Is it or isn't it?

This is a heavily-cropped image of one of the bluetits which we have recently observed flying backwards and forwards to a limb of our oak tree. 

It’s impossible to tell if a nest is being built behind that jagged stump, but it seems entirely likely. I have tried to view the tree from the other side, but there isn’t a convenient spot to do that because our next door neighbour’s house gets in the way. 

So far I’ve not seen any twigs or vegetable matter in the birds’ beaks to back up my theory, so I shall just have to continue craning my neck upwards as I sit in the family room looking out at the oak tree, until I have my answer.