Dispersal of juvenile and immature Bonelli's Eagles in northeastern Spain

Between 1986-93, we wing tagged and banded 122 Bonelli's Eagles (Hieraaetus fasciatus) in northeastern Spain to analyze their dispersal before recruit ment to the breeding population. By 1998, we had obtained 18 band returns a nd 42 incidental observations of juvenile and immature eagles ranging in ag e from 1-3 yr. These eagles were recorded from 1-1020 km from their nests w ith a geometric mean distance (95% C.I.) of 101 km (72-153). Up to 57% of t he birds remained within a radius of 100 km of their nests, whereas 33% wer e found beyond 200 km. Short-distance dispersers were found mainly in north eastern Spain in central Catalonia (<200 km), while long-distance disperser s (>200 km) were found in central and southeastern Spain. No significant di fference in dispersal distance was found between males (101 km, 40-273, N = 15) and females (189 km, 86-419, N = 11), but males were mostly recorded a t shorter distances. Dispersal distances of juveniles (114 km, 68-193, N = 43) and immatures (77 km, 44-135, N = 16) also did not differ significantly , but immatures were mostly recorded at shorter distances. No significant d ifference was found between sighting (82 km, 54-137) and band recovery (167 km, 87-323) distances, but the proportion of band recoveries to sightings was lower for short- than for long-distance dispersers, and it increased wi th distance. The recording rate declined sharply at the end of the first ye ar of life, suggesting high mortality during this period. The main causes o f death were electrocution and human persecution. Most long-distance disper sers were reported dead, suggesting that long-distance movements entailed s ome mortality costs.