To maximize fitness, breeding adults may respond to environmental processes by adjusting their progeny's sex ratios. R. A. Fisher in 1930 hypothesized that frequency-dependent selection would result in equal investment in sons and daughters over the long term, yielding a balanced sex ratio if the costs of raising a son and daughter are equal. Diverse hypotheses have tried to explain population and brood-by-brood deviations from this mean as well as annual variation by focusing on adult sex ratios, resources, abiotic conditions, and female and male quality. We collected data in 2002–2010 to explore population-level variation in nestling sex ratios in 2 migratory grassland songbird species: the Bobolink (Dolichonyx oryzivorus) and Savannah Sparrow (Passerculus sandwichensis). These species differ in migratory strategy (long-distance vs. short-distance), and morphological dimorphism. Fisher's hypothesis was rejected for Savannah Sparrows (n = 684 nestlings; 39% male) but not rejected for Bobolinks (n = 390 nestlings; 53.8% male). No relationship was found between nestling and adult sex ratios measured in the same year. In descriptive analyses at the brood level, male and female body size and age, and ecological conditions (temperature and precipitation) failed to predict nestling sex ratios. Although male nestlings were heavier than female nestlings and resource availability changed through the season, these factors did not influence sex ratios relative to female body size or seasonality. For Savannah Sparrows, larger broods tended to be male-biased. While we were otherwise not able to explain deviation in offspring sex ratio for Savannah Sparrows, our results suggest that the ecological and evolutionary pressures that affect sex ratios may be both species- and population-specific.
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Vol. 131 • No. 2