The issue is that we aren't birds and we do not know their "subtle body language." The only way we can learn their subtle body language is through operant conditioning similar to how we train the parrots. If slick feathers, pinning eyes, or open beak means parrot is scared, we won't realize that these are a warning sign until we have been bitten (positive punishment) or the parrot has flown away (negative punishment). So to calibrate our understanding of their subtle body language, we must first experience the maximum consequences to realize the subtle cues. It's the exact reverse of us training the parrot. We give reinforcement in exchange for behavior and teach the parrot the cue. Well reading body language is the parrot training us and the bite or fly away is the consequence that teaches us the cue (body language).
However, the clipped vs. flighted debate is extremely relevant here because a clipped parrot is limited the freedom of expression (consequence) solely to biting for all purposes. I would say for a flighted parrot, typically flight = scared but bite = antagonistic. However, for a clipped parrot bite can mean either so it is more difficult to tell what the parrot wants as the consequence is used universally. We cannot learn the subtle cues (without specific prior experience) until we have witnessed the more obvious ones.







