Effects of social aggression on tree swallow (Tachycineta bicolor) offspring behavior and physiology

Territorial animals that breed in high-density environments often experience high levels of competition for resources necessary for reproduction and, thus, are likely to engage in social aggression with conspecifics. In many species, increased social aggression among breeding females results in increased concentrations of androgens (e.g., testosterone) in the females which are transferred to their offspring, a phenomenon called a maternal effect. In general, elevated yolk testosterone increases nestling growth rates, decreases immune function, and enhances aggressive and bold behavioral traits. In this study, I altered the breeding density of tree swallows (Tachycineta bicolor) to modify levels of social aggression and yolk testosterone. I measured subsequent effects on nestling growth, overall health, and behaviors (aggression and boldness) using a partial cross-foster design. Females at high-density sites experienced more aggressive interactions and their eggs had higher testosterone concentrations. Nestlings hatched in high-density sites grew faster, were more aggressive, and bolder regardless of where they were reared. Nestling behaviors were not directly correlated; however, there was a positive trend in behaviors for nestlings reared and hatched at the same site. This study is one of the first to demonstrate that naturally-induced maternal effects can create adaptive phenotypes for an anticipated environment.