At about 14:00 whilst the Great Tit female was sitting in NB-2 (nesting box #2) just "relaxing", a Tree Sparrow suddenly entered the nesting box and attacked the female. The fight was brutal and the Sparrow clearly had the upperhand.
I only noticed this later on my display and noticed that the Freat Tit female was perched in the tree in what seemed to be a state of shock. Indeed she was very still, and did not notice what was going on around her. I opened the garden door and whilst the other birds flew away, she remained there without even noticing me. I did not want to distress her further and therefore did not leave the house. She remained in the tree in the very same position for at least 10-15 minutes. It was quite discomforting. In the meantime a Tree Sparrow again entered the nesting box. Some time passed and a Tree Sparrow again entered NB-2. At this point I entered the garden and moving to NB-2 I moved it around it. My intention (though perhaps not very "scientist that does not get involved") was to scare the Sparrow and thus discourage it from again entering the box in the future. It did get scared, that is for sure. From a scientific point of view I am definitely not proud of what I have done, from a maternal point of view I feel that I have done nothing wrong.
The floor of NB-2 is covered in small black feathers, a clear indication of the brutality of the fight which took place in it. The female Great Tit had almost completed the nest as the attack occured. Indeed she had started carrying in the "soft" material used for padding. I wonder how come the Tree Sparrows allowed the Great Tit to get so far with nest building and only just today mounted an attack.
I wonder if the female Great Tit will return, I wonder if the Sparrows will continue with the attacks and I wonder if they will even maybe destroy the eggs should the Great Tit lay any.
Only time will tell. At any rate, at this point in the afternoon, the birds tend to be somewhere else and thus I shall have to wait 'til tomorrow.