Sex and Egg Size in Great-Tailed Grackles

Abstract

Trivers and Willard (1973) suggested that when the variance in reproductive success of one sex is greater than that of the other, females in good condition should produce more offspring of the sex that exhibits the higher variance. This assumes that the condition of the female is reflected in the condition of the offspring and that the survival and breeding success of an individual is at least partially dependent on the condition of the individual at the end of parental care. Although Trivers and Willard's model predicts different strategies for different females, it is clear that nestling quality within individual broods may vary greatly and that females may potentially influence this variability in an adaptive way. For example, sequence-related hatching trends found in Lesser Snow Geese (Chen caerulescens, Ankney 1982) and Bald Eagles (Haliaeetus leucocephalus, Bortolotti 1984) may function to provide a competitive advantage for the larger sex in both species. Mead et al. (1987) recently found that eggs from which male White-crowned Sparrows (Zonotrichia leucophrys) hatched were larger than those from which females were hatched. Since the variance in reproductive success of males in this species is probably greater than that of females and since egg size has been shown in many studies to be correlated with nestling growth and survival, they interpreted these results as being an