I wish I could find the original article that I read some time ago. It was on the concept of using "Nanny birds" as a link between human hand feeding, and going to the new home. They say that in the wild, Greys (and Cockatoos) spend many months past weaning with adult flock members who show them 'how' to be a bird, and that birds raised in this closer to nature way, end up having the best chance of being well adjusted, non-plucking, phobia free birds.
It said that if Greys were really such shy birds, you'd think that the ones who underwent the trauma of being wild caught would surely make them prone to plucking. They found wild caughts were less likely, and that this must have some basis in how they were raised, by who, and for how long.