Overlapping moult and reproduction might be crucial for long-distance migratory birds, which are time-constrained to complete these energy-demanding functions before the onset of migration. However, proximate factors modulating the potential trade-off between moult and breeding, such as haemoparasite infection and stress, have not been studied in wild avian populations. We investigated the occurrence of moult-breeding overlap in females of a Spanish population of pied flycatchers (Ficedula hypoleuca) and its association with female age, haemoparasite prevalences, physiological stress, and condition at initial (incubation phase or beginning of the nestling period) and final breeding stages (days 11–12 of the nestling period). Late-breeding females were more likely to show a moult–breeding overlap than early-breeding females. Female age was not associated with moult status when taking into account laying date and study year. A higher proportion of females infected by Haemoproteus at initial breeding stages showed a moult–breeding overlap. Haemoproteus prevalences at final breeding stages did not differ according to moult status. Females with a moult–breeding overlap also showed better condition and lower stress levels (HSP60 levels) at the end of the season. A higher proportion of moulting females returned to the breeding grounds in the following season compared with non-moulting ones. Conversely, moulting females showed reduced hatching success and numbers of hatchlings and fledglings. Overlapping moult and breeding might be the expression of a shift in resource allocation between present and future reproduction, towards increased self-maintenance and reduced reproductive investment.

Nomenclature: Cramp, Perrins & Brooks, 1993.