Independence and juvenile dispersal distances in wild and reintroduced Spanish imperial eagles

Abstract

Reintroductions have been increasingly used for species restoration and it seems that this conservation tool is going to be more used in the future. Consequently a better knowledge of consequences of this kind of management is needed. Several authors have found differences in dispersal distances among wild and reintroduced individuals; although no common explanation for this general trend (i.e. released animals moving farther than wild ones) has been proposed. Here we compared the dates of dispersal and the distribution of maximum distances during juvenile dispersal between a natural high density population and an alternative situation where young Spanish imperial eagles (Aquila adalberti) were reintroduced in a new area with a very low intra-specific density, and ad libitum feeding until the onset of dispersal. Results showed that maximum dispersal distances were longer in translocated (mean=205km) than in control juvenile eagles (mean=119.70km), and the shape of the distribution changed from leptokurtic right-skewed to quasi-normal. According to our results, for reintroduced young fed ad libitum, we can predict that effective dispersal distances will be longer in reintroduced young during juvenile dispersal than in natural populations. But dispersal distances might also depend on the distance from the release area to the nearest breeding population of the species. For reintroductions close to other existing populations, immigration of released young into existing populations would be greater than expected.