Supplementary feeding as an effective tool for improving breeding success in the Spanish imperial eagle (Aquila adalberti)

Abstract

We studied the factors that led to brood reduction in 1192 nest/years of 143 territories of the Spanish imperial eagle during the period 1988–2003 and we carried out an innovative supplementary feeding programme in perches near nests in order to mitigate nestling mortality and improve fledging rate. Chick mortality affected 9.3% of hatchlings and 13.9% of the nests monitored. Sibling aggression was the most important factor in brood reduction (54.8% mortality of chicks in the nest), and supplementary feeding limited the process of sibling aggression. In addition, we compared the fledging rate in nests with supplementary food to that of non-supplemented control nests. Supplementary food increased the number of fledglings per brood (1.56 vs 0.72 in the control nests). This increase was repeated in subadult pairs (1.57 vs 0.53 in the control nests) and occurred both in high-quality territories (2.32 vs 1.36) and in low-quality territories (1.35 vs 0.54). Our results also suggest that supplementary feeding does not appear to have any effect on the provisioning rate of either males or females or on the wild prey. Food supplementation led to the recovery of breeding success after a period of loss due to a reduction of prey as a consequence of viral haemorrhagic disease. We conclude that the Spanish imperial eagle’s breeding output is food-limited during the nestling period and that food taken to the nest regulates sibling aggression. Supplemental feeding when food is scarce, or under emergency conditions, can be considered a good management tool for the conservation of this endangered species.