The vast wetland area at the Biebrza river is the biggest marsh complex in Poland. It is of particular significance for biodiversity preservation. 273 bird species are registered on the territory of the National Park, including 181 bird species breeding here. 56 bird species found at the Biebrza are included into the Polish Red Data Book. In the extending sedge meadows a numerous population of globally threatened Aquatic Warbler is settled, threatened Great Snipes gather for competitive displays, Short-eared Owls, Corncrakes breed and Jack Snipes stay here.
Aquatic Warbler - by Marius Karlonas
In wet forests globally threatened Greater Spotted Eagles, Black Storks nest, and Eurasian Eagle Owls, Lesser Spotted Eagles are common here. In numerous channels, old river beds, wide reed beds Eurasian Bitterns, Whiskered and White-winged Terns, Great Egrets live, a rich variety of small song birds is found.
In spring migration, big flocks of migrating swans, geese and ducks gather in the flooded Biebrza river valley. The symbol in the logo of the Biebrza National Park is the Ruff, as nearly nowhere else one could see such agglomerations of migrating waders. In autumn migration, all marshlands and nearby fields vibrate with whooping crane sounds. Boggy and inaccessible marshes are safe places for their overnight stay and rest.
In the National Park, besides the unique variety of birds, 48 species of mammals, 12 species of amphibians, 5 species of reptiles, 36 species of fish, 750 species of butterflies, 500 species of beetles, 450 species of spiders are registered. The Biebrza marshes and forests are of particular importance to existence of moose and elks. In Poland, the Biebrza National Park is the most important living place of mouse where ca. 600 animals are concentrated. By the river, beavers prosper in big numbers, and, possibly, Yotvingians named the river Biebrza due to abundance of these animals. Otters are not rare in the river. Over 1000 species of higher plants are found in the Park.