Understanding the causes of sexual differences in aggression is a major goal in studies of evolution. Although males are widely considered the more aggressive sex, females often accrue substantial fitness benefits through competition. Competitive females frequently gain priority access to limited resources that directly impact reproductive success. Despite such benefits, substantial costs can be associated with the hormones that regulate aggressive behaviour. Females aggression is often mediated by testosterone, and females face substantial costs from this hormone because it interferes with breeding physiology. A tradeoff therefore exists between resource acquisition, and the depression of reproductive output. Although female social behaviour has traditionally been understudied, recent emphasis is focused on the adaptive role of female competition. However, little evidence compares positive and negative selective pressures acting concurrently on a single system, and, as a result, we only have a partial view of the evolutionary pressures acting to mold this core female behaviour. The objective of this proposal is to forge an enduring collaboration with international colleague, Prof. Troy Murphy (Trinity University, USA). My goal is to bring Prof. Murphy to The Netherlands for 4 months to evaluate the evolutionary benefits as well as the costs associated with female aggression using a model bird species, the Blue Tit (Cyanistes caeruleus), a species with which I have worked for many years, and a species where female aggression is mediated by testosterone and female-female competition for nesting sites is remarkably violent. This study system presents an ideal opportunity to study fitness outcomes because the population is large and tractable (approx. 150 nesting pairs utilize nest boxes in our study population) and so manipulative experiments will yield large sample sizes that can detect small but meaningful variations in behaviour and fitness. Our research will involve three main procedures: first, we will increase competition for breeding sites by rendering half of the nest boxes inaccessible. Second, we will capture wild females and implant half of them with testosterone pellets, and implant the other half with empty pellets as a control (following methods previously used in my laboratory). We expect that testosterone implanted females will be more competitively successful. Third, we will assess the costs faced by females by comparing testosterone treatment to various measures reproductive performance and output (e.g., incubation and provisioning behaviour, fledging success). This research will add valuable insights into which components of fitness are affected by aggression, and will provide essential information for interpreting the selective processes underlying the female competitive phenotype. Our results will be highly impactful because they will provide detailed fitness assessment based on experimental manipulation of both competition and hormone levels in females, and our use novel techniques (i.e., increasing population wide competition) and modern technologies (i.e., automated video monitoring) will provide us with a solid integration of proximate and ultimate components of this behaviour. This work will yield multiple high impact journal articles, and because we recognize the importance of public education, there is already a plan to publish this research in a popular science book.