Just so that you don't think I have forgotten about Box1, here is a short sequence from that box.
This looks like a bluetit coming in to land on the entrance hole.
It seems to be hovering just outside the hole.
Still hovering!
Then, without ever appearing to touch the box, it stops hovering and is away!
The funny thing is that the box was empty at the time! Perhaps this is a newcomer to the nest box?
Bertie is back to Box2!
That looks like the top of Bertie's head to me.
For about 15 seconds he doesn't seem to do very much, and then, presumably because he has heard Bella outside, he crouches down.
Well, Bella is there all right! Or is it another bluetit?
Whoever it is, it is very interested in the box.
The bird moves to view things from a different angle.
Suddenly, Bertie gets the idea that this is not Bella after all. He starts displaying and from the look of his beak, making threatening noises!
All this fuss doesn't seem to have any affect on Bella - for I think this is Bella in spite of what Bertie may think.
Bertie is still in "repel all boarders" mode!
He must be convinced that this is a foreign bird.
Bella seems quite unfazed by all this fuss. She must be able to hear Bertie's fuss even if she cannot see him - and anyway, she saw him go into the box, didn't she!
Perhaps Bertie has suddenly realised that he has been making a fool of himself. His wings are starting to return to their normal state.
Suddenly, he decides that there is no threat to the box, but neither is Bella going to come and join him - so he leaves, watched by Bella.
"Well, what's all that fuss been about"
Bella gets on with nest building practice. She doesn't leave anything in the box though at the end of the day!