Investigating Eastern bluebird ecology and behavior through models integrating individual and systems perspectives
Schulze, Jill Goldstein
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I investigated the ecological and behavioral factors that contribute to differential fitness in eastern bluebirds. My goal was to identify variables that contribute to variation in reproductive success among female breeders. Using data from a long-term study of eastern bluebirds by Patty Gowaty from 1993-2000 in Athens, GA, I categorized breeders by traits associated with extra-pair offspring: female breeder age male breeder age; nest box density; and female foraging success. I used two approaches to analyze the data. The first method was a standard analysis of reproductive timing and success, and factors associated with foraging. My second approach was to research the problem from a systems perspective. Using modeling software, I created a directed graph with ten state variables, which represented life stages of the bluebirds, in order to track the flow of biomass through the life cycle. I created three sets of sensitivity analyses to investigate the relative effects of direct versus indirect variables; relative food accessibility; and mortality. Nest box density was the most important influence of reproductive success, followed by female breeder age. Breeder pairs at low-density nest boxes attempted earlier first nesting attempts and had a greater percentage of eggs hatch and fledge. Eggs in low-density boxes experienced less predation and less non-predatory mortality. Offspring viability, the percentage of eggs that fledged in nests that were not predated, was greater in low-density boxes. Approximately 60% of nests experienced partial- or whole-nest non-predatory mortality during the egg state. This is much higher than non-predatory mortality rates recorded for other populations of eastern bluebirds. There were low rates of recruitment of fledglings into the breeding population, and this resulted in increases in production having little effect on the growth rate of the local population. It appears that the Athens, GA population of bluebirds may be an ecological sink population, due to high rates of egg non-predatory mortality, and low recruitment rates of fledglings.