This bee has taken up residence in this Rhus typhina (staghorn sumac) stump. If you look closely you'll notice old burrowings at the top. It appears to have been used for many years before it rotted away.
The bee kept reversing out of the hole, flying in a small circle over a log pile then reentering at the same spot.
The Rhus was a key feature of the back garden when we viewed the house 26 years ago. I've noticed there is one in the front garden of our future home.