Corticosterone mediated costs of reproduction link current to future breeding

Highlights • Cort in feathers grown while breeding reflect reproductive effort by giant petrels. • fCort was 1.5 times higher successful versus failed female breeders. • There were no differences in fCort between successful and failed male breeders. • We attribute these sex-differences to known differences in foraging ecology. • High fCort at breeding and slow moult predicted future breeding effort in females.