Balanced Sex Ratios in Dimorphic Altricial Birds: The Contribution of Sex-Specific Growth Dynamics

Abstract

Sex ratio theory holds that the population sex ratio at the termination of parental care should reflect expenditures on males and females. Where sexual dimorphism occurs in required expenditure, the sex ratio should be skewed in favor of the less costly sex. Male fledglings of the yellow-headed blackbird weigh about one and one-half times their female contemporaries, leading to a prediction of a strongly female-biased sex ratio. Data from this study and others show that this species appears to have a sex ratio approximating unity. I show differences in the dynamics of growth of males and females such that females fledge at an earlier age, show earlier maturation of plumage, relatively faster attainment of adult measurements, and lower variability of growth parameters than males. The effect is to sharply reduce the disparity in energetic requirements of males and females below that expected on the basis of weight. Other dimorphic species have yielded similar findings with regard to both sex ratios and growth patterns, suggesting that such mechanisms are general in maintaining a sex ratio of unity in dimorphic species.