Cardinal are territorial, some moreso than others. I’ve seen a male kill his own son because he wouldn’t leave dad’s territory and find one for himself.
They’re pretty well known to attack red things, my lamp, red curtains, red ribbons on wreaths. I thought he’d get over the lamp but he quickly got frantic and looking like he’d hurt himself.
I certainly never knew cardinals were that territorial to kill their own offspring. We never get too old to learn, do we?