Supplementary feeding improves breeding performance in Eurasian Eagle Owl (Bubo bubo)

Abstract

After more than a century of persecution and low reproduction rate, the Eagle Owl is extinct, scarce or still declining in many areas. One possible mitigating action is to supply them artificially with food, but earlier experiments with supplementary feeding of other species have produced varying effects. Supplementary feeding has some caveats and controversies and needs to be tested to avoid counterintuitive management outcomes. Here we present an experimental supplementary feeding trial on a wild population of Eagle Owls on two islands in the middle of Norway. An Eagle Owl population of 27 territories was investigated from 1999 to 2019, and supplementary feeding was provided in three territories over two to seven years. Other important factors for Eagle Owl reproduction that have changed during the 21 years were, in addition to supplementary feeding, included in the GLMM analyses. We found significant earlier egg-laying in territories with supplementary feeding, and a delay in egg-laying in periods with a high corvid population. The probability of producing young increased with supplementary feeding, but was negatively affected by an increased number of pedestrians. This experiment shows that supplementary feeding can enhance breeding performance in Eagle Owls, and that food availability is a limiting factor. Despite that, we would not recommend supplementary feeding as a general mitigating method to help Eagle Owls, because it is time-consuming, the fledglings will have problems surviving if there is not enough food available naturally in the surroundings, and predatory mammals can be attracted to the Eagle Owl territories.