LOL, he sees another male in his territory and he is going to try to get rid of him. The other male obviously is his own reflection. So anything you can do to cut down on that reflection would help.

Decals on the window, they do make some that stick due to static cling. Putting cardboard or other covering over the windows. It's a pain would will also keep your truck cleaner.

I had a bluebird do the same thing against my truck windows and mirrors. I put grocery bags over my mirrors and closed the windows onto large garbage bags. After a couple of weeks he was done with the "other" bird. I also had a male cardinal do the same thing after dark when I would use my desk lamp in the house (he would crash against the bedroom window repededly) I just didn't use the lamp for a while and he quit.

Thanks sweetcountry. I think I saw some of those around.
Does sound like a pita though. I don't mind it so much right now being its still winter and EVERYbody's car looks like hell, but if it continues through spring...

My mother used to have a male robin who would fight with her basement windows to the point of losing many of his feathers and leaving blood on the glass. She had to cover the windows during mating season.

Last week I had a female bluebird that kept trying to fly into a window off my back porch. I couldn't believe that the window was very reflective at that location. She would sit on the back of a chair next to the window and then repeatedly fly up to the window as though she expected to be able to fly into the house. Fortunately, she wasn't flying far or fast, so she wasn't hurt by her efforts. I was surprised that it was the female that was doing this - her mate was with her in the yard by the bluebird house all day but stayed away from the porch.