It
all started with a bluetit arriving on the entrance of Box1.
No
messing about here - in it goes!(I use 'it' because I am not sure of its sex!)
Inside
the box all is as it should be.
Half
a minute later, another bird arrives. It approaches the box somewhat
tentatively as though it wasn't sure what welcome would await it.
Twenty
seconds later, it flies onto the entrance hole and has a look inside.
The
bird in the box is clearly not a friend of the one outside. Watching
what it is up to over a series of pictures, it is fluffing its feathers up
and cheeping vigorously. Not a welcoming posture!
Half
a minute later, the bird on the outside is still there but appears to be
distracted by something outside the box ...
... and flies off. (You can just see a blur of a bluetit's wing in the
bottom left of the picture!)
This
is where it starts to get tricky. Twenty seconds later a bird came
back and landed on the entrance hole again. Is this the same as the
last bird or is it a different one?
They both look very much alike but so do most bluetits at this stage of the season!
Whether
it's the same bird or not, the welcome is just the same! A minute has
passed since the similar picture above, but this bird hasn't relaxed its
guard at all.
This
time, though, it goes inside pretty quickly - about 10 seconds after landing
on the box.
The
birds inside still clearly doesn't like it ...
... so the newcomer turns ...
... completely round ... 
... before deciding to ...
... drop to the floor of the box anyway.Now, is this the way someone wanting to take over the box would behave? It seems to me that the first bird was a newcomer and this is the partner of the bird on the floor of the box. It is even turning it's back on the bird inside! So hint of anger or any preparation for an attack.
"Come on - it's only me!" it seems to be saying.