Increasing the number of birds released enhances success of passerine translocations

Translocation is a conservation tool that has been implemented for decades to mitigate species decline (reinforcement) and local extinctions (reintroduction). However, translocations remain complex because of the wide range of different techniques, the need for substantial resources, and the variability of outcomes. Moreover, it is still unclear which variables influence translocation success, and even how to define what constitutes success. Despite these challenges, translocations have sometimes achieved very positive outcomes, so we need to deepen our understanding of the factors related to their success. In this study, we conducted a review of 50 conservation translocations of Passeriformes, implemented in different conditions and regions. First, we defined variables to be used as proxy of translocation success. Then, our findings suggest that main factors determining success in conservation translocations are mostly related to translocation logistics. Higher success rates were observed when more birds were released, and when birds were soft released. These results should guide the choice of variables that need to be carefully considered to measure success and increase effectiveness of future translocation efforts.