[Sooner or later]

Originally Posted by masraum

My guess is that it has to do with where the sun is this time of year. If there's a dark basement and the sun is just right, that window may be like a mirror to him. Maybe he's thinking "man, that's a damn good looking bird, I'm going to kick his ass!" If you turn the light on in the basement does he stop? Often if there's darkness behind a window, that increases the likelihood that window will act like a mirror.

That is my guess.

https://www.thespruce.com/stop-birds-attacking-windows-386449