Factors affecting the length of the post-fledging period in the Bonelli's Eagle Hieraaetus fasciatus

Abstract

We investigated factors associated with the length of the post-fledging dependence period in 28 radio-tagged young Bonelli's Eagles Hieraaetus fasciatus monitored from 1998 to 2000. Under the assumptions of the Resource Competition Hypothesis (RCH), we predicted that young hatched in high-quality territories would have longer post-fledging dependence periods than young hatched in low-quality territories. As an alternative, we tested the Ontogenetic Switch Hypothesis (OSH), which predicts a better body condition in early dispersing young. For this purpose we measured body condition of young through levels of urea in the blood plasma. We found a weak positive effect of natal habitat quality on the length of the post-fledging dependence period, but no effect of body condition. Siblings were dependent on their parents for a similar number of days, suggesting that habitat and/or parent quality might influence the length of the post-fledging dependence period. Young hatched in 1999 had shorter post-fledging dependence periods than young hatched in 1998 or 2000. The date of independence for young Bonelli's Eagles was correlated with the timing of breeding (laying date), but not with the length of the post-fledging dependence period. Neither brood size nor sex affected the length of this period.