If you have a window that suicidal birds are always choosing as their preferred "Lover's Leap", form a vertical wire fence with about 1/8th of an inch (or the metric equivalent) between strings.
This slows down the birds without breaking their necks and does not badly obstruct your view. Do not string too tightly (you might need the window for a fire escape). Depending on the prevailing winds it does make an interesting harp.