Many people focus on the end result more than the steps that it takes to get there. People admit that parrots are smart, but not realize that certain training aspects are not conducive to learning. Some techniques only teach the parrot what *NOT* to do and fail to even work on teaching a parrot what *TO DO*.
Biting is a clear example of this. It can teach parrots that no matter how they feel in a matter, they have no control over the end result if you simply ignore them when they bite. This leads to unpredictable biting because why should a parrot warn you that they are going to bite when you aren't even paying attention to them that they *WILL* bite? If you can learn to understand your parrot, why they are biting, the signs of "I'm going to bite you if you don't back off", take precautions to avoid getting bitten and then work *with* the bird rather than *against* them, then you have just bridged a communication barrier!
This article is kind of good at explaining the communication barrier.
Living With Parrots Cage Free: My Bird Knows When She Is Bad, Right?? !! (Hmmm)...
Positive reinforcement, applied behavior analysis or clicker training tries to help bridge this communication gap, but it does require thinking outside the box to what you'd normally think. What many people think is positive reinforcement really isn't (if you praise a bird for stepping up, does the bird really find your voice to be rewarding? If you give your bird a treat, does the bird even like the treat?), or it's a mixture. I do not train the same way as I did 10 years ago!
I'm no expert when it comes to training, but the more I read about it, the more I learn of how behaviors may be avoided or changed by changing our behavior - not theirs. Another example is lunging. If every time you walk by the bird's cage and it lunges at you, rather than ignoring the behavior and possibly having it escalate, you can instead try to change the way you approach the cage or at what speed you approach the cage, thus reducing the chance of the lunge.
In regards to Percy's behavior while other people are around... these links may be of some help (please bear in mind I *DO NOT* agree with the "alpha", "boss" or "dominance" theories)
Site Name - Articles - Behavioral - Sex And The Psittacine
https://companionparrotonline.com/sex_csaky.html
RP - Mating
Articles, Handouts & Quick Tips (a few things on there might be helpful to you!)
If Percy is actually viewing you as a mate, then the positive reinforcement/clicker training and redirecting his behaviors as well as socializing him with people may help him to lesson and possibly even stop his aggression. Then again, it might not be your behavior or his, but the other people. Are they too hyper? Is there something about them that is setting him off?
Just some things to think about!
I would love to hear of any updates you have with Percy!