On the whole, the best way to help birds see your windows is by giving them visual cues or markings on the exterior surface. What you’re trying to do is break up the reflectivity.There are many ways to do this. The treatment can be permanent (like an etched pattern) or it can be temporary (such as tape or decals).
Tape or decals on the exterior can be effective if you use enough of them to break up the reflection. Studies show that birds will attempt to fly through an area as small as your hand. You really need enough treatments, whatever they are, to ensure birds don’t just zip between them (and straight into the glass). Best case scenario? Ideal spacing is about 4” apart horizontally and 2” apart vertically. Certainly some markings are better than none! Apply as desired, monitor for problems and add more if needed.
ABC BirdTape is a translucent tape created by the American Bird Conservancy and can be installed in thin or wide strips or in pre-cut squares.
WindowAlert™ decals have ultraviolet coating (that birds can see put people cannot), come in a variety of shapes and are less obtrusive than standard “decals.” The UV coating does wear off over time.
What about hawk decals?
A couple things are worth noting about the classic hawk silhouette decals. First, the shape itself doesn’t really matter. Your image can be a hawk or a butterfly or a circle – the point is to make some kind of marking on the glass surface, preferably on the exterior. Second, one hawk decal is typically installed on one corner of a very large window. That leaves a lot of space that isn’t marked (that shows dangerous reflections) for a bird to fly into. So, you can use hawk decals but you’d probably have to use far more than you want to look through out of your picture window. We know more now and subtler, semi-transparent tape and decals are now available.
If you have kids in your house or are an artist yourself, you can use a variety of temporary applications on the window surface to create your own art. Window markers or tempura paint are easy and fun to apply and can be very effective for accessible windows. Both wash off easily with water.