I'm not sure now whether this bluetit is coming or going, but whatever it is, he making a bit of a meal of it!
The birds in Box2 are easy to tell apart.
Here is the male bluetit ...
... and here is the female.
No problem there you will think.
This sequence in Box1 shows that it isn't always that easy.
Here the female is entering. And how can I be sure it's the female? By what happens next.
The female was following the male into box. The male is the bird in the middle with the female in the top of the picture.
These two are much more difficult to tell apart from their looks. However, telling them apart by their behaviour is much easier.
Directly she arrives, he prepares to leave ...
... leaving her alone in the box.
Exit one male bluetit.
He doesn't go far though. He perches on the roof of the box and probably chats to the female while she looks around.