For birds, glass windows are worse than invisible. By reflecting foliage or sky, they look like inviting places to fly into. And because the sheer number of windows is so great, their toll on birds is huge. Up to about 1 billion birds die from window strikes in the U.S. each year, according to a 2014 study.
In daylight, birds crash into windows because they see reflections of vegetation or see through the glass to potted plants or vegetation on the other side. You can help prevent this by making your windows more visible to birds with some of these options. Keep in mind the following recommendations for maximum effectiveness:
Vertical markings on windows need to be spaced no more than 4 inches apart and horizontal markings no more than 2 inches apart across the entire window.