Supplementary feeding during the chick-rearing period is ineffective in increasing the breeding success in the bearded vulture (Gypaetus barbatus)

One of the most widespread tools in the conservation of scavenger species is the provision of supplementary food. However, scientific studies on its effectiveness have been rarely conducted. Here, we present the first results of an experimental specific supplementary feeding programme applied from hatching to 45-60 days, aimed at increasing the breeding success of an obligate brood reducer, the threatened bearded vulture Gypaetus barbatus. We tested its effectiveness in the Spanish Pyrenees, the locale of the most important population of this species in the European Union, in which a regressive trend in breeding parameters has been observed in recent years. We compared the breeding success in nests with supplementary food to non-supplemented control nests. Supplementary food did not significantly increase global breeding success (supplemented nests, 0.793 +/- 0.193 chicks per pair with eggs hatched vs non-supplemented nests, 0.771 +/- 0.185) or the individual breeding performance of the territories (supplemented period, 0.712 +/- 0.307 vs non-supplemented period, 0.642 +/- 0.311). The similar values obtained suggest that the specific supplementary feeding programme applied during the chick-rearing period is apparently ineffective at increasing breeding success. The results suggest that, at least in the study area, factors that provoke breeding failure after hatching continue to operate independently of the supplementary feeding programme. Although more research on this subject is required, these preliminary conclusions should be taken into account by managers in order to optimise the investment of economic resources and to better prioritise the future establishment of alternative conservation actions.