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


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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 …

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.

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  • Summation of what we know
  • habitat specialist on both ends of its migratory range. This talk focuses on the winter grounds, but first wanted to start with some breeding grounds background.
  • The Problem. Apparent skewed sex ratio found throughout the breeding range. No difference in capture probability of either sex via mark-recapture N= years
  • Sample size very small: 14 Vermont nests (2000-2002) and 6 Quebec nests (1999-2000) Sex of nestlings: 18 females and 8 males in Vermont (69% female); 8 females and 8 males in Quebec (50:50). Overall nestling sex ratio = 67% female Sex of fledglings: 8 female and 4 males in Vermont (67% female); 8 females and 7 males in Quebec (53% female) Overall fledgling sex ratio = 59% female Sex ratios in nest do not appear to account for skewed breeding adult sex ratio No data on post-fledging survival or survival during migration
  • BITH is monochromatic BUT MALES SIG BIGGER-BODIED , and this is the crux of beh. dominance, so I see no a priori reason to discount poss. of beh. dom.
  • Generalized linear model with proportion male as response and year and site as predictors, there was a sig diff between the two sites 2 Main Conclusions: the sex ratios are different between the two sites, the sex ratio at the Mid-elevation shows greater interannual fluctuation
  • Data do not fit neatly into ONE MECHANISM FOR habitat segregation, nor do they resolve the issue of whether beh. dom. operates in monochromatic spp. HIGHLIGHTS IMPORTANCE OF LOOKING AT MULTIPLE SITES OVER MULTIPLE YEARS
  • Transcript

    • 1. 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
    • 2.  
    • 3. 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
    • 4. 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
    • 5. n=14 nests n=6 nests Nestling Sex Ratios n=16 chicks n=26 chicks Gaspe data provided by Yves Aubry, Canadian Wildlife Service
    • 6. Females are limited somewhere in the annual cycle: Where?
    • 7. “ 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
    • 8. 2004 Island-wide Surveys
    • 9.  
    • 10.  
    • 11.  
    • 12. 2004 PCR Analysis Results: Proportion male by site 50% n=25 80% n=18 77% n=25 (18% in 2000) (67% in 2000)
    • 13. 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)
    • 14. 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
    • 15. 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
    • 16. High-elevation “Male Site” = PUVI e ~185 KM Dominican Republic Study Sites Mid-elevation “ Female Site” = Canela
    • 17. Dominican Development Pressure
    • 18.
      • 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
    • 19. Different Diets
      • Significantly different delta carbon 13 in blood
      • (F 41.8, p = 0.0003)
      Fruit Arthropods (MID) (HIGH)
    • 20. SEX RATIO COMPARISON 2000 2004 2008
    • 21. GLM p = 0.02
    • 22. 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
    • 24. GLM p = 0.81
    • 25. SEX RATIO OF JUVENILES GLM p = 0.21
    • 26. 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)
    • 27. Are there measurable habitat-specific consequences?
      • Radio Telemetry
      • Adjusted Body Mass
    • 28. N = 25 N = 11 N = 12 N = 5 TERRITORY SIZES BY SITE AND SEX
    • 29. 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)
    • 31. 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
    • 32.
      • 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
    • 33. Female Population Limitations?
      • **The area where females compete equally with males is also the most developed area in the DR
    • 34.  
    • 35.  
    • 36. 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
    • 37. 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.
    • 38. 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
    • 39.