Do you mean "coming to" or "coming through"? If you mean "coming to your windows" then you probably mean the pecking behavior that territorial birds exhibit when they confuse their reflection in the window as an adversary. In this case, you need to make it so the bird doesn't see its own reflection in the window. I'm sure you can think of some ways to do that. If you mean "coming through your open windows" then the only effective way to stop birds from flying through an open window is to close the window or install a screen (which also deters insects from invading). If you mean "flying into your closed window" and this is a common problem, then birds in quick flight incorrectly discern the window to be an open space. (You ever walk into a closed glass/screen door?) Make the window more noticable. One popular technique is to decorate your window with something that can be easily spotted from the outside. If you don't want to decorate your window, try placing items near the windows. This will block the access to the window as well as cause the item to appear in the reflection of the window. A window box with flowers "kills two birds with one stone" in that it is pretty from the inside, and doubly pretty when the window reflects the beautiful flowers.