Summary Leach's Storm-Petrel (Oceanodroma leucorhoa) chicks accumulate fat through most of the nestling period, near the end of which they typically weigh 50–100% more than their parents. Much of this excess mass is lost abruptly prior to leaving the nest and fledging itself marks the termination of parental care. In this study, we evaluated the hypothesis that the onset of the prefledging mass loss period coincides with the end of the development period and the completion of structural growth. We characterized the pattern of prefledging mass loss and the timing of fledging with respect to a point (T), based on wing growth, at which structural development stops. Variation in T suggested that development rate is partially influenced by parental food provisioning, particularly early in the nestling period (age 16–20 days) just prior to the onset of rapid primary feather growth. The timing of mass loss and fledging in relation to T suggested that prefledging mass loss begins upon completion of wing growth and that fledging occurs after chicks have reached a mass compatible with sustained flight. Thus, the time to fledging is positively correlated with nestling mass at the end of the development period. The prefledging period of mass loss is a unique phase of anorexia placed between parental feeding and self-feeding in the life of the young petrel. Functional Ecology (2005) 19, 73 –80