Biases in Diet Study Methods in the Bonelli's Eagle

Abstract

We compared 3 methods of studying diets of Bonelli's eagle (Hieraaetus fasciatus): (1) recent prey present in the nest; (2) remains collected in the nest after breeding; and (3) pellet contents, with the delivered prey by the eagles in 2 nests. Remains collected after breeding was the only method that differed (P = 0.001) from delivered prey by underestimating small prey, especially red squirrel (Sciurus vulgaris) and ocellated lizard (Lacerta lepida), and overestimated pigeons (Columba spp). Study of prey consumption patterns showed that small prey frequently were fully consumed but remains were often left from medium and large prey. Rabbits (Oryctolagus cuniculus), the largest prey, appeared in similar proportions to those in which they were delivered, probably due to the fact that the eagles removed the largest bones from the nest. Although the recent-prey-present method did not show significant differences (P = 0.730) from delivered prey, the method could hide some biases linked to the greater probability of detecting large prey such as rabbits. Considering the accuracy and the effort involved in obtaining information, we conclude that pellet analysis is the most efficient method of monitoring the diet of Bonelli's eagle.