It's because you must have used Windex®

It's a male and he is seeing his reflection in the window and "fighting" with the other bird for territory.

Lisa

Thank you, Lisa. I may not be able to stop him, but at least I know why he's there now.

Hang a picture in the window, like an owl, he will stop.

No; hand something, anything on the outside of the window to break up the reflection.

From the Bird Forum:

Birds are Flying Against/Hitting Closed Windows.

Birds can see their reflection in a window. During the breeding season, when the birds are most territorial, they view the reflection as another bird intruding on their territory, and try to chase it away. Most species do not recognize the reflection as their own self. Many species of birds engage in this activity. The best way to discourage this behavior is to place a non-reflective material in front of the window on the outside to prevent the reflection. This is only a temporary behavior that will cease when the breeding season in over.

LOL. What great entertainment for your cats!

Check out a local wild bird store. They have stuff to discourage amorous birds from attacking their own reflections!

Thank you, everyone. This bird has a girlfriend, so I can see why he'd be territorial at this time.