The scope and adaptive value of modulating aggression across breeding stages: case study in a competitive female songbird
Description
In seasonally breeding animals, the costs and benefits of territorial aggression should vary over time; however, little work thus far has directly examined the scope and adaptive value of plasticity in aggression across breeding stages, at least not at the individual level where selection acts. We explore these issues using the tree swallow (Tachycineta bicolor), a bird species in which females compete for limited nesting sites each spring, before producing a single brood. We measured the aggressiveness of nearly 100 females across three different stages: shortly after territory-establishment, during incubation, and while caring for young chicks. We used k-means clustering to categorize females into four distinct plasticity ‘types’ based on the timing, direction, and magnitude of their changes in aggression between stages. We then tested whether plasticity type and stage-specific aggression varied with age, morphology, and other performance metrics. Two of the four behavioral types became less aggressive over the breeding season, consistent with population-level patterns, though these plasticity types did not differ from one another in survival or reproductive success. A third behavioral type was characterized by hyper-plasticity in aggression; these females hatched marginally fewer eggs, had lower body mass while parenting, and were less likely to return to breed the following year. A final behavioral type, which applied to about a third of all females, exhibited very little plasticity, with moderate to low levels of aggression across all breeding stages; surprisingly, we did not identify any costs or benefits associated with this phenotype per se. However, females with lower aggression during territory-establishment were more likely to be older, experienced breeders, suggesting a potential long-term benefit to a low-aggression, low-plasticity phenotype. Together, these results reveal substantial among-individual variation in behavioral plasticity, at least some of which may reflect diverse solutions to trade-offs between current reproduction and future survival.