Słońsk, Nature reserve in Gmina Słońsk, Poland

Słońsk is a nature reserve in western Poland featuring wetlands, meadows, and a network of waterways created by the Warta River. These water landscapes provide habitat for numerous bird species that inhabit the area year-round.

The area was protected in 1977 and later formed a core part of the Warta Mouth National Park established in 2001. This designation marked the beginning of comprehensive protection for the region's valuable water landscapes.

The name Słońsk comes from the town within whose commune the reserve lay. Before protection, the meadows were mown for hay and grazed by cattle, and from 1953 the Polish Hunting Association leased the grounds. Autumn gatherings of over 100,000 birds of many species were the sight the reserve was known for.

The reserve lay in the fork between the Warta River and the Postomia Canal, with Kostrzyn Lake entirely inside its borders and about 80% of the Kostrzyn retention reservoir. Its eastern boundary was the Królewska Wyspa island, surrounded by the Skarbowe Meadows. About 1,100 ha were under strict or seasonal protection and 3,066 ha under partial protection.

The reserve was one of Poland's largest, covering 4,244 hectares (the founding act lists 4,166.22 ha); only the Milicz Ponds reserve, at 5,298.15 ha, was bigger. It lay between the Warta River and the Postomia Canal and took in all of Lake Kostrzyn and about 80% of the Kostrzyn retention reservoir. In autumn more than 100,000 birds of various species could gather there.