Differential habitat selection by immature and adult Grey Eagle-buzzards Geranoaetus melanoleucus

Abstract

During the 1992 breeding season, we studied the habitat selection, behaviour, aggressive interactions and diet of immature and adult Grey Eagle-buzzards Geranoaetus melanoleucus in an area of the Argentinean Patagonia. Immature eagles selected areas rich in prey, had no preference for flat or rugged areas and did not avoid areas close to active nests or those used by adult eagles. The density of the European Hare Lepus europaeus was the best predictor of the distribution of immature eagles. Adult eagles preferred rugged to flat areas but were not significantly affected by prey distribution. Immature eagles used flapping flight more frequently than did adult eagles. Wing-flapping frequency of immatures did not differ when flying over slopes or plains. In contrast, adults used a more economic flight with a lower wing-flapping frequency than that of immatures, especially when flying over slopes. Adults, in contrast to immatures, tended to select slopes when flying over predominantly flat country. Only in 22% of the occasions when adult and immatures were seen together were immatures attacked by adults, a rate of aggressive encounters similar to that observed between immatures (32%). This similarity indicates that adults do not actively exclude immatures from certain areas. The main prey of both immature and adult eagles was the introduced European Hare followed by native rodents and birds. Immatures ate significantly more carrion and fewer birds than did adults. These observations suggest that differences in flight behaviour and in the flight silhouette between adult and immature eagles may be responsible for their different habitat selection in relation to topography. While immatures apparently cue on prey density for habitat selection, adults select areas that allow more economic foraging flights