We have almost the whole south wall of our living room in large double pane windows.  Birds hit them often in summer. 

Daughter loves to decorate windows with clings, those thin plastic images that you place on the windowpane and they stay in place.  We have noticed during a season, such as 4th of July, Thanksgiving, Christmas, etc, when she has clings on the windows, the frequency of birds hitting the windows decreases. 

We probably should find some clings that are generic we could leave up all year around, we are pretty sure those things sticking on the windows keep the birds from hitting them.

EDIT:  I forgot to add that we had problems with bedroom windows, birds kept walking next to the windows, leaving bird mess on the sills, plus flapping their wings against the glass, glass was dirty all the time.  I hung some wind chimes over the windows, have had no more problems.