Potential Effects Of Aggression, Site, And Proximate Determinants That Facilitate Tree Swallow Range Expansion

Breeding range expansion occurs when individuals disperse to novel environments over successive breeding seasons and it is expected that both range expanding and native species will be affected. Additionally, native species can experience increased agonistic interactions and competition for limited resources, but they may be able to adapt by adopting behaviors such as increased aggression. Currently, tree swallows, Tachycineta bicolor, are undergoing a range expansion to the southeastern US. In Chapter 2, I investigate the effect expansion may have on a native species of eastern bluebird, Sialia sialis, by comparing territorial defense behavior of bluebirds currently experiencing the tree swallow range expansion (North Carolina) and a more southern bluebird population that is not yet living in sympatry with tree swallows (Alabama). In Chapter 3, I investigate differences between behavior and physiology across tree swallow populations throughout their historical (Wisconsin, Ontario, Nova Scotia) and new (North Carolina, Indiana, Iowa) sites. My results support the prediction that tree swallows on the edge of expansion exhibit aggressive phenotypes and have elevated glucocorticoids. My thesis improves our understanding of differences between geographic populations, expansion effects on native species, and how individuals undergoing expansion are able to survive despite the assumed costs of novel colonization.