Meeting Abstract
Our understanding of the proximate and ultimate mechanisms shaping competitive phenotypes primarily stems from research on male-male competition for mates, even though female-female competition is also widespread. Obligate secondary cavity-nesting provides a useful comparative context to explore the phenotypic effects of competition because this reproductive strategy has evolved repeatedly across avian lineages, and it is thought to generate strong competition for a pre-made cavity in which to nest, for both males and females. We tested the hypotheses that cavity-nesting elicits more robust aggressive responses to conspecifics and that this behavioral trait is facilitated by elevated testosterone levels in circulation in both sexes. We assayed aggression in males and females in two obligate cavity-nesting species and two related non-cavity-nesting species in the same avian family: tree swallow (Tachycineta bicolor) vs. barn swallow (Hirundo rustica); Eastern bluebird (Sialia sialis) vs. American robin (Turdus migratorius). We found that both male and female cavity-nesting species were more aggressive than their non-cavity-nesting close relatives. However, we did not find higher testosterone in cavity-nesting females or males, despite some correlative evidence that testosterone is associated with territorial defense. These patterns support the long-held hypothesis that cavity-nesting may select for greater territorial aggression in both sexes, but parallel increases in aggression are not associated with greater testosterone secretion in either sex.
