Good morning John,..and Marie.
       I live in the country, and had the same thing happening with birds, especially the Robins.    When I first saw this happening, they were going at my shed windows, and was wondering like you,..."What`s wrong with this bird" ?
         Like with anything in nature there is only so much room,.......and in my garden,..I guess there are so many earth worms ??  (smile)   I think they are only claiming their area,.......and they only see their reflection in the glass (especially if it is dark from the inside,......which becomes like a mirror)...........and the Robin is trying to only get at his own reflection,(thinking it is another Robin)........but doesn`t know it.  They would do this for hours at a time,......and day after day.     Hope this helps, some, and  I guess I could be wrong, too ???   Chip