I have tried many different methods to stop birds from striking my windows including sticking white label dots all over the window. I found these UV decals that go on the exterior of your window to be the easiest and most successful of any solution.
Birds read the reflection of nature in your windows as real and think they are traveling towards trees or sky or another bird–whatever is reflected. The WindowbAlert decals are nearly invisible (so you can barely see them when looking through the window) but the birds see UV light reflected back from the decal, thereby deterring them from flying into the window. Before applying the decals I made sure to wash the window and used rubbing alcohol to remove any residue.
It also helps if you place feeders and bird baths either very close to the window or away at an angle.
Window Alert U.V. Decals