There are two types of window collisions involving birds. Birds that continually fight the glass, are fighting their own refection, believing that the bird they see in the glass is a competitor bird. These birds do not get hurt.
The second type of collision is when birds fly into windows because they see a reflection of the outdoors in the window, such as the woods outside the house. They often hurt themselves, and sometime die. The way you can reduce or prevent either of these kinds of collisions is by removing the reflection. Some strategies that work include soaping the outside of the window, hanging objects on the outside to distract the birds from hitting the glass, or aiming the glass up or down to keep the birds from seeing the reflections. Some people have prevented collisions by placing silhouettes of hawks and owls in the windows, which keep the birds at a distance.
We hope this helps,eNature Naturalists