Sex and age distribution of Bicknell's Thrush in the Dominican Republic

Jason Townsend: SUNY-ESF Chris Rimmer and Kent McFarland: VERMONT CENTER FOR ECOSTUDIES Sex and Age Distribution of Bicknell’s Thrush at Two Wintering Sites in the Dominican Republic

Breeding Season Background
Harsh Conditions, Habitat Islands, Short Time Window: 2-4 weeks behind congeners breeding at lower elevations
Polygynandrous Breeding (Goetz et al. 2004)
Highly Male-skewed Sex Ratio

n=4 n=4 n=10 n=3 n=5 n=7 Overall mean = 2.2 males : female Breeding Adult Sex Ratios Gaspe and Gosford data provided by Yves Aubry, Canadian Wildlife Service n = years

n=14 nests n=6 nests Nestling Sex Ratios n=16 chicks n=26 chicks Gaspe data provided by Yves Aubry, Canadian Wildlife Service

Females are limited somewhere in the annual cycle: Where?

“ Differential mortality of females during the non-breeding season, resulting from effects of dominance, may thus also contribute to the male-biased sex ratio observed in breeding populations of redstarts and perhaps other species.” (Marra and Holmes 2001) Hispaniola Dominican Republic Haiti

2004 Island-wide Surveys

2004 PCR Analysis Results: Proportion male by site 50% n=25 80% n=18 77% n=25 (18% in 2000) (67% in 2000)

Winter Intersexual Competition: 3 Major Theories
Habitat specialization = each sex chooses sex-specific optimal habitat: COMPETITION AVOIDANCE
*Hooded Warblers (Morton 1990)
Behavioral Dominance = larger, more aggressive males relegate females and sub-dominant males to sub-par habitat
*American Redstarts (Marra et al. 1993)
Equal Competition = lack of behavioral dominance, with males and females competing equally for resources.
*Willow Flycatcher (Koronkiewicz et al. 2006)
*Ovenbird (Brown and Sherry 2008)

Monochromatic/Monomorphic species?
WIFL: “…lack of sexual habitat segregation may be common life-history traits in Empidonax, and possibly other monochromatic migrants.”
* Koronkiewicz et al. 2006
OVEN: “…little evidence involving monomorphic species exists that age- or sex- based dominance is operating…”
*Brown and Sherry 2008

Where does Bicknell’s fit in?
Our Approach:
Document sex and age ratio at multiple sites over multiple years
Radio telemetry to document differing levels of territoriality
Mid-winter body condition: adjusted body mass

High-elevation “Male Site” = PUVI e ~185 KM Dominican Republic Study Sites Mid-elevation “ Female Site” = Canela

Dominican Development Pressure

Male Site
Female Site
High Elevation ~1800m
Prisitine Cloud Forest
Dense Understory
Precip = Clouds
Arthropods High**
Fruit Low
Mid Elevation ~500m
2 0 Rainforest
Open Understory
All-Year Trade Rains
Arthropods Low
Fruit High**
Biotic and Abiotic Site Differences

Different Diets
Significantly different delta carbon 13 in blood
(F 41.8, p = 0.0003)
Fruit Arthropods (MID) (HIGH)

SEX RATIO COMPARISON 2000 2004 2008

GLM p = 0.02

Sex Ratio Conclusions
The sex ratio differs significantly between mid and high elevation forests
High-elevation cloud forest is a consistently male-biased habitat
Mid-elevation rainforest sex ratio fluctuates interannually and is best characterized as an equal-ratio site

AGE RATIO COMPARISON

GLM p = 0.81

SEX RATIO OF JUVENILES GLM p = 0.21

Age Ratio Conclusions
Overall age ratios do not differ between the sites and in fact trend together interannually
Non-significant trend = higher mean proportion of male hatch year birds at the high elevation site
*Island-wide, for all years = sex ratio of hatch year birds does not differ from 50:50 ( χ 2 = 15.5, p = 0.5)

Are there measurable habitat-specific consequences?
Radio Telemetry
Adjusted Body Mass

N = 25 N = 11 N = 12 N = 5 TERRITORY SIZES BY SITE AND SEX

Telemetry Conclusions
Males, females and all ages defend territories of similar size
Females and juveniles can maintain exclusive territories, even surrounded by adult males
Both arthropods and fruit are defensible resources
Incidence of floaters low: just 2 of 53 =4%
*WOTH: 27/61=44% (Rappole et al. 1989)
*HETH: 7/50=14% (Brown et al. 2000)

BODY CONDITION BY SITE AND SEX

Body Condition Conclusions
Females @ the mid-elevation fruit 50:50 site hold more mass for their body size than females @ the high-elevation male-dominated arthropod site
Mid-elevation rainforest with an abundance of fruit may be preferential habitat for wintering females

PUVI: High-elevation
Canela: Mid-elevation
Male-dominated, evidence for behavioral dominance
Little territorial overlap
Arthropod-heavy diet
Females in poor condition (low body mass residuals)
Equal sex ratio, no evidence for behavioral dominance
Little territorial overlap
Fruit-heavy diet
Females in good condition (high body mass residuals)
Different Sites, Different Dynamics

Female Population Limitations?
**The area where females compete equally with males is also the most developed area in the DR

Where Are Females Limited in Annual Cycle? Nest: 1:1 sex ratio chicks Fledge Post-fledging dispersal 1:1 1:1 1:1? Fall Migration Winter Habitat and Territory Selection 1:1? High >3:1 Mid ~1:1 Spring Migration Breeding Grounds >2:1 ***? **Juveniles do not differ 1:1 **VT spring juveniles 2:1

Future Directions
Quantify available habitat via remote sensing
Focus on late-winter diet and condition
Large push by Vermont Center for Ecostudies to establish a thorough conservation plan for the northeastern Dominican Republic in collaboration with local conservationists and cacao growers.

Thanks!
Laboratory support and guidance:
Irby Lovette and Laura Stenzler, Cornell Evo Bio Lab
Funding support provided by
Cooper Ornithological Society
Wilson Ornithological Society
Association of Field Ornithologists
Eastern Bird Banding Association
National Science Foundation
US Fish and Wildlife Service
The Nature Conservancy

Sex and age distribution of Bicknell's Thrush in the Dominican Republic

by Vermont Center for Ecostudies on Sep 01, 2009

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Sex and age distribution of Bicknell’s Thrush in the Dominican Republic — Presentation Transcript