REGULATION OF THE PERIOD OF POSTFLEDGING DEPENDENCE IN THE SPANISH IMPERIAL EAGLE AQUILA-ADALBERTI

From 1986 to 1988, 30 Spanish Imperial Eagle Aquila adalberti nestlings, equipped with radio transmitters, were studied in Donana National Park, southwestern Spain, to evaluate factors that determine when the young become independent. The results support the view that the dependence period is composed of two different periods, each representing almost half the total, and so the variations produced in one affect the overall period. The first part of the dependence period begins with leaving the nest and ends with the first soaring flight. The duration of this period is related to the physical condition of the young. The second period extends from the first soaring flight to the date of independence. Its duration is strongly related to hatching date. The young are not forced to become independent because of parental aggression. Aggressive behaviour allows the parent to evaluate the flight capacity of the young.