Last year I observed Black-capped Chickadees nesting in an old fence post. Today I had a chance to watch them at work excavating the hole.
They started with a rotten Sumac about 6″ in diameter, broken off a few feet from the ground. Nice punky wood in the middle. They had obviously been working at this for a while.
In for a landing —
Repeat, taking turns, until the photographer gets too intrusive. They scolded me and after my retreat to the car, it was back to work.