No bird bites for no reason at all. I had one client whose young Pionus would, on occasion, bite her for seemingly no reason. We humans often don’t understand why parrots do what they do, but this does not mean that there is no cause behind their behavior. Behaviors never happen for no reason. Instead, parrots often do things for no reason that we people understand. This point is critical. Without appreciating the motivation for aggression, we cannot figure out how to change things.
Find out the sex of your Pionus even if you have no intention of breeding it, because its gender will influence behavior. Is it coming into sexual maturity? If so, your bird’s behavior toward you could be changing, and you might need to change your training methods or go back to basic training.
According to Pionus breeder Russ Shade in his excellent book, The Practical Pionus, Vol. 1 (Mindo Press, 2003), you are describing the classic behavior of a male Pionus that is becoming sexually mature. According to Shade, a male Pionus coming into puberty is more likely to act unpredictably and aggressively.
If you have not had your bird DNA-sexed, do this during the next veterinary checkup. Unfortunately, many people (including some veterinarians) will tell you that a parrot’s sex does not matter if you do not intend to breed it, but I emphatically disagree. Male and female animals often have different jobs in the wild, and their behaviors differ as a result.
Look for the warning that precedes the bite. Parrots might be noisy creatures, but their primary communication tool is not aural but visual. In other words, parrots do most of their communicating through body language. In my experience, parrots always warn prior to a bite. Always. The warning might be quite subtle and only last a millisecond, but the warning is there for us to see and understand, if we are paying attention.
Because it is impossible to clearly see psittacine body language when the bird is on your shoulder, cease to allow her there until you have a better idea of what is happening in your relationship. Besides, it’s never a good idea to putting a biting bird next to a part of your anatomy that you don’t want damaged (again).
Prior to the bite, did someone else enter the room or move close to you? If so, then you were likely the recipient of redirected aggression. Your Pionus really wanted to bite the other person to drive them off; however, she was unable to reach them with her beak, so she bit you instead.
Perhaps she saw this person as a threat? According to Shade, Pionus also might bite to try to drive their beloved person away from something that they perceive as dangerous. Did something like a bird flying by a window frighten her, so that she lashed out in a panic? That is what I do if something startles me, though I mean no hostility by it. Did she lose her grip on your shirt and start to slip? If so, she likely grabbed you to prevent her from falling, which is a reflexive reaction of all animals.
Was your parrot just trying to get your attention with its behavior? If not taught to be gentle, parrots often don’t realize that they hurt us with their beaks. After all, if they grab at each other, they only get a beakful of feathers!
By carefully analyzing the situation, you will be able to recognize why these incidents happen, enabling you to avoid these conflicts in the future.
Attempting to change a behavior without understanding the underlying cause does not work in the long run, because your bird will find another behavior that facilitates getting what she needs. From my experience, it’s better that you teach her to fulfill her needs with a behavior that you don’t mind rather than one that she chooses!