Sex-specific mortality in nestling great-tailed grackles

Growth and survival of male and female Quiscalus mexicanus was monitored 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. Sex ratio bias in other than last-hatched eggs could be a consequence of predation not being independent of starvation. The fledging sex ratio in individual nesting colonies was negatively correlated with the level of starvation in those colonies. 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. Observations provide strong evidence of differential mortality among sons and daughters of a dimorphic species. -from Authors