Sex differences in early determinants of lifetime reproductive success in a polygynous bird: Should mothers adjust offspring sex ratios?

Abstract

ABSTRACT The Trivers–Willard theory postulates that if a male in good condition at the end of the period of parental investment reproduces more successfully than a sister in similar condition, the mother should overproduce sons. Although this theory has been tested empirically in a wide variety of species, often producing equivocal results, few studies have analyzed whether the study populations conform to its assumptions. Here, we analyzed sex differences in lifetime reproductive success in a wild population of a polygynous bird, the Spotless Starling (Sturnus unicolor), and tested the Trivers–Willard prediction. We found that lifetime reproductive success was influenced by the body mass at fledging in males but not in females. However, although mothers with higher body mass produced heavier fledglings, they did not increase the proportion of sons. By contrast, we found that mothers with higher body mass produced heavier male fledglings and, after fledging, sons increased their body mass more than daught...