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

► We examined personality and behavioural plasticity in tree swallows, Tachycineta bicolor. ► Differences in personality between individuals explained approximately 55% of variation in aggressiveness. ► More aggressive individuals were better able to respond to variation in ambient temperature and tended to fledge more young. ► However, plasticity conferred a reproductive advantage only for nonaggressive males. ► Our results suggest that personality and plasticity are context-dependent components of fitness in tree swallows.