Ontogeny of dispersal distances in young Spanish imperial eagles

Summary I studied factors influencing dispersal distances in 30 young Spanish imperial eagles (Aquila adalberti) radio-tagged in southwestern Spain in 1986–1990. The mean dispersal distance between the natal nest and the settling area was 138 km. Every young bird reached its maximum dispersal distance within 4 months of its departure from the natal population. No significant differences between the sexes were detected in maximum dispersal distance, but females spent more time in the more distant settling areas. Dispersal distance was not related to the date of departure from the natal population. Dispersal distance was significantly longer for birds that hatched earlier and that had lower blood urea levels, indicating that better-nourished young had longer displacements. These results are not in accordance with the competitive displacement hypothesis.