IBTCG Sponsored Symposium
Ecology and conservation of a rare, declining species: Bicknell's Thrush
Presentations:
BICKNELL'S THRUSH – CONSERVATION IN THE FACE OF LONG ODDS
Rimmer, C. C., Vermont Center for Ecostudies, Norwich, Vermont, USA
Hart, J. A., Vermont Center for Ecostudies, Norwich, Vermont, USA
Dettmers, R. P., U.S. Fish and Wildlife Service, Hadley, Maasachusetts, USA
Whittam, B., Bird Studies Canada, Sackville, New Brunswick, Canada
McFarland, K. P., Vermont Center for Ecostudies, Norwich, Vermont, USA
Aubry, Y., Canadian Wildlife Service, Ste. Foy, Québec, Canada
Abstract: Bicknell’s Thrush is among the Nearctic-Neotropical migrants of highest conservation priority in North America. Its
rarity (estimated global population of < 50,000 individuals), restricted breeding and winter distributions, habitat
specialization, and vulnerability to multiple rangewide threats are compounded by incomplete knowledge about its
overall status. The species exhibits a complex mating system and a highly skewed breeding adult sex ratio. Sexual
habitat segregation may occur in winter, and females appear to be limited at some point in the annual cycle. Recent
population trend data show conflicting results, with sharply declining populations in Maritime Canada and stable or
slightly increasing trends in the U.S. On the species’ Greater Antillean wintering grounds, where an estimated 90% of
the global population occurs on Hispaniola, loss of forested habitats has been severe and is ongoing. The International
Bicknell’s Thrush Conservation Group (IBTCG) formed in 2007 to develop and implement a scientifically-based
conservation action plan for the species. We will present an overview of the conservation challenges facing Bicknell’s
Thrush, highlight the IBTCG’s primary goals and recommendations, and assess the prospects for this at-risk species.
GENDER AND AGE DISTRIBUTION OF BICKNELL'S THRUSH AT TWO WINTERING SITES IN THE 
DOMINICAN REPUBLIC: EVIDENCE FOR SEXUAL HABITAT SEGREGATION
Townsend, J. M., SUNY-College of Environmental Science and Forestry, Syracuse, NY
Rimmer, C. C., Vermont Center for Ecostudies, Norwich, VT
McFarland, K. P., Vermont Center for Ecostudies, Norwich, VT
A number of studies have shown sex and age-based habitat segregation to be an important component of winter
season population limitations among migrant songbirds. In this study, we investigate the age and gender distribution
of wintering Bicknell’s Thrushes in the Dominican Republic. Between 2000 and 2008 we focused on two floristically
and climatically distinct sites: one in high elevation cloud forest, and the other in mid elevation rainforest. We found
the cloud forest site to be significantly male-biased in comparison to the rainforest site. The mean cloud forest
proportion male was 74% ± 11%. In contrast, the rainforest site did not differ significantly from a 1:1 male to female
ratio and, rather than being characterized as a “female” site, is best labeled as having an equal sex ratio. The mean
proportion male at this site was 53% ± 2%. The distribution of juvenile birds did not differ between the sites. We
identified several differences in the spatial behavior, diet, and body condition of birds at the two sites and we discuss
these in the context of the differing gender distributions.
MODELING POPULATON GROWTH OF BICKNELL'S THRUSH ACROSS A 
LATITUDINAL GRADIENT
McFarland, K. P., Vermont Center for Ecostudies, Norwich, Vermont, USA
Rimmer, C. C., Vermont Center for Ecostudies, Norwich, Vermont, USA;
Aubry, Y., Canadian Wildlife Service, Ste-Foy, Québec, Canada;
Sillett, T. S., Smithsonian Migratory Bird Center, Washington, DC, USA;
Paradis, S., Canadian Wildlife Service, Ste-Foy, Québec, Canada
Bicknell’s Thrush tops most North American lists of species of conservation concern because it occupies a limited,
fragmented distribution and faces multiple habitat threats. Between 1993-2003, Bicknell’s Thrush in New Hampshire
experienced annual declines of 7% per year. Monitoring in Canada’s Maritime provinces (2002-2008) indicated
declines as steep as 20% annually, while surveys at Mont Gosford, Quebec from 2001-2007 showed a decline in the
number of occupied sites. Here we present the first robust estimates of annual survival and fecundity for this species,
and we examine population growth rates spanning a latitudinal gradient from Stratton Mountain, VT (1997-2008) in
the southern portion of its breeding range to Mount Mansfield, VT (1993-2008) in the central and Gaspe, Quebec
(1998-2003) in the north. We used capture-mark-recapture models to estimate survival of adults. Estimates of annual
fecundity (females/female) were determined with the aid of radio telemetry to calculate renesting rates and molecular
methods to determine fledgling sex ratios. Our estimates of finite rate of population increase indicated negative
growth following cone mast years (high red squirrel abundance) and positive growth after mast failures.
BICKNELL'S AND SWAINSON'S THRUSHES IN REGENERATING NEW BRUNSWICK CLEARCUTS:
NICHE
SEPARATION BY NEST SITE AND DIET
McKinnon, E. A., University of New Brunswick, Fredericton, New Brunswick, Canada,
Fraser, K. C., University of New Brunswick, Fredericton, New Brunswick
Diamond, A. W., University of New Brunswick, Fredericton, New Brunswick
Bicknell’s Thrush (BITH) (Catharus bicknelli) is sympatric with Swainson’s Thrush (SWTH) (C. ustulatus) in
regenerating clearcuts in north-central New Brunswick. We determined if BITH and SWTH show niche separation by
nest site and or diet. Vegetation was characterized in 5-m radius patches around nests and ‘non-use’ points. The ratio
of stable nitrogen isotopes (deltaN) was used to determine dietary niche-width. Initial analysis of blood using stable
hydrogen isotopes revealed an effect of wintering grounds until mid-June; thus deltaN of feces was analyzed to obtain
a value for local diet. BITH but not SWTH nest patches differed from non-use patches. Fecal samples are currently
being analyzed and we predict that BITH will show a smaller trophic niche width than SWTH, reflecting relative
specialist versus generalist diet. Despite evidence that regenerating clearcuts are relatively poor in biodiversity, we
found that closely related, morphologically similar species still chose different nest sites. Our results suggest that
stable isotope values should be interpreted with caution, since blood collected early in the breeding season may be
influenced by diet on the wintering ground or during migration.
POPULATION-LEVEL IMPLICATIONS OF CLIMATE CHANGE FOR A MONTANE FOREST SONGBIRD
,
BICKNELL’S THRUSH
Frey, S. J., Oregon State University, Corvallis, Oregon, USA
Strong, A. M., University of Vermont, Burlington, Vermont, USA
McFarland, K. P., Vermont Center for Ecostudies, Norwich, Vermont
High elevation species are among those predicted to be at greatest risk from climate change. Identifying critical
montane habitat patches is essential to promote conservation of these species. We examined population-level
responses of a montane forest songbird, Bicknell’s Thrush, to climate warming. We combined local (stand
characteristics) and landscape (patch size) scale habitat models to predict spatial and temporal distributions of habitat
suitability. Using these models, we predicted probability of occupancy, local colonization and extinction across
Vermont under current conditions and with a 1 deg C increase in mean July temperature. An increase of 1 deg C
results in a dramatic reduction of montane habitat with substantial loss of small patches. Although there were fewer
patches under the future scenario, those that remained were of highest local quality (high boreal basal area). We found
that average probability of site occupancy and colonization decreased slightly, however, extinction probability
increased considerably. Under limited warming, Bicknell’s Thrush could persist given favorable local habitat
conditions in remaining patches; however, increases of more than 1 deg C threaten the future persistence of the
species.
IBTCG members present at the AOU Meeting.
