Sexual Dimorphism in Egg Size and Implications regarding Facultative Manipulation of Sex in Mountain White-Crowned Sparrows

The volume of male eggs in Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha) was slightly larger than that of female eggs in every year of a 5-year study. The volume difference was highly significant (P < 0.01) when data for all years were combined. Male nestlings also grew faster than female siblings. Natural history observations on adults suggest that reproductive success is more variable in male than in female White-crowned Sparrows. Taken together, these data are consistent with facultative manipulation of offspring sex as proposed under the Trivers and Willard hypothesis (1973). In avian species it may be necessary when testing this hypothesis to consider the relationship between sex and parental investment for individual offspring rather than focusing solely on differences between broods.