Feather corticosterone in Northern Bald Ibis Geronticus eremita: a stable matrix over time able to predict reproductive success

Measuring corticosterone concentrations in feathers (FCC) has been increasingly used as an integrated index of bird's response to stressors, offering valuable insights into subsequent carry-over effects. However, it is still unclear to what extent corticosterone levels deposited in feathers during the moulting period relate to individual physiology in high-energetic demanding situations. In addition, the stability and repeatability of FCC over the same feather generation and across successive generations are still unresolved questions. In this study, we assessed FCC in 24 captive Northern Bald Ibis, Geronticus eremita. We sampled body feathers on three occasions during two consecutive years (2015-2016) to explore within-individual stability of FCC in feathers grown during the same generation and test whether FCC was repeatable or varied between two consecutive generations. Then, we explored whether FCC (a) was associated with individual health status, (b) correlated with previous reproductive success and (c) was related to future reproductive success. Results showed high stability of FCC among feathers of the same generation, while FCC differed between successive generations. We also observed that FCC, age and health status were negatively associated with subsequent reproductive success the following season, while no association was found with previous reproductive success. Overall, our study supports the use of feathers as a stable measure of corticosterone over the same feather replacement generation and emphasizes the usefulness of FCC as a biomarker of subsequent reproductive success.