Deb, this probably isn't the most appealing suggestion, but what if you just hung some brown shipping paper or something similar and blocked that window from the inside and see if they keep doing it. If not, then they must either see something inside your home, or some reflection or light play from the outside that attracts them. At least you can narrow it down, before they end up braining themselves on the window. Maybe they want to build a nest, and they see a plant or something in your home that they want to pick at to build it?
