I used a supplemental feeding experiment to determine whether adult Tufted Puffins (Fratercula cirrhata) would decrease provisioning effort in response to a reduction in nestling nutritional requirements, and to investigate the relationship between parental provisioning effort and timing of fledging. As predicted, parents of the supplemented nestlings decreased the number of provisioning trips but did not alter bill-load size or prey composition. Supplemental feeding significantly increased the growth of the culmen and tarsus but had no detectable effect on wing growth or body mass of chicks. Supplemented nestlings fledged significantly older than control nestlings. The maximum mass attained by the nestling and the age at which mass peaked also influenced timing of fledging. These results demonstrate that fledging age is influenced by both the energy provided by the parents and the nestling's developmental state and are consistent with the hypothesis that nestlings time their departure from the nest based on the costs and benefits of remaining.