Sexual dimorphism in house sparrow eggs

Recent evidence has revealed an apparently high degree of control by female birds over the physiological aspects of their reproduction and offspring s ex allocation, consistent with adaptive hypotheses of sex allocation and di fferential investment in their offspring. In the house sparrow, we investig ated possible mechanisms that may be used by females to enhance the fitness returns from a reproductive effort. Using molecular techniques, we demonst rate that house sparrow eggs containing male embryos are significantly larg er than those containing female embryos. We also found that male embryos we re laid randomly with respect to laying order. We speculate that this sexua l dimorphism of eggs is adaptive, because male house sparrows show greater variance in condition-dependent reproductive success than females. More imp ortant, the result provides further evidence of the ability of females to d etect or control ovulation of either male or female ova and to differential ly invest in one sex over the other.