Sex‐Specific Mortality in Nestling Great‐Tailed Grackles

Abstract

The growth and survival of male and female Great—tailed Grackles (Quiscalus mexicanus) was monitored between 1985 and 1987 to test the hypothesis that, because of their large size, sons are more likely than daughters to suffer when food is limited. Larger egg and clutch sizes, as well as lower levels of starvation, indicated that food was more abundant in 1985 than in 1986 or 1987. Fledgling sex ratios were 1:1 in 1985 but female—biased in the latter 2 yr, suggesting that males suffered higher mortality during years of food shortage. Biased fledging sex ratios were due primarily, but not exclusively, to higher male starvation among last—hatched nestlings. Indirect evidence suggests that sex ratio bias in other than last—hatched eggs could be a consequence of predation not being independent of starvation. The fledging sex ratio (males: females) in individual nesting colonies was negatively correlated with the level of starvation in those colonies. Finally, an experiment that subjected male and female nestlings to a similar and potentially food—stressed situation suggested that males were affected more severely than females. Together, these observations provide strong evidence of differential mortality among sons and daughters of a dimorphic species. Although these data are qualitatively consistent with Fisher's (1930) hypothesis of biased sex ratios balancing investment in sons and daughters, the data are also consistent with the hypothesis that biased sex ratios are a nonadaptive consequence of one sex being more costly to produce and of a 1:1 primary sex ratio.