Birds are amazing. At least on your pictures. Your first picture brings back some sad memories. Once I had nesting box like yours with little Great Tits in it. One nice morning I heard from some distance a banging on wood. As I found out it was a woodpecker who beat a hole in the side bord. When arrived at the scene two little birds where taken one little left. I fixed the bord and ... ah it was not a nice ending. Bad woodpecker.
Thanks. It was my first chickadee. Didn't even knew them.