Sexual conflict among polygynous pied flycatchers feeding young

In the polygynous pied flycatcher, Ficedula hypoleuca, reproductive success of females is constrained by male food provisioning during the nestling period. Hence, there will be conflicting interests among the male and each of his mates as to how male feeding effort should be shared among broods. This paper describes three experiments designed to examine the parental behavior of the members of a bigynous trio, i.e., the male and his two mates, in light of these conflicts. In all experiments, primary and secondary broods were manipulated to hatch on the same day to reduce the difference in brood-reproductive value due to age. Males divided their effort equally when the two broods were the same size. However, males did not allocate their investment in proportion to brood size when brood sizes differed, but invested more heavily per young in the larger broods. This finding suggests that males tried to optimize the joint effort of their two mates. Males and females showed similar responses to experimental reduction in brood demands, which indicates no difference in their willingness to invest in offspring. When one of the male's mates was removed temporarily, the male increased his total feeding rate and provided proportionately more food to the "motherless" brood. Through flexible allocation of parental investment, males seem able to optimize their reproductive interests in the two broods. The only way a polygynously mated female might successfully increase the amount of male assistance at her nest is to make her own brood more valuable for the male, relative to the other broods he might have. We discuss some ways this might be achieved. [Behav Ecol 1991;2:106-115]