[eeyore]

Originally Posted by GuyF

My theory is that it is because it is springtime. I think he sees the reflection of himself in the window and thinks it is a rival so trys to attack it and out-do it. It's funny to watch, and I don't think it does the bird too much harm. So basically I think it's these birds being territorial and finding rivals (reflections) to try and scare off. I don't think it does any harm, but to stop him you could probably put a sheet over the window so there aren't any reflections.
Guy

I think you are right - i saw a male blue tit today kicking hell out of its own reflection in my car wing mirror