After hitting the window, birds can be stunned, break a wing, or even be killed. Not all of these ideas work for all people. It depends on the house, birds, and surroundings. If one tip doesn't work, try another. They think they can fly to whatever is reflected, and then hit the window.
Dirt reduces reflections.
Put a screen on the window. Thumb tack shade cloth over the window. Shade cloth can be bought at hardware stores or garden centers. It's a plastic mesh that is easy for you to see through and will keep the windows from having reflections. Thumb tack string diagonally across the windows, from corner to corner.