A decrease in reproductive performance in older birds, despite the likely benefits of increased experience, is often attributed to senescence. Senescence is the progressive deterioration of structure and function over time, and in vertebrates is manifested as an age-related increase in mortality, decrease in fecundity, or both. The physiology of senescence is complex, and involves age-related changes in function of somatic and reproductive cells and tissues, and subsequent changes in interactions among these systems. Analyses of seventeen years of data from our study population of Florida Scrub-Jays (Aphelocoma coerulescens) reveals a quadratic relationship between breeder age and number of fledglings produced, with the youngest and oldest birds having the lowest reproductive success. The Florida Scrub-Jay is a cooperatively breeding passerine that is restricted to remnant scrub oak habitat of peninsular Florida. In this project, the investigators will assess reproductive aspects of senescence in this species; taking into consideration reproductive endocrinology, stress physiology, egg hatchability, sperm morphology, life history trade-offs, and age-related variance in parental behavior. Specifically, the investigators will conduct experiments to determine which component of the reproductive (hypothalamic-pituitary-gonadal: HPG) axis exhibits senescence by recording physiological changes (primarily endocrine measures) as individuals age. The results from this project which will combine observational and manipulative experiments will ultimately contribute valuable insight into the mechanisms that underlie (or are a result of) avian reproductive senescence. The broader impacts of the project will primarily be realized through the continuing education of a graduate student. The investigators will present and publish findings in appropriate national and international venues (e. g., at meetings and in respected journals). Similarly, both investigators regularly present research findings to lay organizations, such as ornithological and nature clubs.