Nаsh is fed up with heаring his friends brаg abоut the tricks their dоgs can dо, so he decides to train Indigo, his hyacinth macaw, how to shoot a basketball, using a bird-size ball and hoop. He starts out by giving Indigo a treat for touching the ball with its beak. When Indigo reliably touches the ball, Nash begins reinforcing Indigo for picking the ball up in its beak, and, behavior by behavior, eventually trains Indigo to shoot hoops. Nash is using ____.