I had a cardinal that used to peck on my window, I'd open the curtain and it would fly away. Repeat cycle. I figured out it was a game to it and when I stopped opening the curtain it went away after a while.
not a mirror, something thats shiny and makes flashes of lights. like those ribbon things they use in vineyards. some sorta wind chime device that has reflective things on it.
lol its actually the opposite, he has a broad and some kids and is building a nest nearby. Cardinals do this in my area alot so I looked it up, its because they see a reflection and are trying to ward off other birds from their nest.
It's mating season. The bird sees its reflection in the window, so it tries to take it out. It isn't retarded. It isn't trying to fly through an invisible wall. It's just trying to be macho, cause it is horny as shit.
Or it lost it's mate and went nuts. There was a pair of cardinals with their nest outside our house.. The female just vanished one day, probably eaten by my cat, and the male just went balls crazy.. Cracked our basement window trying to kill it's reflection.