield suggests that incubating
or brooding females become habituated to nearby foot traffic, and that male and female
movements are unaffected by narrow openings (Rimmer and McFarland, unpubl. data).
Studies of other avian species, however, have shown that recreational activities may
reduce singing activity (Gutzwiller et al. 1994), nest placement (Knight and Fitzner 1985)
nesting success and survival (Knight and Gutzwiller 1995), and population size (Yalden
1992). These findings underscore the need for investigations of hiker effects on
Bicknell’s Thrush and other songbirds breeding in montane forests.
9. Experimental research should be carefully considered to determine whether it is possible
to enhance Bicknell’s Thrush habitat in a manner that mimics the types of natural
disturbance to which the species is adapted. Bicknell’s Thrush occurs in regenerating
forest stands following timber harvest at middle elevations in New Brunswick and Nova
Scotia and at upper elevations in Maine and southern Quebec. However, the value of
these areas as breeding habitat is unknown. A rigorous investigation would require
replicated habitat treatments in a region where Bicknell’s Thrushes are sufficiently
abundant to provide adequate sample sizes. The larger habitat units of the GMNF are
well suited for such an investigation. If techniques can be developed to improve
Bicknell’s Thrush habitat, they could potentially offset effects of habitat loss due to ski
area expansion and telecommunications or wind power development. However, it is
important to emphasize that such habitat manipulations should not substitute for
maintaining the natural integrity of montane forests, whose dynamics provide a
continually shifting, but unpredictable, mosaic of suitable thrush habitat.
10. In winter, distribution and habitat use of Bicknell’s Thrush in Cuba and Haiti, and to a
lesser extent Jamaica, need to be better understood. The protected status of core
wintering areas must be carefully assessed, and needs for further protection specifically
identified. Occupancy of primary vs. second-growth winter habitats needs study, as does
existence of possible habitat segregation by sex and/or age. Demographic and ecological
studies are needed to investigate microhabitat use, overwinter survival and site
persistence by age and sex, between-winter site fidelity and survivorship. Spacing
patterns and movements of age and sex classes throughout winter need further study, as
do possible seasonal shifts in diet and body condition. The possibility of restoring
degraded winter habitats holds promise but needs careful study.
Action Plan
While long-term conservation of Bicknell’s Thrush and its breeding habitats may ultimately
depend on a multilateral, international effort to halt or reverse the effects of acid precipitation
and global climate change in the Northeast, a short-term strategy is needed to monitor the
species’ status and minimize the risk of its becoming imperiled. The following proposed actions,
while not exhaustive, apply to both the GMNF and the full breeding range of Bicknell’s Thrush,
and are minimal components of such a strategy. Most follow from the needs and
recommendations identified above. Those recommendations most specific to the GMNF are
listed first, followed by those in which GMNF’s role would likely be more collaborative than
1. A species management team should be formed to oversee and guide conservation
planning for Bicknell’s Thrush on the GMNF, and throughout its breeding range.
2. Each ski area on the GMNF should develop a management plan targeted towards
Bicknell’s Thrush habitat. This should include spatially explicit identification of existing
habitat and its occupancy by Bicknell’s Thrush, a prioritized list of opportunities for
habitat enhancement, and an annual agenda of specific management activities. Each plan
should be annually reviewed by a GMNF Bicknell’s Thrush management team, and its
outcomes carefully evaluated.
3. The Mountain Birdwatch program should be expanded, both within the GMNF and
throughout the northeastern U.S. breeding range of Bicknell’s Thrush, and annual trend
analyses should be conducted.
4. Spatially explicit density estimates should be obtained throughout the GMNF and entire
breeding range of Bicknell’s Thrush. These should be correlated with quantifiable
habitat variables and used to construct a GIS model that will enable reasonable estimates
of total population size at different spatial scales, following a model recently developed
for the WMNF (Hale 2006).
5. Small-scale experimental manipulation of forest stands above 2500’ should be considered
in designated areas for the purpose of developing habitat improvement techniques for
Bicknell’s Thrush. Such manipulation should strive to mimic natural disturbance and be
limited to hand-felling or girdling of mature trees to encourage softwood regeneration.
Soil disturbance and removal of trees or debris would be avoided by this method.
6. Long-term demographic and ecological research and monitoring at VINS’ Stratton
Mountain study site, part of which lies within GMNF boundaries, should be continued.
This is the only intensive, long-term Bicknell’s Thrush study site within the species’ U.S.
breeding range. Maintenance of a core research program on Stratton will be critical to
understanding the long-term population dynamics of Bicknell’s Thrush.
7. Collaborative efforts should be made to develop informational materials on the GMNF
and elsewhere that showcase Bicknell’s Thrush habitat management efforts and educate
the public about montane forest ecology and conservation.
8. An international consortium of GMNF, USFSIP, VINS, The Nature Conservancy, and
conservation partners on Hispaniola should be formalized and diversified. This group
should identify, prioritize, and plan Hispaniolan conservation projects that target
Bicknell’s Thrush.
9. A spatially explicit PVA should be conducted and parameterized with long-term
demographic data that can be annually updated. Periodic sensitivity analyses should be
conducted to guide further research.
Unpublished data from the breeding and wintering grounds of Bicknell’s Thrush were
generously shared by Yves Aubry, Dan Busby, David Evers, John Faaborg, Steven Latta, Joe
Marshall, Ghislain Rompré, Gilles Seutin, Allan Strong, Ann and Robert Sutton, and Joseph
Wunderle. We express sincere thanks to the many individuals who endured trying field
conditions to assist in VINS’ data collection on the species’ breeding and wintering grounds.
Funding for our field studies since 1992 was provided by sources too numerous to list, and we
are grateful to all of them. Funding for preparation of this report was provided by the USDA
Forest Service, Green Mountain National Forest.
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Figure 1. Generalized breeding and wintering distribution of Bicknell’s Thrush, with probable
migration routes.
Figure 2. Documented Bicknell’s Thrush occurrences in the U.S., 1992-2004.
Figure 3. Bicknell’s Thrush habitat on the Green Mountain National Forest.
2001 2002 2003 2004
# individuals per point (+SE)
Figure 4. Relative abundance of Bicknell’s Thrush on 47 Mountain Birdwatch survey routes
monitored each year between 2001 and 2004. Column numbers represent proportion of survey
routes on which Bicknell’s Thrush was detected by 5-minute point count in each year.
0.51 0.47 0.45
y = -0.024x + 0.315
= 0.8727
2001 2002 2003 2004
# of BITH per point
Figure 5. Relative abundance of Bicknell’s Thrush on 47 Mountain Birdwatch survey routes
monitored each year between 2001 and 2004. Slope of regression line corresponds with an
annual decline of 9.0% (t = 3.703, P = 0.066, R² = 0.873).
Table 1. Green Mountain National Forest peaks with confirmed detections of Bicknell’s Thrush,
Abraham, Mount
Battell Mountain
Bloodroot Mountain
Boyce Mountain
Bread Loaf Mountain
Cape Lookoff Mountain
Cleveland, Mount
Cutts Peak
Deerfield Ridge
Dorset Peak
Ellen, Mount
Farr Peak
Gillespie Peak
Glastenbury Mountain
Grant, Mount
Haystack Mountain
Horrid, Mount
Lincoln Peak
Monastery Mountain
Nancy Hanks Peak
Peru Peak
Romance Mountain
Roosevelt, Mount
Round Mountain
Searsburg Ridge (or Unnamed ridge 8.8 km SW of Haystack Mountain)
Snow, Mount
Stratton Mountain
Styles Peak
Unnamed peak 0.5 km S of Gillespie Peak
Unnamed peak 0.9 km N of Styles Peak
Unnamed peak 0.9 km S of Lincoln Gap
Unnamed peak 0.9 km S of Mount Abraham
Unnamed peak 1.0 km NW of Farr Peak
Unnamed peak 1.4 km NE of Mt. Roosevelt
Unnamed peak 1.5 km SW of Stratton Mtn.
Unnamed peak 2.0 km NW of Farr Peak
Unnamed peak 2.7 km E of Goshen Mtn.
Unnamed peak 3.2 km W of Glastenbury Mtn.
Unnamed peak 4.6 km S of Glastenbury Mtn.
Unnamed peak 6.4 km NE of Glastenbury Mtn
Wilson, Mount
Worth Mountain
... The logistics of obtaining data on individual-level processes (birth, death, immigration, emigration) for a large number of subpopulations are unrealistic. The need for a more reliable estimate of population size, identification of population sources and sinks, factors affecting occupancy patterns, and within-patch habitat suitability are all crucial next steps to improve conservation efforts and management strategies for this species (Rimmer et al. 2001b, Frey 2008. ...
... Among Neotropical migrant birds in the northeastern United States, Bicknell's Thrush (Catharus bicknelli) is ranked as the species most at risk of extinction, and thus of highest conservation priority (Pashley et al. 2000, Rimmer et al. 2001a, 2001b. Bicknell's Thrush is also one of the least-known breeding species of eastern North America, a fact that has precluded its formal consideration for federal endangered or threatened status. ...
... Catharus bicknelli (Ridgway) (Bicknell's Thrush) is a rare, forest-dwelling passerine that nests at upper elevations in New York, Vermont, New Hampshire, and Maine (Atwood et al. 1996), and in scattered highland and coastal areas of southeastern Canada (Nixon 1999, Ouellet 1993 ). Its global population may number fewer than 50,000 individuals; however, habitat and density data are too scarce to permit an accurate estimate (Rimmer et al. 2001Rimmer et al. , 2005a). A Bicknell's Thrush abundance model for New Hampshire's White Mountains, derived from point counts, habitat measurements, and satellite imagery , produced an estimate of 5000 birds (95% CI = 900–23,000; Hale 2006). ...
Catharus bicknelli (Bicknell's Thrush) is a rare inhabitant of mountain forests in the northeastern United States and southeastern Canada. Conservation planners consider the species to be at risk, although evidence of population decline has thus far been localized or inconclusive. In order to assess the status of Bicknell's Thrush in the White Mountains of New Hampshire, we conducted point-count surveys on 40 forested, high-elevation routes from 1993 to 2003. Non-linear regression oil aggregate counts revealed a 7% annual decline over this period (P < 0.1). We discuss possible threats to Bicknell's Thrush, including winter habitat loss, pollution of mountain ecosystems, climate change, and human intrusion during breeding. A range-wide monitoring program that incorporates new survey methods is needed to help identify limiting factors and reduce potential sources of error and bias. This study underscores the importance of efforts to monitor and conserve Bicknell's Thrush.
In 1989, 1990, and 1991, we conducted experiments on 30 circular 1.0-ha sites to assess whether human intrusions during a 10-week period influenced the occurrence and consistency of primary song in breeding subalpine birds. Using only those weekly censuses during which a species was present at a site, we computed song occurrence as the percentage of censuses during which a species sang, and we calculated singing consistency as the maximum number of consecutive censuses during which a species sang. An intrusion bout involved one person who walked through a site for 1 or 2 h. We used a priori contrasts, involving habitat covariates when appropriate, to assess differences in song occurrence and singing consistency between control and intruded sites and between sites at which the inner 25% of the site was disturbed (S25) and those at which 100% of the site was disturbed (S100). Singing by a number of species did not appear to be influenced by intrusion. For several species, however, song occurrence and singing consistency were higher on control sites than on intruded sites, indicating intrusion reduced singing activity. Song occurrence was higher on S100 relative to S25 sites as well. This latter pattern may have emerged because all of the individuals using the S100 sites were able to observe us during repeated intrusions and discern that we were not predators, whereas most of the individuals using the S25 sites likely did not have this opportunity. Thus, some of the individuals using S25 sites may have reduced their singing to avoid detection by us. Because song is essential in territory defense, mate acquisition, and in other reproductive activities, levels of intrusion that alter normal singing behavior have the potential to lower the reproductive fitness of males that are sensitive to this form of disturbance.
The volume is broadly split into two main sections. The firsts consists of seven introductory chapters: biodiversity and priority setting; identifying endemic bird areas; global analyses; the prioritization of endemic brid areas; the conservation relevance of endemic bird areas; endemic bird areas as targets for conservation action; and regional introductions. The second, and larger part of the text looks at the endemic bird areas in detail. The section is split into six subsections, by region: North and Central America; Africa, Europe and the Middle East; continental Asia; SE Asian Islands, New Guinea and Australia; and the Pacific Islands. Within each regional subsection the endemic areas are detailed, providing information on : general characteristics; restricted-range species; threats and conservation; and location maps.