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Most bird species have low survival rates in their first year of life, and the highest losses occur when juveniles become independent and disperse. Young great horned owls (Bubo virginianus), monitored by telemetry in the southwestern Yukon, Canada, survived well during the peak of the population cycle of snowshoe hares (Lepus americanus). Subsequently, juvenile survival collapsed parallel to the decline in hare densities. The proportion of starving owls did not increase, but there was a significant increase in mortalities involving parasitism and predation, probably as an interaction with food shortage. The mortality rates of juvenile great horned owls peaked before, not during, dispersal. We propose that extended parental care makes the postfledging stage safe during optimal conditions, but that the relatively slow development during this stage incurs the cost of increased susceptibility to disease and other mortality factors under environmental stress.