Birds? Squirrels? Raccoons?
I don't know, but it looks like a nice place to live if you're a critter.
When I lived in CA, before I net Hubby, I lived in an area called Forest Lakes; in the mountains, in the redwoods. Across the creek, which ran through the backyard, and up the hill a bit, was a huge redwood tree that had a hole in the trunk. Every afternoon, around 4:30 or 5:00, I could see something stirring in that hole. With the help of some binoculars, I saw that it was a raccoon, just waking up, stretching, cleaning up to make a night of it.
I do love catching a glimpse of wildlife in their own habitat.