This male cardinal keeps landing on my windowsill and pecking at my window–two different windows, btw, around a corner from each other and 20 feet apart. I know it is the same one because I have named him Bill. No, really, I know it is him because he has a scar on his belly.
What’s up, bird behavior experts? Has spring sprung and he is looking for a mate (maybe he read my thread on the topic)?
I had an American Goldfinch doing the same thing one year. Its likely mating season for the cardinal as well and due to the reflection he thinks he’s fighting off a rival male.
We had a truly ugly gecko in west Africa who would attack his reflection in the kitchen window all day long. There was also some sort of bird that would land there and preen in front of the reflective coating, thinking (I guess) that it was a female.
Move anything red inside that he might be seeing from outside, away from the window.
He is pecking at his shadow/reflection.
I had a Honda CRX once, red, and the fuckers would be attracted to it and find themselves in the REAR VIEW MIRROR! no shit, and attack themselves all day long.
Had a girlfriend with a red car, when she’d visit, they’d find her rear views and attack themselves at her car all day, too.
You have to eliminate the reflection, or you will have nasty bird-peck-saliva-snot stains on your window until like July and shit.
He’s baaaack. I just saw a female near him, so he probably does think he is defending his mate. What a birdbrain. 
I had a light-gray colored bird that would come peck at several different windows on my house and poop purple poop all over my light-colored wooden windowsills. He’d wake me up early in the morning pecking on the windows beside my front door. He also would peck at the side mirrors on my truck.
I have a little brown bird that was doing that on my office window this morning. I knocked right in front of him, and he backed away a step, fluffed himself up, and went right back to attacking that other bird/reflection. It reminded me of Henry the Chickenhawk.
<Brooklyn accent> “No other little brown boid is gonna muscle in on MY territory!” </Brooklyn>
Hate to break it to you (or maybe you won’t mind), but you may have made a long-term friend. My mom has a cardinal that pecks at all her first-floor windows, and he’s been doing it for at least two or three years now. Moving various reflective and/or red objects away from the windows doesn’t make any difference. You may just have to get used to it!
I had some birds last year that made a nest right outside my living room window. They were attacking their reflections all day long. I’m surprised they didn’t break their bills/necks smacking against that window over and over again for hours on end. Eventually they smartened (?) up and moved to a location with less threatening (!) neighbors.
My cats would so love this, but the bird and I are at the office. He has kept it up nearly all day, off and on.
As opposed to the really cute, hot geckos?
I’ve heard if you put an stuffeed owl or even a picture of an owl in your window it’ll scare them off.
StG