Reinforcement is not enough to establish a viable population of the globally threatened <i>Acrocephalus paludicola</i> (Aquatic Warbler) at the margin of its breeding range

Abstract Population viability analysis is a valuable tool to guide the reinforcement of threatened bird species. In migratory birds, demographic data required for population projections are often deficient, making reinforcement outcomes challenging to predict. The globally vulnerable Acrocephalus paludicola (Aquatic Warbler) is a long-distance migratory passerine that breeds in sedge fen mires in Central Europe and spends the nonbreeding season in sub-Saharan Africa. Although conservation measures, such as protecting breeding habitats from drainage or succession have stopped the decline in the core breeding sites, the westernmost breeding population (Pomeranian population) is currently threatened with extinction. In this study, we used integrated population models to determine the demographic parameters that contributed to the past decline, and then explored different management scenarios to identify which combination of reinforcement, habitat restoration, and survival improvement are required for achieving a stable population in the future. We found that the prevention of second broods due to habitat deterioration and grassland mowing during the breeding season adequately explained the observed decline of the Pomeranian population. To re-establish a viable population, adapted grassland management that allows second broods, the restoration of breeding habitat (&amp;gt;1,000 ha), and an increase in annual survival probability are required in addition to several years of releases of juveniles translocated from the core breeding area. Without an improvement in survival, populations may decline after releases end, but our projections are surrounded by large uncertainties due to unknown spatial dispersal and recruitment processes. In long-distance migrants, most causes for mortality are found outside the breeding area. Therefore, simultaneous conservation actions along the whole flyway are needed to increase the success of A. paludicola population recovery at the margin of the breeding range.