Daily energy expenditure and cell-mediated immunity in pied flycatchers while feeding nestlings: interaction with moult

Ecological immunology posits a trade-off between parental effort and immunocompetence underlying the cost of reproduction. The moult-breeding overlap observed in several bird species represents a conflict in resource allocation between two energy-demanding processes. Moult processes have been associated with enlargements of immune system organs. In the present study. we measured simultaneously daily energy expenditure (DEE) and the T-cell-dependent immune response of pied flycatchers, Ficedula hypoleuca, caring for grown nestlings. We used the doubly labelled water technique and the phytohaemagglutinin (PHA) injection assay on both males and females, while recording provisioning rates and moult scores. DEE and the PHA response were negatively correlated for females, but not for males. A significantly higher proportion of males than females initiated moult. Provisioning rates were strongly correlated with DEE for females but only for non-moulting males. The DEE of moulting males was marginally correlated with moult score. For moulting males, there was a marginally significant positive correlation between moult score and immune response. The trade-off between DEE and immunity for females could underlie the cost of reproduction. However, the moult-breeding overlap found in males may offset this tradeoff, thereby reducing the implications of immunosuppression for parental survival.