Window strikes are a fact of life when you feed birds. It is estimated that approximately 1 Billion (yes, with a "b") birds are killed annually due to collisions with glass. The closer to your windows the feeders are, the more bird strikes you are likely to have. So a frequent question is, "how do I stop birds from hitting my windows?"
Birds fly into the windows because the reflection makes it look like they can fly straight through. When the birds are startled from a feeder, they are prone to take off quickly to escape and splat!
I fight window strikes by placing my feeders in view of, but well away from the windows and also by allowing my windows to get a little dirty, reducing the window's reflective properties. There are other things you can do, although none are foolproof. The American Bird Conservancy has a list of tips that may be useful to you.