This seems to be the most common position adopted by fighting bluetits - both birds lying on their sides, head to head (or should I say, beak to beak as this is probably this little bird's most fearsome weapon?) They use their claws to kick at each other and try to take chunks out of each other's belly. In doing this they frequently remove some of the other bird's under feathers.
You can also clearly see that the bird on the left is male. His head and neck feathers are raised, presumably to make him look more fearsome. However, this doesn't appear to have the slightest effect on Joy!
For two birds that didn't appear to want to fight to begin with, they are making a pretty good show of it now.
They just go on ...
... and on ...
... and on.
Neither bird appears
to have the advantage.
Is this when Joy on the right gets the upper hand?
Joy is still on her side, but the male appears to be leaving the fight.
Yes, he is getting up ...
... and going! He's had enough.
Joy goes to the hole to see him on his way ...
... before returning to the scene of her great and victorious battle.
Finally, she gives a celebratory nesting wriggle - commonly done by a successful female after a fight.