Breeding statistics of the Aquatic WarblerAcrocephalus paludicola on the Biebrza marshes, northeast Poland

The study population of the Aquatic Warbler showed a low proportion of unfertile eggs (8.6 %), very low nest losses (17.9 %), including losses from predation (11.1 %), and high average production of fledglings per nest (3.25 ±1.9) compared with populations of its congeners and other related species. As adult mortality also seems low, it is concluded that the main threat to this endangered species in the study area may be change of habitat.