Differential dispersal, local resource competition, and sex ratio variation in birds.

Local resource competition arising through differential natal dispersal of daughters and sons may be an overlooked selection pressure on progeny sex ratio variation in birds. Passerines have predominantly daughter-biased natal dispersal; anseriforms have predominantly son-biased natal dispersal. Therefore, the local resource competition hypothesis predicts that passerines will have daughter-biased sex ratios and anseriforms will have son-biased sex ratios. These predictions are supported by observations of between-order variation in progeny sex ratios. Across latitude and within season, variation in progeny sex ratios of given species may also reflect selection through local resource competition arising in association with variation in relative son and daughter philopatry.