The Slavic paradise where birds migrate in winter is Vyraj.
Vyraj—also spelled Iriy, Vyriy, or Irij—is a mythical otherworld associated with wintering birds and, in some traditions, the destination of human souls after death. Birds were imagined to leave the visible world for this distant realm and return in spring, linking seasonal migration with the cycle of life, death, and renewal.
Descriptions vary across Slavic traditions. Vyraj could be imagined beyond the sea, at the edge of the Milky Way, or as a garden beyond a guarded gate. Some accounts place it in the crown of the cosmic tree, whose branches shelter birds that may represent human souls. Veles is sometimes said to guard its entrance, holding the keys to the otherworlds.
A common mix-up is treating Vyraj as one universally fixed heaven. Folklorists have noted that “Iriy” may function as a broad name for the otherworld rather than a precise location. The related terms Yav, Nav, and Prav are instead commonly used in modern retellings for different aspects of reality, not as the standard name of this migratory paradise.