EXPERIMENTALLY REDUCED PATERNITY AFFECTS PATERNAL EFFORT AND REPRODUCTIVE SUCCESS IN PIED FLYCATCHERS

The question of whether, and how, male birds should change their paren tal effort in response to reduced paternity is a controversial issue a mong behavioural ecologists. We report a study on pied flycatchers, Fi cedula hypoleuca, in which paternity was manipulated through experimen tally induced mate switching during the female's fertile period. The p aternity 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 effec t on female reproductive success. For 17 monogamous males, provisionin g 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 curviline ar decline, whereas nestling body mass showed a linear decline. Two im portant assumptions of the theory of optimal parental effort seem to b e fulfilled in this case: that males had reliable cues to their patern ity, and that paternity is likely to be increased in future (normal) b reeding attempts. The fact that males reduced their effort at the expe nse of the welfare of the offspring suggests that there are significan t costs of parental care in this species. Whether a similar response t o cuckoldry occurs in unmanipulated breeding situations remains to be studied. (C) 1998 The Association for the Study of Animal Behaviour.