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.