Decoys?
Interesting, they are smart birds, I bet a decoy is why. I saw Jays screaming at the crows last year because they raided their nest. I saw the baby Jay on the road.
Yeah, I've considered that. Also, maybe a backup nest if needed in the event of destruction of the primary one. Or possibly, one mate demonstrating his/her prowess in nest building. Cardinal males will feed the female during courtship to theoretically demonstrate his ability to care for the young. I saw this earlier in the week in my backyard. It's an impressive spectacle every time I see it.