Sex-dependent responses to increased parental effort in the pied flycatcher

The optimal number of offspring for males and females may differ, as males and females invest differently in different aspects of reproduction. This creates potential for a sexual conflict leading to reduced residual reproductive value of the sex that experiences exploitation by the other sex. We experimentally investigated (by changing the brood size by one offspring) the effects of increased or decreased parental effort on the future local survival and breeding success of adult pied flycatchers (Ficedula hypoleuca), and on the future expression of male ornamentation. In addition, we studied how experimentally changing the brood size affected offspring condition and their recruitment. We show that females (but not males) rearing enlarged broods had lower local survival compared to those rearing reduced or control broods. This indicates a sexual conflict concerning optimal brood size. However, even though brood size manipulation did not affect the local survival of males, it did have an influence on the dorsal melanin coloration of young males. Young males rearing enlarged broods showed a stronger increase in plumage darkness from the current to the next breeding season than those rearing reduced or control broods. This suggests that stress experienced during reproduction might have carry-over effects that influence the complicated melanocortin system and lead to changes in the expression of melanin-based coloration. Alternatively, successful breeding might have stimulated young males to further invest in reproduction in the following season. Taken together these results indicate that both sexes are affected by brood demands, but in different ways.