Sex differences in avian yolk hormone levels: Maternal investment

Female birds may invest more in breeding attempts when paired with preferred males and may consequently produce chicks that are in better condition 1 . 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 sons 1 . 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.