Zwolenka is one of the wildest small rivers in Poland. In 1996-2006, an area of 15 km2 of the valley was surveyed along a 25-km section of the river. In total, 110 breeding species were recorded, including the Hen Harrier Circus cyaneus. Probably breeding were Ruff Philomachus pugnax, Great Snipe Gallinago media, and Crane Grus grus. A pair of Pintails Anas acuta was observed in the breeding season. In addition, 5-6 individuals of Aquatic Warbler Acrocephalus paludicola were noted in 2005. Numbers of Corncrakes Crex crex (30 singing males) qualify this area as an Important Bird Area of European rank.