Well, for a variety of reasons, the boxes were not attractive to barn owls. In time, the plywood deteriorated and the boxes fell from the trees. In the meantime, the walnut trees continued to age and now they provide what looks like plenty of owl habitat where old branches used to be. We hear the owls at night.
And, much to my surprise (though I realize the irony here) we DO have a barn owl...
living in the barn.