Understanding the forces that create variation in individual fitness is fundamental to predicting population and evolutionary dynamics. Yet major questions regarding the causes and consequences of variation in fitness remain unanswered. Variation in male ‘paternity success’ (i.e. success in siring offspring) is a major fitness component that underpins multiple key evolutionary processes. Understanding the causes and consequences of this variation is consequently an ongoing challenge in evolutionary ecology. Progress requires integration of multiple disciplines, including physiology and quantitative genetics, to quantify determinants of paternity success in natural populations.In PatSuccess, I will provide new insights into key forces that shape variation in male paternity success and reproductive fitness. My objectives are to 1) quantify effects of inbreeding on key sperm performance traits and oxidative damage, thereby linking inbreeding with physiology and fitness in a wild population; 2) quantify relationships between key sperm performance traits and oxidative damage and male paternity success, thereby identifying physiological predictors of reproductive fitness; and 3) estimate heritability of male paternity success and additive genetic covariance with female multiple mating, thereby testing key hypotheses explaining the evolution of male and female reproductive strategies.I will achieve these objectives by developing and applying new measures of sperm performance and oxidative damage and state-of-the-art quantitative genetic analyses within an outstanding long-term field study on song sparrows (Melospiza melodia) where >20 years of paternity and pedigree data are already available.PatSuccess will thereby provide novel empirical understanding of the forces that shape variation in male paternity success and consequent selection on reproductive strategies, and substantially advance my skills and opportunities to become an independent research leader.