FEATHER CORTICOSTERONE VALIDATIONS
Feather corticosterone (CORT) has the potential to serve as a biomarker of disturbance, habitat quality, reproductive effort, and carry-over effects. However, the response of feather CORT to changes in workload or environmental stress remains an important gap, hindering interpretation of individual variation in this trait in the wild. In our recent paper in Oecologia, we combined multi-year reproductive monitoring, a manipulation of workload, and measurements of feather CORT to provide an experimental examination of whether feather CORT levels reflect prior effort or future reproductive potential in female tree swallows. We found that CORT levels of feathers grown during moult did not reflect past breeding experience, predict reproductive output, or respond to a manipulation of flight effort during reproduction. Higher feather CORT levels predicted higher return rate (a proxy for survival), but they did so only in the manipulated group, and this relationship was opposite to expected. Our results indicate that CORT levels or disturbances experienced during one time (e.g., breeding) may not carry-over to subsequent stages (e.g., moult).