Includes bibliographical references (pages 50-55).
Contents:
Methods: -- Study area -- Habitat characterization -- Captures and banding -- Site fidelity; Factors affecting fruit abundance -- Food availability per study site -- Ocean Hole space use and food resources -- Analyses and definitions -- Statistical tests -- Fruit abundance model selection -- Overwinter site persistence model selection. Results: Habitat variation among study sites -- Fruiting phenology -- Fruit abundance modeling -- Variation in food availability -- Fruit -- Foliage arthropods -- Ground arthropods -- Seasonal change in food resources within a winter -- Correlation of food-resource measures -- Diet composition -- Sex and age differences in captures -- Overwinter site persistence -- Annual return to a study site -- Site persistence in relation to food abundance and sex and age -- Modeling overwinter site persistence: models with arthropod and fruit biomass -- Modeling overwinter site persistence : models with fruit biomass -- Study-site shifts in relation to food abundance -- Late-winter warbler abundance at the fruit-rich OH site -- Late-winter warbler abundance and food-resource biomass -- Changes in physical condition -- Changes in corrected body mass -- Changes in fat scores -- Departure schedules -- Changes in physical condition with rainfall -- Effect of rainfall on corrected body mass in different periods -- Effect of rainfall on fat levels during different periods -- Annual return in relation to corrected body mass. Discussion: -- Factors affecting fruit abundance -- Variation in study-site food abundance -- Habitats with low arthropod biomass -- Overwinter site persistence -- Site fidelity and survival -- Annual return -- Role of dominance -- Late-winter warbler abundance and food resources -- Differences in corrected body mass and fat -- Rainfall and body condition -- Annual return and corrected body mass -- Conservation implications. Appendices: 1. Radiotelemetry methods used to study Kirtland's Warbler movements -- 2. Resight detections of color-banded radiotagged Kirtland's Warblers -- 3. Details of methods used to study abundance and phenology -- 4. Phenology models for the response variable "unripe fruit" and "ripe fruit" for three shrub species -- 5. Model-fit results for the global model for each set of models using the likelihood ratio test statistic ... -- 6. Temporal model parameter estimates for unripe and ripe Chiococca alba ... -- 7. Candidate models for overwinter site persistence in Kirtland's Warblers on southern Eleuthera, the Bahamas -- 8. Summary of captures and observations of color-banded Kirtland's Warblers ... -- 9. Color-banded Kirtland's Warblers detected at sites other than the site where originally captured on southern Eleuthera, the Bahamas.
Summary:
"Distribution of nonbreeding migrant birds in relation to variation in food availability has been hypothesized to result from the interaction of dominance hierarchies and variable movement responses, which together may have sex- and age-specific consequences. We predicted that site fidelity, movements, and abundance of Kirtlands Warblers (Setophaga kirtlandii) wintering on the island of Eleuthera in the Commonwealth of the Bahamas (hereafter S2The BahamasS3) would be correlated with food abundance but vary by sex and age. We found that the species food resources (fruits and arthropods) typically declined during a winter but varied between winters (years) and study sites. Rainfall is a driver of variation in fruit abundance, as indicated by an information-theoretic evaluation of abiotic factors that influence fruit abundance. Despite variation in food availability, the proportions of fruits and arthropods in the diet of Kirtlands Warblers (88% of 90 fecal samples with both) varied little within or between winters or with sex or age class. Overwinter site persistence was low and variable among study sites (average = 43%, range: 1167%); as predicted, site fidelity within and between winters differed by sex (males> females) and age class (adults> juveniles). However, knowledge of only sex and age was insufficient to predict site persistence in a model-selection framework in the absence of other contributing variables from the confidence set of models (i.e., food resources and/or habitat structure) for two model sets. These analyses further indicated that measures of food resources, either foliage arthropods or fruits, were reliable positive predictors of site fidelity, given the respective confidence set of models. Birds that shifted between study sites within a winter moved to sites with higher biomass of ripe fruit and ground arthropods, such that late-winter densities of Kirtlands Warblers were positively related to the biomass of fruits and ground arthropods. Sex and age differences in corrected body mass and fat were significant from midwinter through late winter, consistent with expected outcomes of dominance and experience. Differences in corrected body mass were evident by 16 April, when males had greater corrected mass than females, and by 26 April, when corrected mass of males was greater for adults than for juveniles. Late-winter rain had a positive effect on corrected body mass, corraborating previous Kirtlands Warbler studies that showed carryover effects on the breeding grounds and that survival in the following year was positively correlated with March rainfall in The Bahamas. Given that drought reduces the food resources and body condition of Kirtlands Warblers in The Bahamas, which negatively affects survival and breeding of Kirtlands Warblers in North America, conservation efforts in the Bahamas archipelago should focus on protecting the least-drought-prone early-successional habitats and sites with favored fruit species."
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