Hanging cords, strings or ribbons in front of your windows can be effective and unobtrusive, even pleasant. Make them yourself or order a product like Acopian BirdSaversTM which are custom-made based on the size of your window(s).

Another ingenious invention along these lines is called The Bird Crash Preventer!TM and uses a series of monofilament line to create an effective, yet invisible warning system to keep birds from hitting windows.

See also Blinds and shades, Surface treatments, and Screens and netting.