Identifying the causes of individual variation in fitness should improve predictions about population dynamics and responses of populations to environmental change. Precise predictions may require long-term studies to parameterize models when the fitness of individual phenotypes depends on environmental conditions. We used a 37-yr study of a resident Song Sparrow (Melospiza melodia) population to identify traits that predicted individual variation in female lifetime reproductive success and to test for context dependence in trait–fitness relationships. Specifically, we asked how individual inbreeding coefficient, maternal age, and a suite of natal morphological traits influenced 2 components of lifetime reproductive success: (1) the probability of surviving to breed, and (2) the lifetime number of offspring produced, given that a female bred locally. We then tested whether population density influenced trait–fitness relationships. We found that differences in natal traits had life-long impacts on female fitness. Lower maternal age, a higher inbreeding coefficient, later laying date, lower nestling body condition, and longer tarsi were all negatively related to lifetime reproductive success. Maternal age and the inbreeding coefficient influenced both components of lifetime reproductive success, whereas other factors only influenced one. Therefore, traits that predict the probability of surviving to breed locally may differ from those that predict the number of offspring produced. We also observed larger effects of the inbreeding coefficient on fitness in years of low population density, which were often preceded by cool winters. Our findings demonstrate that natal traits and the environment experienced early in a bird's life can have life-long effects on individual fitness, primarily independent of population density.
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