Scope and adaptive value of modulating aggression over breeding stages in a competitive female bird

In seasonally breeding animals, costs and benefits of territorial aggression should vary over time; however, little work thus far has directly examined the scope and adaptive value of individual-level plasticity in aggression across breeding stages. We explore these issues using the tree swallow (Tachycineta bicolor), a single-brooded bird species in which females compete for limited nesting sites. We measured aggressiveness in nearly 100 females within three different stages: (1) shortly after territory-establishment, (2) during incubation, and (3) while caring for young chicks. Based on the timing, direction, and magnitude of behavioral changes between stages, we used k-means clustering to categorize each female’s behavior into a ‘plasticity type’. We then tested whether plasticity type and stage-specific aggression varied with key performance metrics. About 40% of females decreased aggressiveness across consecutive breeding stages to some degree, consistent with population-level patterns. 33% of females exhibited comparatively little plasticity, with moderate to low levels of aggression in all stages. Finally, 27% of females displayed steep decreases and then increases in aggression between stages; females exhibiting this pattern had significantly lower body mass while parenting, they tended to hatch fewer eggs, and they had the lowest observed overwinter survival rates. Other patterns of among-stage changes in aggressiveness were not associated with performance. These results reveal substantial among-individual variation in behavioral plasticity, which may reflect diverse solutions to trade-offs between current reproduction and future survival.