Our house, which sits in the middle of the woods, has several large plate glass windows. Every few days we'll hear a loud thump as another bird smacks himself into the window, even when we have the blinds down so it clearly looks like an obstacle. More often than not, they merely stun themselves, leave a few feathers on the glass, and are gone by the time we can get there.
Occasionally, however, they are either more stunned and sit there for a while, or are injured to the point the cat causes them to disappear. I hate it when that happens, but Niko rather enjoys it, I think.