A mysterious hole

My husband told me as I arrived home from work that we needed to go for a walk because there was something he wanted to show me.

We went to a local stretch of woodland, crouched down, and stared at a cut-off stump. A few seconds later, a little flash of blue appeared.

These blue tits have made a nest in the stump, only a food off the ground, and were flying back and forth with caterpillars and bugs to feed to their young. 

After many pictures of a plain hole in a tree stump, I finally managed to take this.