Reintroducing endangered raptors: A case study of supplementary feeding and removal of nestlings from wild populations

1. Supplementary feeding is a common practice to raise reproductive output in raptors and other species; nevertheless, its application in conservation has only recently been discussed critically. Here, we analyse the effect of supplementary feeding in territorial raptors, taking advantage of two long-term datasets for the Spanish imperial eagle (Aquila adalberti) and bearded vulture (Gypaetus barbatus). In both species, supplementary feeding was used over fouryears, allowing the extraction of eggs or nestlings for reintroduction programmes. 2. Both populations increased during the last 20 years. In 2001, only 10 Spanish imperial eagle pairs were found in Sierra Morena, increasing to 91 pairs in 2015 (810% of increase). The Bearded vulture population in Aragon increased from 15 occupied territories in 1988 to 67 in 2012 (347% of increase). Density-dependent breeding productivity on habitat heterogeneity was established in both populations. 3. Results of generalized linear mixed model analysis with relative productivity as the dependent variable, species and supplementary feeding as fixed factors, and territory as random factor showed a significant effect of supplementary feeding on relative productivity in both species as well as in the interaction between territory and supplementary feeding. This implied a different response among territories to supplementary feeding. Birds in poor-quality territories with low productivity levels responded more strongly to supplementary feeding than birds in territories with higher levels of natural productivity. 4. A reintroduction programme based on supplementary feeding and extraction of nestlings costs eight times less than the same programme based on captive breeding and takes 10 years less. 5. Synthesis and applications. Supplementary feeding in territorial raptors could be useful (1) in an episodic main prey collapse and (2) in poor-quality territories in a high-density population, to produce extra young for reintroduction programmes. For greatest efficiency, supplementary feeding needs to be targeted at poorer territories in which the reproductive rate has the potential to be raised by provision of extra food. The extra young produced can then be used in reintroduction programmes in which chances of recruiting to a breeding population are high.