Mass loss in brooding female Pied Flycatchers Ficedula hypoleuca: No evidence for reproductive stress

In altricial birds, mass loss among brooding females may reflect nutritional stress due to reduced time available for foraging or the cost of warming young. Alternatively, it may be an adaptive adjustment to future energetic demands. To distinguish between these hypotheses we conducted a brood replacement experiment with Pied Flycatchers. Female Pied Flycatchers brood their young during the first 7 days after hatching. Nine broods with chicks 4 days old were replaced by broods that hatched on the day of manipulation. Females of the first group thus brooded for a longer period (''double brooding''), while the second group of females experienced a shortened brooding period (''non-brooding''). Other broods were not manipulated. Chicks were returned to their original nest after one week. Females adjusted their brooding and feeding response to the age of the brood being cared for. ''Double brooding'' females lost significantly less mass than control females during the first 8 days after hatching of their brood, although there was no difference between control and ''non-brooding'' females. There was no difference between treatments in nestling growth and survival. Total mass loss during the nestling period in the three treatments was equal. Female mass loss during the brooding period is not due to brooding effort, and presumably not a symptom of reproductive stress.