Tribulations of a cardinal

Every morning, just like clockwork...tap, tap at my bedroom window.
At first it was interesting ... even though I am an early riser, I became annoyed at his frequent visits, each time he would actually attack the window, leaving and coming back several times.
After about a week I started to wonder why he did that...with some investigation, I found out that the male cardinal will attack another one in his territory.
While hanging clothes one morning, I happened to look up at my bedroom window ... a light bulb came on in my head..the trees were a perfect reflection on the window...
Each time the protective male cardinal flew over his territory he saw his reflection in my window and naturally attacked the intruder.
I put newspaper over the window and had no more tap, tap .. just like that.
Well, anyway, I learned something in my quest.. saved the poor cardinal's beak too...