Experimentally reduced paternity affects paternal effort and reproductive success in pied flycatchers.

The question of whether, and how, male birds should change their parental effort in response to reduced paternity is a controversial issue among behavioural ecologists. We report a study on pied flycatchers, Ficedula hypoleuca, in which paternity was manipulated through experimentally induced mate switching during the female's fertile period. The paternity of care-giving males ranged from 0 to 100% of the brood. The number of parental males per nest varied between zero and two, and the amount of male assistance in nestling provisioning had a marked effect on female reproductive success. For 17 monogamous males, provisioning effort and the body mass of nestlings on day 12 were reduced at low levels of paternity. However, the shape of the effort function was not unambiguously determined. Male provisioning effort showed a curvilinear decline, whereas nestling body mass showed a linear decline. Two important assumptions of the theory of optimal parental effort seem to be fulfilled in this case: that males had reliable cues to their paternity, and that paternity is likely to be increased in future (normal) breeding attempts. The fact that males reduced their effort at the expense of the welfare of the offspring suggests that there are significant costs of parental care in this species. Whether a similar response to cuckoldry occurs in unmanipulated breeding situations remains to be studied.Copyright 1998 The Association for the Study of Animal Behaviour.