Sexual size dimorphism (SSD) is one of the most apparent and puzzling differences between males and females across many different taxa. This dissertation explores the evolution and maintenance of female-biased SSD (females larger than males) in Magnificent Frigatebirds (Fregata magn.ificens) breeding on Barbuda, in the eastern Caribbean. I tested two hypotheses to explain SSD. The first, the resource division hypothesis, implicates natural selection in the evolution of SSD through selection for reduced intersexual competition via trophic niche divergence. Using prey, stable isotope, and foraging location data, I tested specific predictions relating to larger female size. My results did not support the resource division hypothesis in frigatebirds, given the similarities in breeding season prey, stable isotope values, and foraging locations between males and females. A second hypothesis attempting to explain smaller male size is the aerial agility hypothesis, which proposes that smaller males have an advantage during mating displays or other aerial acrobatics. Wing traits affecting flight performance and predicted to be under selection were measured from breeding birds, and fledging success was used as a measure of fitness. Projection pursuit regression and cubic splines were used to explore the strength and shape of selection acting on wing traits, respectively. Male wing traits influencing manoeuvrability were under stronger selection than in females and correlated with nest volume, providing support for the aerial agility hypothesis maintaining small male size. This likely reflects the male's role in collecting nest material. Large female size may be a result of extended parental care relative to males, and requires further study. Because of low fledging success early in the study, I also conducted an experimental study and meta-analysis on the effects of wing tags, a common avian field marker. Wing tags had a significant negative effect on nest success in Magnificent Frigatebirds, and on survival and hatch and nest success in other birds. Based on these findings, I strongly recommend against the use of wing tags in future studies.
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