We've experienced a bird flying into one of our windows on only two occasions. They flew off again without any serious injury.
As you can imagine, being in the New Forest we have loads of birds flying about so it's surprising that it doesn't happen more often.
Having large picture windows, or patio doors, are the main problems as there is a bigger surface area for reflections. It is most likely to be the reflection of the bird hitting the window, seeing it as another rival in its territory and wanting to attack it. Sometimes they just get confused and just don't see the windows.
Either way Eric, if it's happening on a very regular basis then you may have to consider putting silhouette bird stickers on your windows. The type that resemble birds of prey would be most effective, as this is the outline that is best known to them as 'danger'.