The white-breasted nuthatches that nested in our yard this year and last have behaved in an obvious way that seems to have no known explanation.
It’s called bill-sweeping. The bird holds an insect or other item in its mouth and vigorously sweeps it over the nest cavity entrance and for several inches around that entrance. I’ve photographed it at least twice. Usually, it appears to be an insect the bird is holding. This spring the most obvious item used was what appeared to be a cluster of feathers, as might have been pulled from the breast of a large bird. We have ducks in our pond. They groom themselves, and the feathers might be theirs.
I’ve searched ornithological records and reports printed in ornithological journals (sora.umn.edu). There was one short item from the journal The Auk, published in 1970. I’ve asked a professor of ornithology. The answer from both was speculation that the action might be meant as protection against predation or insect infestation (like mites). I can’t find a definitive answer. “Why” is obviously a tough question.
In the photos the bird holds an insect (last spring), and a clump of feathers from about a week ago. The insect might be a beetle.