Who's a clever bird?

I had to take this picture through the patio doors because everytime I even attempted to open the door the rook (or is it a crow?) made a quick exit!

I spent a good few minutes today watching it trying to undo the string that fastens this home made fat ball to the tree. It did eventually give up and just resorted to reaching down and taking huge mouthfuls of it! But it always amazes me how clever these birds are.