On 26 March 2016, we watched this female White-breasted Nuthatch (Sitta carolinensis) approach a cavity on the underside of a large oak branch.
At first we thought she might be interested in using it as a nest site. But when she backed out with a mouthful of material, we realized that she was gathering nesting material from this cavity:
I don't know if there was a former nest in the cavity, or if softer materials had simply collected there and she was gathering them to use at a new site. Before this, I hadn't considered what nuthatches use to make their nests, so it was intriguing behavior to watch.