Not fighting, biting to draw blood. And it's parrots in question, not other birds. Please refer to: Brice, February, 1994, Munn, July 1998, Gilardi, February, 1999, English, November, 2000, May 15, 2001. In 35 years of field research, only two researchers ever heard of a parrot biting hard enough to draw blood.
Positive reinforcement does not mean you do not set boundaries for your birds.