I'm not sure the decals for the window would do the trick for a bird who is attacking his reflection. The decals do work to let birds know they can't fly through there, as happens at my patio door pretty often.
Last year two hummingbirds were chasing each other, male and female, and crashed into the glass door. Killed them both. I found them lying on the deck, about a foot apart. I was very upset.
Rubber snakes, or perhaps an owl cutout, from black construction paper, might help.
Good luck! We have had male orioles bloody their heads by attacking the kitchen window so violently.