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 δ13C and the frequency of Eurasian red squirrels in the diet, and a significant negative correlation between δ13C and the frequency of Red-legged Partridges, which are species that occur in forested and open habitats, respectively. The values of δ15N 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, δ34S 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 δ13C, δ15N, and δ34S may be useful for assessing nestlings' dietary patterns in terms of main prey consumption and prey trophic level.
El águila Aquila fasciata es una de las rapaces más amenazadas de Europa debido a la elevada tasa de mortalidad adulta y preadulta, la degradación y pérdida del hábitat, así como una disminución de sus principales presas como el conejo europeo (Oryctolagus cuniculus) o la perdiz roja (Alectoris rufa). Durante la temporada de cría de 2008 se estudió la dieta de 15 parejas reproductoras de A. fasciata en Catalunya, noreste de la Península Ibérica, a través del análisis convencional de egagrópilas y el análisis de isótopos estables (AIE) en las plumas de los pollos. Nuestros objetivos fueron investigar la dieta de los pollos de A. fasciata, así como determinar si el AIE permite representar con exactitud sus patrones tróficos. El análisis de egagrópilas reveló una dieta variada que incluyó palomas (Columba spp.; 31.1%), conejo europeo (27.9%), “otras aves” (16.2%), perdiz roja (13.1%), la ardilla Sciurus vulgaris (5.2%), el lagarto Timon lepidus (2.6%), la gaviota Larus michahellis (2.2%) y “otros mamíferos” (1.7%) como principales categorías de presas. Sin embargo, la composición de la dieta fue heterogénea y se hallaron diversos patrones tróficos entre pollos pertenecientes a diferentes territorios de cría. Asimismo, se halló correlación positiva entre δ13C y la frecuencia de ardilla roja en la dieta, y nega
