INVESTIGATING THE LIMITING FACTORS OF A RARE,
VULNERABLE SPECIES: BICKNELL’S THRUSH
JASON M. TOWNSEND,1,3 CHRISTOPHER C. RIMMER,2 AND KENT P. MCFARLAND2
Abstract. Bicknell’s Thrush (Catharus bicknelli) is a migratory songbird of high conservation concern.
Populations may be limited by events on the northeastern North American breeding grounds or by
events on wintering grounds in the Greater Antilles. We use mist-net captures of individuals on
both the breeding and wintering grounds to investigate sex ratio variation throughout the life his-
tory of Bicknell’s Thrush. We identify variation in the sex ratio at two distinct sites in the Dominican
Republic and discuss the possibility that females may be limited by dominance interactions with
larger, more aggressive males on the winter grounds.
Key Words: Bicknell’s Thrush, migratory songbird, montane forest, Dominican Republic, sex ratio,
winter habitat segregation.
INVESTIGANDO LOS FACTORES LIMITANTES DE UNA RARA Y
VULNERABLE ESPECIE: EL ZORZALITO DE BICKNELL
Resumen. El Zorzalito de Bicknell (Catharus bicknelli) es un ave cantora migratoria de gran preocupa-
ción entre los conservacionistas. Su población puede estar hoy viéndose limitada debido a acontec-
imientos sucedidos tanto en sus zonas de reproducción del nordeste norteamericano como en áreas
de invernada en las Antillas Mayores. En este examen utilizamos capturas de individuos en redes de
niebla, tanto en las zonas de reproducción como en las de invernada, para investigar la variación de
proporción sexual durante el ciclo de vida del Zorzalito de Bicknell. Identifi camos la variación en la
proporción de sexos en dos sitios distintos de la República Dominicana y discutimos la posibilidad
de que las hembras, podrían estar limitadas a causa de interacciones de dominio, que ocurren con
machos mayores y más agresivos, en las zonas de invernada.
Proceedings of the Fourth International
Partners in Flight Conference: Tundra to Tropics
91–95
INTRODUCTION
Bicknell’s Thrush (Catharus bicknelli) is one of
eastern North America’s most rare and range-
restricted breeding passerines. Considered one
of the Nearctic-Neotropical migrants of highest
continental conservation concern (Pashley et
al. 2000, Rich et al. 2004, Wells 2007), Bicknell’s
Thrush is classifi ed as globally “vulnerable” by
the International Union for the Conservation of
Nature (BirdLife International 2000). Limited
information on the species’ ecology and popu-
lation trends has held back its formal consid-
eration for federal endangered or threatened
status in the United States, although its federal
status in Canada is to be reviewed in 2009. In the
United States, the breeding range of Bicknell’s
Thrush is naturally restricted to coniferous
forests of New York and New England above
915 m elevation (Atwood et al. 1996, Lambert et
al. 2005). In Canada, the species is patchily dis-
tributed in coniferous forests of the Laurentian
Mountains, St. Lawrence River Valley and
Maritime Provinces (Rimmer et al. 2001). The
species’ wintering range is confi ned to the
Greater Antilles, with an estimated 90% of the
global population concentrated on Hispaniola
(Rimmer et al. 2001).
On both the breeding and wintering grounds,
Bicknell’s Thrush faces multiple threats. These
include climate change (Rodenhouse et al. 2008),
acid ion deposition (Johnson et al. 1992, Hames
et al. 2002), mercury contamination (Rimmer et
al. 2005a), mountaintop development (Rimmer
et al. 2001, 2004), forestry operations in Canada
(Chisholm 2005, Gardiner 2006), and habitat loss
and degradation on the species’ Hispaniolan
wintering grounds (Rimmer et al. 2001, 2005b;
1SUNY-College of Environmental Science and Forestry, Department of Environmental and
Forest Biology, 1 Forestry Drive, Syracuse, New York 13210, USA; and
2Vermont Center for Ecostudies, P.O. Box 420, Norwich, Vermont 05055, USA
3 E-mail: jatownse@syr.edu

Proceedings of the Fourth International Partners in Flight Conference92
Latta et al. 2006). Recent data provide confl ict-
ing results on Bicknell’s Thrush population
trends throughout its breeding range. On New
Hampshire’s White Mountain National Forest,
which supports an estimated 25% of the species’
United States population (Lambert et al. 2005),
a signifi cant annual decline of 7% was recorded
from 1993–2003 (King et al. 2007, Lambert et
al. 2008). In New Brunswick and Nova Scotia,
annual declines of 19% have been documented
over a 5-year period from 2003–2007 (Campbell
et al. 2008). In contrast, data from 31 survey
routes monitored in every year from 2001–2006
across the United States breeding range showed
no consistent trend (Hart and Lambert 2007),
although an annual decline of 9% was recorded
on 47 routes surveyed annually from 2001–2004
(Lambert 2005). A more recent analysis of
United States survey data indicated a signifi -
cantly increasing trend of 10% from 2001–2007
(Dettmers, Lambert and Hart, unpub. data).
To address the conservation needs of
Bicknell’s Thrush, the International Bicknell’s
Thrush Conservation Group formed in 2007. Its
initial charge is development of a conservation
action plan for the species. Ongoing research
examining the factors that limit Bicknell’s
Thrush survival on both the breeding and win-
tering grounds will contribute to an informed
and effective conservation plan. Here we pres-
ent preliminary summer and winter demo-
graphic data comparing sex ratios during both
seasons, and we discuss intersexual dominance
interactions during the winter period as a pos-
sible limiting factor affecting the survival of
females. We also outline recommendations for
future research.
METHODS
On the breeding grounds, we have conducted
long-term demographic research and monitoring
of Bicknell’s Thrush at two montane study sites
(900–1200 m) in the Green Mountains of Vermont
since 1995: Mt. Mansfi eld (44°32’N, 72°49’W)
and Stratton Mountain (43°05’N, 72°55’W). On
the wintering grounds, we have conducted both
broad-scale presence/absence surveys in Haiti
and the Dominican Republic, and intensive stud-
ies at two sites in the Dominican Republic. One
site is located in the Sierra de Bahoruco (18°12’N,
–71°32’W; 1700–1800 m elevation) and is charac-
terized by pristine cloud forest, while a second
site consists of moderately disturbed second-
ary rain forest in the Cordillera Septentrional
(19°25’N, –70°8’W; 300–500 m elevation). At
both the breeding and wintering grounds sites,
birds were captured in passive arrays of mist-
nets and by using playback of conspecifi c song
to lure birds into nets. Approximately 80 μL of
blood was taken from each captured individual
with heparinized capillary tubes via brachial
venipuncture using sterile 27-gauge hypoder-
mic needles. Blood samples were stored in
0.5 mL blood lysis buffer [100 mM Tris-HCl, pH
8; 100 mM Na2 EDTA; 10 mM NaCl; 0.5% SDS;
White & Densmore 1992)]. We extracted DNA
from each sample using Perfect gDNA Blood
Mini kits (Eppendorf) following the manufac-
turer’s protocol. Extracted DNA was then ampli-
fi ed by polymerase chain reaction to determine
gender (Griffi ths et al. 1998). All birds were also
banded with aluminum bands and a unique
combination of color bands for mark–recapture
and resighting analysis. At wintering grounds
sites, we also conducted radio telemetry stud-
ies from 2005–2008 to identify any differences
in the degree of territorial behavior between the
sexes. Birds were fi tted with 1.2 g radio trans-
mitters (Model BD2G, Holohil Systems, Ltd.).
Transmitters were attached using the backpack
harness method (Rappole and Tipton 1991). All
birds were checked for full leg and wing move-
ment before release. Locations were achieved for
radio-tagged birds by stealth homing to within
an estimated distance that did not force the birds
to fl y from the area.
RESULTS
Based on capture data in Vermont, the mean
sex ratio among breeding adults was 2.5 males:1
female (n = 1,231; 1993–2007). The mean sex
ratio of fl edglings leaving the nest, however,
was 1 male: 1.5 females (n = 27 fl edglings). On
the Hispaniola wintering grounds, the mean sex
ratio for birds sampled island-wide in 2004 was
1.9 males: 1 female (n = 142). At intensive sites,
the Sierra de Bahoruco (SDB) population had
a highly male-skewed sex ratio of 3.8 males: 1
female (n = 25), whereas the population in the
Cordillera Septentrional (CS) showed a 1:1
ratio (n = 25). There was no signifi cant differ-
ence between male (n = 29) and female (n = 12)
mean territory size at either site. Territoriality is
the prevailing form of resource defense at both
the SDB male-skewed site and the CS balanced
sex ratio site, and individual core use areas
show minimal overlap. Raw banding return
rates at SDB indicated that 23% of birds banded
between 2004 and 2007 returned in at least one
subsequent winter, whereas return rates at CS
during this four-year period were 7%.
DISCUSSION
The male-biased sex ratio of adult Bicknell’s
Thrushes on both the species’ breeding and

Limiting Factors for Bicknell’s Thrush—Townsend et al. 93
wintering grounds, compared to the nearly
equal sex ratio among nestlings, suggest that
limiting factors act disproportionately on
females at some point in the annual cycle. On
Hispaniola, males predominate in pristine cloud
forest at SDB, where site fi delity as refl ected by
raw banding return rates is more than 3 times
greater than in secondary rainforest at CS.
Stronger philopatry at the male-dominated
SDB site, coupled with a higher rate of within-
season recaptures at this site, suggest that SDB
may provide higher quality habitat than the
CS site. Additionally, preliminary analysis of
fecal samples and stable isotopic composition
of blood samples suggest that birds at SDB con-
sume an arthropod-heavy diet whereas birds at
CS, where available arthropod biomass is lower
than at SDB, rely primarily on fruit (Townsend
and Rimmer unpubl. data). There may be an
elevated nutritional value to omnivorous birds,
especially during pre-migratory fattening, of
a diet dominated by arthropods (Long and
Stouffer 2003).
It is possible that female Bicknell’s Thrushes
wintering in the Dominican Republic tend to
occupy sub-optimal habitat, such as the CS site,
and that this may compromise their overall fi t-
ness. Females may preferentially occupy these
sites, or they may be forced to inhabit them as
the result of dominance interactions with males.
For example, in American Redstarts (Setophaga
ruticilla) wintering in Jamaica, females are lim-
ited by agonistic interactions with larger, more
aggressive males (Marra et al. 1993, Marra
2000). In this system, females are relegated to
lower quality habitat by the behavioral domi-
nance of males who maintain territories in the
highest quality habitats. Females wintering in
sub-par habitat display lowered body condi-
tion at the end of the wintering season which
can lower chances of survival during migration
(Marra and Holmes 2001) or cause carry-over
effects during the breeding season that nega-
tively affect nesting success (Marra et al. 1998,
Norris et al. 2003). It is possible that a similar
dynamic could be operating among populations
of wintering Bicknell’s Thrushes.
Further investigations will more closely
examine differences in Bicknell’s Thrush body
condition, survivorship and the quality of
dietary items between our SDB and CS study
sites. If habitat quality at the CS site, where
females at least equal males in abundance and in
some years outnumber them, is poor compared
to that at the male-dominated SDB site, females
may end the winter in poor condition. As a
result, females may experience lower survival
during the energetically-taxing spring migra-
tion. This could have signifi cant impacts on
this species’ overall population dynamics and
might account in large part for the male-skewed
adult sex ratio on the breeding grounds. Further
research priorities will include investigation of
post-fl edging ecology and demography, migra-
tion stop-over ecology, migratory connectiv-
ity, seasonal carry-over effects, and the use of
remote sensing to quantify available habitat on
the Greater Antillean winter range. We believe
that protection and restoration of mid-elevation
secondary forests on Hispaniola may be crucial
to the conservation of Bicknell’s Thrush.
ACKNOWLEDGMENTS
We gratefully acknowledge funding sources
too numerous to list for support of our studies
on Bicknell’s Thrush over the past 15 years, both
in the U.S. and on Hispaniola. We also thank an
international legion of fi eld biologists and vol-
unteers for assistance in collecting data. Many
partners have provided invaluable logistical
help in enabling us to conduct our work, and
we offer sincere thanks to all. Our participation
in the McAllen Partners in Flight conference
was made possible by a generous grant from
the Stewart Foundation. We thank R. Dettmers
and S.L. Jones for constructive reviews of this
manuscript
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