Why is your red scarlet macaw pulling his feathers out on belly?

Stress from it's enviroment or it is not getting enough attention from you. Birds bond to their owner and need attention from them. A Macaw is going to need at least 4 hours of contact. This doesn't mean you have to hold them that long, but they need to be around you and be able to see you. You really can't give them too much attention. I have a Macaw, Cockatoo, African Grey, Amazon and 3 Conures and they all need attention.

Once a bird starts plucking, they tend to continue it. Keeping the bird with you as much as possible and with several toys or things it can tear up may help it stop. Birds can become insane if they are stressed enough or do not get the attention they need.