MASS-LOSS IN BROODING FEMALE PIED FLYCATCHERS FICEDULA-HYPOLEUCA - NOEVIDENCE FOR REPRODUCTIVE STRESS

In altricial birds, mass loss among brooding females may reflect nutri tional stress due to reduced time available for foraging or the cost o f warming young. Alternatively, it may be an adaptive adjustment to fu ture energetic demands. To distinguish between these hypotheses we con ducted a brood replacement experiment with Pied Flycatchers. Female Pi ed Flycatchers brood their young during the first 7 days after hatchin g. Nine broods with chicks 4 days old were replaced by broods that hat ched on the day of manipulation. Females of the first group thus brood ed for a longer period (''double brooding''), while the second group o f females experienced a shortened brooding period (''non-brooding''). Other broods were not manipulated. Chicks were returned to their origi nal nest after one week. Females adjusted their brooding and feeding r esponse to the age of the brood being cared for. ''Double brooding'' f emales lost significantly less mass than control females during the fi rst 8 days after hatching of their brood, although there was no differ ence between control and ''non-brooding'' females. There was no differ ence between treatments in nestling growth and survival. Total mass lo ss during the nestling period in the three treatments was equal. Femal e mass loss during the brooding period is not due to brooding effort, and presumably not a symptom of reproductive stress.