At my house, they crash into the windows because they are so damn aggressive. They see themselves in the reflection and attack the other bird. They shat all over my cars this year because they kept seeing themselves in the side view mirrors. Then shat all over the back of my car because it has a chrome bumper. I have watched robins sit on the side of my car for an hour just attacking the sideview mirror over and over. They regularly crash into the one window in my house that has a tree next to it, because they land in the branches, then decide to attack the other bird in the reflection. They will sit there for hours doing this until they finally hit the window hard enough to scare themselves off.

I've used the UV reflective "anti-collision" stickers with reasonable success. You can get discrete (to humans) ones that look like etched bird silhouettes. Just make sure to put them on the outside.