I have to disagree with that, Michael. I know that your theory sounds as if you were right but I have never, ever seen this and I've dealt with hundreds of rehomed birds. In my personal experience [and this is without a single exception, mind you], birds that were cared for correctly do a million times better when rehomed than birds that were not. Some of them don't even seem to go through any significant stress at all! I am not saying they are not suffering from it, I am sure they are but, externally, you don't see anything. No plucking, no screaming, no biting, no lack of appetite, no getting sick, no nothing but a sort of over-vigilance where they watch you closely without approaching you - but it lasts only a few days. I think that this is because for parrots as well as for humans stress is cumulative and, as we know that parrots live with chronic stress in captivity no matter what, exposing them to a larger amount regularly does not help them in the short or the long term. As a matter of fact, science tells us that being regularly exposed to stress affects not only our brains but our internal organs and that some of these changes cannot be reversed even if the stress disappears. It literally changes parts of our brain! It has to do with the effect of cortisol [the fight or flight hormone] on it and the necessary balance between the sympathetic and the parasympathetic nervous system [a balance that disappears with chronic stress] as well as 'normal' stress versus 'abnormal' [normal stress being the one that nature already took into consideration and gave the species natural mechanisms to cope - and abnormal stress being the one that nature never took into consideration]. Cortisol is a doozy of a hormone! It's necessary for the fight or flight and, as such, is essential to life BUT allow cortisol to flow too often and/or too much and you end up with what they call the 'cortisol domino' effect where the brain goes in a loop and cannot stop the fight or flight. Dogs and cats are the same [and I have a lot of experience on being on both rehoming ends of these two species, too]. You take an animal that has lived a life without any undue stress and, even when they go through the traumatic experience of losing their home and human, they 'bounce back' quickly and without any lasting consequences because their brains are 'healthy' and their brain has the right sympathetic and parasympathetic response to stress - meaning, it starts and ends when it's supposed to BUT you take an animal that has been exposed to stress for a significant period of time and you have an animal that is going to require a lot of work to be made 'whole' again because it will take a loooong time to get the brain to go back into 'cortisol balance'.
See below:
https://psychcentral.com/lib/the-physic ... rm-stress/https://www.psychologytoday.com/us/blog ... nnectivityhttp://www.tuw.edu/content/health/how-s ... the-brain/http://news.berkeley.edu/2014/02/11/chr ... l-illness/