Behavioural and fitness effects of experimental immune activation during incubation in a wild passerine

Incubation is vital for avian embryo development but demands significant parental investment, potentially at the cost of self‐maintenance and immune function. For example, the acute phase response (APR), a systemic reaction against inflammation, infection or tissue injury, can trigger sickness behaviours such as reduced activity and foraging. At the same time, immune activation may serve as an internal cue that prompts birds to prioritize current reproduction over potential future reproductive opportunities. In this study, we experimentally exposed wild European Pied Flycatcher Ficedula hypoleuca females to an immune challenge through lipopolysaccharide (LPS) injection to investigate the effects of immune activation on female incubation behaviour and its reproductive consequences. We found a clear reduction in the average duration of incubation and recess sessions in those birds administered with LPS, as well as a tendency to increase the number of incubation sessions, while other behaviours remained unaffected. These changes had no apparent effects on fitness‐related traits of females, but they were related to fledging success, which was higher in females administered with LPS. These findings suggest that females can maintain sufficient incubation conditions while simultaneously sustaining their own fitness during immune activation, as well as a compensation for potential future reproductive failures by the prioritization of immediate reproduction.