A trade‐off between two resource‐demanding functions: post‐nuptial moult and immunity during reproduction in male pied flycatchers

Abstract

Summary Costs of reproduction might be mediated by a resource‐allocation trade‐off between immunity and reproductive effort. Recent studies have suggested that the moult–breeding overlap observed mainly in males may affect this trade‐off. In order to test whether increased investment in immunity may affect the onset of moult or reproductive output in a species with moult–breeding overlap, we immunized breeding male pied flycatchers Ficedula hypoleuca (Pallas) with a non‐pathogenic antigen (sheep red blood cells) in the first week post‐hatching and compared their moult score, parental effort and reproductive success shortly before fledging with that of control males injected with sterile phosphate‐buffered saline. Immunized males provisioning nestlings delayed the onset of post‐nuptial primary moult compared to control males. The experimental activation of the immune defence of males affected their stress levels (heterophile/lymphocyte ratio, heat‐shock proteins) on day 13 after chicks hatched. An activated immune response tended to depress reproductive output, although the experimental design was not efficient enough to find a significant effect because the manipulation was not performed early enough in the nestling period. The results suggest that experimental activation of the males’ immune system decreased the resources necessary for initiating post‐nuptial moult. The trade‐off between immunity and moult onset may imply costs of delayed moult in cases of infection and may interact with reproductive immunosuppression in cases of moult–breeding overlap. Moult, immunity and reproduction are locked in a three‐way interaction which may markedly affect avian life histories.