Disentangling the effect of parental care, food supply, and offspring decisions on the duration of the postfledging period

The postfledging dependence period (PFDP) is a crucial stage in the development of altricial birds. This period is regulated by parental investment, in terms of food provisioning and protection, and the demands of young associated with their development and physical condition. We examine the relative role of parental investment, food provisioning, and offspring decisions on the PFDP regulation in the Spanish imperial eagle (Aquila adalberti) by comparing the PFDP timing among young from non-manipulated territories, food supplemented territories, and birds translocated by hacking methods in the absence of adults and with ad libitum food supply. We found that extra food homogenized the nutritional condition of young and reduced the length of the first stage of PFDP, which is related to flight development and thus dependent on body condition. However, hacked birds did not reduce this stage despite ad libitum food, likely due to the lack of parental stimulus to develop advanced flights. Although the presence of adults might accelerate young becoming independent, hacked birds did not extend significantly the whole PFDP and all birds eventually started dispersal. Thereby, the PFDP regulation was primarily under offspring control, and modulated secondarily by parental effects independently of food provisioning and laying date. The length of this period seems to be constrained mainly by the inherent benefits of early dispersal on ultimate fitness in accordance with ontogenic hypotheses. In addition, hacking was shown to be an effective translocation method when properly used, without negative drawbacks for young development during the PFDP.