USING STABLE ISOTOPES TO DETERMINE DIETARY PATTERNS IN BONELLI'S EAGLE (AQUILA FASCIATA) NESTLINGS

Bonelli's Eagle (Aquila fasciata) is one of the most endangered raptor species in Europe due to high adult and subadult mortality rates, habitat loss, and a decrease in populations of its most important prey, European rabbits (Oryctolagus cuniculus) and Red-legged Partridges (Alectoris rufa). During the breeding season of 2008, we studied the diet of Bonelli's Eagles at 15 breeding territories in Catalonia, northeastern Iberian Peninsula, through a conventional pellet analysis and stable isotope analyses (SIA) of nestlings' feathers. Our objectives were to investigate the diet of Bonelli's Eagle nestlings and to determine whether SIA allowed accurate representation of their dietary patterns. The pellet analysis revealed a broad diet including pigeons (Columba spp.; 31.1%), European rabbits (27.9%), "other birds" (16.2%), Red-legged Partridges (13.1%), Eurasian red squirrels (Sciurus vulgaris; 5.2%), ocellated lizards (Timon lepidus; 2.6%), Yellow-legged Gulls (Larus michahellis; 2.2%) and "other mammals" (1.7%). Diet composition was heterogeneous and varied markedly among nestlings from different breeding territories. We found a significant positive correlation between delta C-13 and the frequency of Eurasian red squirrels in the diet, and a significant negative correlation between delta C-13 and the frequency of Red-legged Partridges, which are species that occur in forested and open habitats, respectively. The values of delta N-15 were not correlated with the consumption of any prey category. However, its wide range of values suggested a global diet with a broad diversity of prey species from at least two different trophic levels. Finally, delta S-34 were higher for those nestlings that fed on Yellow-legged Gulls. Our study provided the first isotopic approach to the trophic ecology of Bonelli's Eagle nestlings, and we concluded that delta C-13, delta N-15, and delta S-34 may be useful for assessing nestlings' dietary patterns in terms of main prey consumption and prey trophic level.