Sex and age distribution of Bicknell's Thrush in the Dominican Republic
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.
Recommended
More Related Content
What's hot
What's hot (20)
Similar to Sex and age distribution of Bicknell's Thrush in the Dominican Republic
Similar to Sex and age distribution of Bicknell's Thrush in the Dominican Republic (20)
Recently uploaded
Recently uploaded (20)
Sex and age distribution of Bicknell's Thrush in the Dominican Republic
- 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
- 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
- 12. 2004 PCR Analysis Results: Proportion male by site 50% n=25 80% n=18 77% n=25 (18% in 2000) (67% in 2000)
- 16. High-elevation “Male Site” = PUVI e ~185 KM Dominican Republic Study Sites Mid-elevation “ Female Site” = Canela
- 20. SEX RATIO COMPARISON 2000 2004 2008
- 21. GLM p = 0.02
- 24. GLM p = 0.81
- 25. SEX RATIO OF JUVENILES GLM p = 0.21
- 28. N = 25 N = 11 N = 12 N = 5 TERRITORY SIZES BY SITE AND SEX
- 30. BODY CONDITION BY SITE AND SEX
- 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
Editor's Notes
- 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
- OVERARCHING QUESTION
- CAN ALSO BE APPLIED TO AGE-BASED SEGREGATION OF HABITATS
- 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
- HAVING IDENTIFIED SIGNIGICANT SEXUAL HABITAT SEGREGATION BETWEEN THE SITES
- 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