"Macaw parrots scream constantly for absolutely no apparent reason." In the cage, out of the cage, playing with friends, alone, eating, drinking, eating fresh treats, doing nothing, sleeping etc.
Unless there is the theory that they only scream constantly whilst being observed.
I wonder if it's like the tree falling down in the forest - does it make a noise if no one is there? Is this screaming just to irritate / entertain during my observation? Must take detailed notes on camera duty later..., hummmm...
Oh and ummm... Buy some ear plugs.