All organisms must allocate limited resources towards competing functions. These functions can be grouped into two broad categories: reproduction and self-maintenance. Reproduction can increase fitness through production of offspring, whereas self-maintenance can increase fitness by improving survival to a time in the future when the individual can reproduce. One central factor influencing strategies of investment in reproduction or self-maintenance is "reproductive value", or the relative fitness benefit of current versus future reproduction. We currently do not understand the mechanisms allowing animals to adaptively allocate resources to reproduction and self-maintenance in the face of changing environmental conditions, resource availability, and reproductive value. Glucocorticoids, or stress hormones, are crucial for maintaining energetic balance, and might play an important role in mediating allocation to reproduction and self-maintenance. This project will test three hypotheses about the role of glucocorticoids in mediating resource allocation through experimental manipulations of glucocorticoids, reproductive effort, and blood parasites in free-ranging Tree Swallows. Results of this work will improve our understanding of the mechanisms underlying variation in reproduction and survival and the evolution of strategies of resource allocation. This information is valuable because it will improve our understanding of evolution and physiology, and will also have implications for management of species of conservation concern. Glucocorticoid levels increase in response to habitat disturbance, and so, an understanding of how these hormones affect reproduction and survival can inform our understanding of dynamics of populations in disturbed habitat. Finally, because glucocorticoids are found in all vertebrates, including humans, the findings from this study might have important implications for human health.