In our last house we didn't have this problem. Birds never flew into the windows! But we moved to this house on Samhain (Oct. 31st) 2007, and ever since then we've had trouble with birds and our windows.
They usually recover, and fly off. However, I remember one particular time that a cat took off with the bird while it was still helpless.
Don't think that we've done nothing to try to deter the birds, though, because we have. Tall One cut out and laminated bird shapes, and put them up on the windows and sliding glass door. First we had them up inside the glass. That didn't work, so on some advice mom got somewhere I put them up on the outside. That hasn't worked, either. We've still got them up, but they don't seem to be doing much good.