There’s something inside…

There’s something inside the space in this Populus suaveolens.
A walnut seems to have been placed below the hole intentionally.
I’ve often heard that Dendrocopos major prepare their meals in advance. But perhaps this is the work of Sciurus vulgaris orientis or crow.
Crows are thought to eat on the spot, so maybe the walnut was hidden there by the Sciurus vulgaris orientis.
But what lives in the hole?
Sciurus vulgaris orientis? Martes zibellina brachyura? Strix uralensis coreensis?
One of the fun things about a nature walk is discovering mysteries like this in the forest and then letting your imagination run wild as you try to explain them.
Photo: The work of Sciurus vulgaris orientis? Mar. 17