The relationship between personality and plasticity in tree swallow aggression and the consequences for reproductive success

Both personality and plasticity can influence fitness, but few studies have investigated these two sources of individual variation simultaneously for the same behaviour. The individual quality hypothesis proposes that individuals with high personality scores will also be better able to respond to the changes in the environment (i.e. have a high degree of plasticity). Alternatively, the ‘compensatory’ hypothesis proposes that personality and plasticity are negatively correlated, because only individuals with low personality scores need to be able to rapidly adjust to environmental conditions. To examine these two hypotheses, we investigated the overall level of aggressiveness (personality) in nest defence, the ability of individuals to adjust this behaviour to respond to changes in temperature (plasticity), and their consequences for reproductive success in male and female tree swallows, Tachycineta bicolor. Using linear mixed effect models with individual as a random effect, we found that consistent differences between individuals explained approximately 55% of variation in aggressiveness and that more aggressive individuals were better able to adjust to variation in temperature, providing support for the individual quality hypothesis. However, although more aggressive males tended to fledge a higher number of young, the degree of plasticity only conferred a reproductive advantage for nonaggressive males, providing support for the compensatory hypothesis. For females, neither personality, nor plasticity was a good predictor of reproductive success. Our results suggest that personality and plasticity in aggressiveness are important components of individual variation but that the fitness advantages of each are context dependent.