A Red-Breasted Nuthatch hollows out a nest in a tree. It would tap at the wood from inside the cavity and then carry it out of the hole one beakful at a time. This cavity was deep enough to completely hide the nuthatch when it dropped down to excavate.
Mated pairs build the nest together, while unmated males make up to four nests to impress potential mates with their work ethic and engineering skills. The female selects the nest from the options her new mate presents.
Hopefully this nest is judged worthy of housing tiny nuthatches!