"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 recent study.
Often a collision will temporarily stun a bird but it will fly off, seemingly recovered, a few moments later. Many times these birds die later, from internal bleeding or bruising, especially on the brain."
-The Cornell Lab of Ornithology
Read what you can do to prevent birds from hitting your windows!