Older or heavier Rhinoceros Auklet Cerorhinca monocerata chicks are less likely to remain in their nest during pre-fledging mass recession. Older or heavier chicks have longer wings but the individual variation in wing length at fledging is small. To identify proximate triggers of fledging from a mix of candidates when chicks experienced mass recession, we selected 30 nest boxes each containing a chick and provided half of them with 30-60 g of supplementary foods in an attempt to expand their variation in growth rate. When chicks reached the minimum wing length required for fledging (130 mm), we stopped all provisioning (supplementary and parental) by closing a hatch on the nest box entrance thereby simulating mass recession and inducing fledging. With these treatments, we examined whether timing of fledging (=duration of mass recession) was only triggered by wing length to a threshold size or whether timing of fledging was delayed by younger age at peak mass or lighter peak body mass. Our results indicate that younger chicks remained in their nests longer than older chicks at the beginning of pre-fledging mass recession, regardless of the small variation in wing length among individuals. Annual variation in the duration of mass recession was also detected but body mass was not the trigger for fledging. Measurement of baseline corticosterone level indicated a negative trend between the corticosterone level and the number of days between the measurement date and the fledging date.

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