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RESEARCH PRODUCT

A Trade-Off Between Current Reproduction and Moult in the Pied Flycatcher- an Experiment

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1. Mechanisms causing costs of reproduction in birds are poorly understood. 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 reproductive effort of the Pied Flycatcher (Ficedula hypoleuca) has any effect on the initiation and progress of the moult in the late nesting period. 3. The absolute timing of breeding did not affect the onset of parents' moult, i.e. latebreeding individuals did not start moulting before nestlings were fledged more frequently than early ones. This indicates that moult was closely related to the breeding schedule of each individual bird. 4. The timing of moult was both sex and age related. Males and young birds were more likely to start moulting during the nestling period than females and older birds. 5. Both females with large original 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 could be one possible mechanism causing costs of reproduction in birds.