DIFFERENTIAL HABITAT SELECTION BY IMMATURE AND ADULT GREY EAGLE-BUZZARDS GERANOAETUS-MELANOLEUCUS

During the 1992 breeding season, we studied the habitat selection, beh aviour, aggressive interactions and diet of immature and adult Grey Ea gle-buzzards Geranoaetus melanoleucus in an area of the Argentinean Pa tagonia. Immature eagles selected areas rich in prey, had no preferenc e for flat or rugged areas and did not avoid areas close to active nes ts or those used by adult eagles. The density of the European Hare Lep us europaeus was the best predictor of the distribution of immature ea gles. Adult eagles preferred rugged to flat areas but were not signifi cantly affected by prey distribution. Immature eagles used napping nig ht more frequently than did adult eagles. Wing-napping frequency of im matures did not differ when flying over slopes or plains. In contrast, adults used a more economic flight with a lower wing-napping frequenc y than that of immatures, especially when flying over slopes. Adults, in contrast to immatures, tended to select slopes when flying over pre dominantly nat country. Only in 22% of the occasions when adult and im matures were seen together were immatures attacked by adults, a rate o f aggressive encounters similar to that observed between immatures (32 %). This similarity indicates that adults do not actively exclude imma tures from certain areas. The main prey of both immature and adult eag les was the introduced European Hare followed by native rodents and bi rds. Immatures ate significantly more carrion and fewer birds than did adults. These observations suggest that differences in night behaviou r and in the flight silhouette between adult and immature eagles may b e responsible for their different habitat selection in relation to top ography. While immatures apparently cue on prey density for habitat se lection, adults select areas that allow more economic foraging flights .