Behavioral correlations across breeding contexts provide a mechanism for a cost of aggression

Identifying correlations among behaviors is important for understanding how selection shapes the phenotype. Correlated behaviors can indicate constraints on the evolution of behavioral plasticity or may reflect selection for functional integration among behaviors. Obligate cavity-nesting birds provide an opportunity to examine these correlations because males must defend limited nest cavities while also competing for mating opportunities and providing parental care. Here, I investigated the role of behavioral correlations in producing a counterintuitive relationship between nest defense and reproductive success in western bluebirds (Sialia mexicana) such that males that defended their nests most intensely had the lowest reproductive success, measured as the number of within and extrapair offspring that fledged. By experimentally measuring aggression across contexts, I show that this cost of nest defense was due to the correlated expression of aggression across the contexts of nest defense and male-male competition coupled with a trade-off between male-male aggression and parental care. In particular, more aggressive males provisioned their females less during incubation and this led to disrupted incubation patterns and fewer fledged offspring. However, aggressive males did not benefit from avoiding parental investment by gaining extrapair fertilizations, and thus, it is unclear how high levels of aggression are maintained in this population despite apparent costs. These results suggest that there are constraints to the evolution of plasticity in aggression and emphasize the importance of considering the integrated behavioral phenotype to understand how variation in behavior is linked to fitness.