This Prothonotary Warbler’s nest is in that cavity, and the parent spent several minutes poking his head in, like he was moving something around.
I did not see if he brought something on this trip to the nest, like he did when I spotted the parent two days before: Prothonotary Warbler Bringing Food Home
The hole is about my eye level and I did see one chick beak when I passed by.
The parent did fly off with a fecal sac, something I hadn’t seen up close before.