We often get calls from people with a different problem. Instead of birds hitting their windows inadvertently, they are flying against the glass on purpose – attacking their own reflection. Cardinals and Robins are notorious for this behavior. While not harmful outright, this behavior is disruptive to you and to the bird – it distracts them from normal behaviors like nesting and also costs them valuable energy.
To deter territorial birds at your window you need to hide that reflection. The Window Alert Company also has a product called Cardinal Alert (External site) for this purpose. You can also eliminate this reflection using screens or with a variety of window films mounted on the exterior. See Surface treatments.