Sex-specific energy requirements of great-tailed grackle (Quiscalus mexicanus) nestlings

Oxygen and food consumption of 4-11 day-old male and female great-tailed grackles were monitored. At 12 days post-hatching, female mass was 65.7% that of males, while female energy requirements over the 12 day period were 1181 kJ or 83% those of males. Thus, while male nestlings required more energy absolutely, female nestlings required proportionately more energy per unit body mass. There was no evidence that adults compensated for energy requirement differences of offspring by bringing male nestlings larger or more nutritious prey items. Sons are thus more costly to rear. Cases of differential mortality of male and female offspring are due to males suffering higher starvation when food is scarce. Biased sex ratios at fledging probably result from the cost to males of increased size due to sexual selection. -from Authors