Sex differences in avian yolk hormone levels

Abstract

Female birds may invest more in breeding attempts when paired with preferred males and may consequently produce chicks that are in better condition¹. But if females favour one sex when mated to preferred males, either in quantity^2,3 or quality, could this explain these differences in investment? We have shown that female mallards lay larger eggs for preferred males but do not produce more sons¹. This increased investment is not directed at one particular sex, and here we point out the importance of distinguishing between differential investment in the sexes per se, as suggested by Petrie et al., and differential investment in the sexes for different males.