Some of us have a hard time understanding why birds collide into windows, but it's not their fault, and it's not because they are "stupid," but because birds perceive things differently than humans. Sometimes they see their reflections and think it is a threatening intruder who they have to attack. Other times, because of their sensitivity to polarized light, they may not see the glass. Whatever the case, window collisions are a real problem for birds. To prevent collisions you can stick things on your windows, or buy clear decals.
But window collisions are not always fatal.
A lot of times birds are just stunned and need time to gather their wits before they fly off. If you find a bird that is stunned like this DO NOT handle the bird unless it is necessary. When is it necessary?
Here are a few websites that sell window decals:
Here are a few websites that I found with more information: