Effect of wing tags and testing hypotheses of sexual size dimorphism in frigatebirds
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Date
2015
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University of New Brunswick
Abstract
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.