Fairly big hole. Have you seen what made it? Had one almost that big in a dead limb of the apple tree made by a flicker. Several things lived in it before that branch totally rotted including a small round faced owl.

A Pileated Woodpecker made it about 3yrs ago and raised two chicks/birds. The following year honey bees made use of it. Nothing is using it at the monent, but for two consecutive early mornings last week,(low light,fog) I saw a pair of birds circle around, they lined up and side by side, flew straight to the tree. One went in, the other kept going. As i watched their flight my first thought was ducks, what are they doing here?? Their short, stubby wings and rapid beat. I watched the hole with binoculars for the next 20min or so, both mornings. Nothing happened.
Nothing more this week. I suspect they could have been flickers. I see these birds from time to time, but I haven't seen them in flight. Go figure.

Wood ducks do nest in trees however just south of my house is a creek. I have seen mallard ducks, male and female, standing on tree branches in the woods along side it. I have never heard that they nest in trees but since the can stand on branches, why not?

Wood ducks crossed my mind too, as there is a "wild" 4 acre
pond nearby and I've got 2-3 acres of bottomland in the back ajoining it.
Surely, I would have spotted some markings on wood ducks as they flew past, even in the poor light. I would be estatic to have those.
Whatever it was they haven't been back.
That particular snag is close to the camp.
To much "activity."