I've had several birds hit my windows - most recently a goldfinch that bounced off into the birdbath and drowned, poor thing. This is despite having stained-glass butterflies stuck inside the glass. The house I grew up in had a constant stream of casualties - there was a big bay window, and the birds just seemed not to see it. They often break their necks, and then of course there's nothing you can do. Other times, they hit the glass so hard that you get a ghostly image formed out of the dust in the feathers - very strange.