The Wilson Journal of Ornithology 123 ( 2 ): 367 — 372 , 2011 
SEX AND AGE RATIOS OF BICKNELL’S THRUSH WINTERING 
IN HISPANIOLA 
JASON M. TOWNSEND, 14 CHRISTOPHER C. RIMMER, 2 
ANDREA K. TOWNSEND. 1 AND KENT P. McFARLAND 2 
ABSTRACT.—We investigated sex and age ratios of wintering Bicknell's Thrush (Catharus hicknelli) across a 
geographic gradient of sites on Hispaniola. The island-wide proportion male was 0.64 (;i = 127), which is comparable to 
ihe known male bias in breeding areas. The proportion male varied geographically on Hispaniola, suggesting some level of 
habitat segregation. Male-biased ratios occurred at two sites whereas sex ratios at five sites did not differ from parity. The 
island-wide proportion adult was 0.72 and age ratios were significantly adult-biased at two sites. We assessed vegetative 
structure at all sites and the proportion of male thrushes increased significantly with density of understory vegetation. Age 
ratios were not associated with vegetation characteristics. Neither sex nor age ratios varied significantly with elevation. Our 
data suggest the possibility of sexual habitat segregation with males preferentially occupying cloud forest sites 
characterized bv a thick understory of vines and saplings occurring at densities >10.000 stems/ha. Received 19 April 2010. 
Accepted I January 2011. 
Males and females of many migratory song¬ 
birds compete directly for territories and accom¬ 
panying resources during the non-breeding sea¬ 
son. One possible outcome of intersexual 
competition is habitat segregation, in which 
disproportionate numbers of either males or 
females occupy a habitat-type distinct from that 
occupied predominantly by the other (Lynch el al. 
1985). Winter sexual habitat segregation has been 
confirmed in at least 11 species of Nearctic- 
neotropical migratory birds (Ornat and Greenberg 
1990, Wunderle 1995, Greenberg et al. 1997, 
Latia and Faaborg 2001. Marra and Holmes 2001, 
Ltta and Faaborg 2002, Roberts 2007). Age- 
based habitat segregation, although not as well 
documented, has been reported for at least two 
species (Stutchbury 1994, Marra and Holmes 
2001 ). 
Habitat segregation by wintering individuals 
attempting to maximize exclusive access to 
limited resources may he an important component 
01 overall population regulation (Marra and 
Holmes 2001, Lalta and Faaborg 2002, Runge 
and Marra 2005). Winter habitat quality can 
influence survival probability during the non¬ 
breeding period and affect fitness during the 
breeding period (Norris et al. 2004, Reudink et al. 
Stale University of New York. College of Environmcn- 
l; ‘l Science and Forestry, I Forestry Drive. Syracuse. NY 
IT210. USA. 
Vermont Center for Ecostudies. P. O. Box 420. 
Norwich, VT 05055. USA. 
Cornell University, Department of Ecology and Evolu- 
""nary Biology. Ithaca, NY 14850. USA. 
Corresponding author; e-mail: jatownse@syr.edu 
2009). Consequences for individuals relegated to 
sub-par winter habitat can include loss of body 
mass over the course of die winter and lower 
interannual survival (Marra and Holmes 2001). 
Several studies have indicated that female surviv¬ 
al may be particularly limited during the winter 
period due to competitive exclusion from the 
highest quality habitats. potentiaUy contributing to 
male-skewed sex ratios frequently documented for 
migratory passerines (Benkman 1997, Marra and 
Holmes 2001, Donald 2007). 
Bicknell's Thrush (Catharus bicktwlH), a Ne- 
aretic-neotropical migrant of high conservation 
concern (Wells 2007, Lebbin et al. 2010). is a Red 
List species considered globally Vulnerable by the 
International Union for the Conservation of 
Nature (IUCN) (BirilLife International 2000). 
The winter range of the species is limited and 
highly fragmented with most of the population 
wintering on Hispaniola (Rimmer cl al. 2001). 
The species’ breeding disUnbution is also highly 
fragmented, and several breeding populations 
have disappeared or are known to be declining, 
leading to listing as a Threatened species in 
Canada and as a Species of National Concern in 
the U.S. (Rimmer et al, 2001). Males outnumber 
females in breeding areas by ~2:1 (Rimnier et al. 
2001, Townsend et al. 2009), and it is possible 
that events in winter areas influence this biased 
sex ratio (Marra and Holmes 2001). Species that 
are listed by the IUCN as Vulnerable. Endangered 
or Critically Endangered (together. "Globally 
Threatened”) have sex ratios significantly more 
male-biased than non-threatened species, and the 
skew becomes greater as populations dwindle 
367 
