Associations of anthropogenic activity and disturbance with fitness metrics of eastern bluebirds (Sialia sialis)

To predict the long-term consequences of increasing human activity and environmental disturbance on natural populations, it is imperative to understand how human activity relates to fitness variation in wild populations. Here, we report associations of avian fitness metrics with multivariate indices of human activity and bird behavioral responses in eastern bluebirds (Sialia sialis). We quantified patterns of human activity, adult bird behavioral time budgets, and four avian fitness metrics (brood growth, brood condition, brood survivorship and nest box productivity) at 52 nest boxes placed along a human disturbance gradient. We found that distance of human activity relates nonlinearly to chick survival and productivity. Nests were more productive at intermediate levels of human activity. The number of box-visits completed by parent bluebirds was positively related to box productivity. We also found a significant negative relationship of intense and variable human activity with adult bird self-maintenance behaviors. These results suggest that adult bluebirds generally buffer their developing broods from human activity in the environment, sacrificing self-maintenance behaviors before compromising the growth and survival of their brood. If adult birds are absorbing the costs of anthropogenic disturbance in lieu of their young, there could be negative sub-lethal consequences of increasing human activity for future breeding attempts. The nonlinear relationships we found between human activity and avian fitness indicate that eastern bluebirds are somewhat pre-adapted to intermediate levels of human disturbance. As local populations have gone through a recent population bottleneck and experienced intense selection for breeding in man-made cavities in the past 40–50 years, we speculate that local populations may have adapted rapidly to human altered habitats. Hence, we advocate a more inclusive evolutionary ecology approach to understanding responses of natural populations to human disturbance.