All birds are born with a certain amount of built-in stress.   As most birds have little defense against predators,  their only option is to keep a sharp eye out for anything 
that might eat them.  They know they're one of the main food sources for all other animals.  They are under "normal stress"   Once a bird is captured and confined,
the stress level increases.  Depending on it's new environment,  this slightly higher 
stress level can also be within a "safe" range.   However,  start adding all the many 
types of stressful situations and soon the meter rises beyond the "breaking point" of stress. 

For instance... in the graph above there are all sorts of different types of possible stress
related issues.  One is not all that much more important than the other.  (except for
possibly health related stress).  But when the different types of stress start adding up...
your parrot is reaching the breaking point of how much stress he can handle.  I believe
this is the main reason for plucking & mutilation.   For instance:  I heard of a person
who's cockatoo started to pluck.  She took the bird to a vet and he did all the tests and asked all the normal questions.  The tests came back pretty normal,  so it didn't appear to be health related.   The one thing that stood out above all others was that the owners smoked in the house.   They were told to stop smoking in the house,  and so they did.
Within days the bird stopped plucking.   For the next 3 months or more everything was
back to normal.   Then one day the bird started plucking again.   Now the question was
what caused this?  What was new in the house or the birds life?  What was different?

The point of the story is that there can be a combination of different stresses that a bird
might contend with.  But ONE MORE STRESSFUL SITUATION and the bird is now
"over the edge" and one of the results is or can be plucking and mutilation.  So your job is to try to reduce that level by figuring out what is new...different..or lacking.  This
at times can be frustrating but the riddle must be solved before the bird gets any worse.

Birds are not that much different than humans in this respect.  We have nervous breakdowns and doctors must find out what caused it.  Naturally,  there is something wrong in your life and once that's discovered and changed,  you can lead a productive life again.   Same goes with your parrot if you catch it in time.  The only problem is of course that he cant tell you what his problem is.  So it's up to you to think like a bird and try to figure out what would cause YOU stress if you were on his perch.  This isn't a page to solve all your problems.  It's just something for you to think about the next time you suspect that there might be a problem developing... or to remind yourself that anything you add...change...or otherwise suspect might have an effect on your bird, probably will...  at least on his stress meter.