The European breeding range of the three-toed woodpecker Picoides tridactylus overlaps with the distribution range of coniferous and mixed forests with the dominance of Norway spruce Picea abies or European silver fir Abies alba. The paper describes a new breeding site of the three-toed woodpecker in the Sobibór Forest, eastern Poland outside the distribution range of these two tree species and its possible relationship with Eurasian beaver Castor fiber. The birds occurred in dead alder forests and were recorded 3–4 years after the forest started to decline due to the high level of surface waters associated with the keystone role of the beaver. The decay cycle of trees was possible because the area settled by woodpeckers was protected as a nature reserve.

Keywords: isolated breeding site, keystone species, suboptimal habitat, deciduous swamp forest, decaying stands, Western Polesia