A TRADE-OFF BETWEEN CURRENT REPRODUCTION AND MOLT IN THE PIED FLYCATCHER - AN EXPERIMENT

1. Mechanisms causing costs of reproduction in birds are poorly unders tood. Here we focus on the renewing of feathers (= moult) which may be a possible physiological link between successive breeding attempts. 2 . We performed clutch size manipulations to study whether the reproduc tive effort of the Pied Flycatcher (Ficedula hypoleuca) has any effect on the initiation and progress of the moult in the late nesting perio d. 3. The absolute timing of breeding did not affect the onset of pare nts' moult, i.e. late-breeding individuals did not start moulting befo re nestlings were fledged more frequently than early ones. This indica tes that moult was closely related to the breeding schedule of each in dividual bird. 4. The timing of moult was both sex and age related. Ma les and young birds were more likely to start moulting during the nest ling period than females and older birds. 5. Both females with large o riginal clutches and females with enlarged clutches delayed the onset of moult. This suggests a brood size-dependent regulation of moult. 6. We suggest that a trade-off between moult and current reproduction co uld be one possible mechanism causing costs of reproduction in birds.