I have never been one to want to train my birds to poop on command - I can see too many things go wrong if I'm not there to issue the command (and I know of bird who will keep from relieving themselves until either out of their cage or until the command is said).
I have a few shirts at home that I wear around the birds so if they make a mess of them, it's fine.
These last few days, I have noticed a change in Piper. Almost every time he wants to poop and he's on me, he'll fly away to one of the stands, do his business there, and come back. This is not something that I have in anyway trained - and at first, I was too curious about it to even think of positively rewarding him. He just kept doing this on his own!
This is not a behaviour I'll object to!