As a medical person, " quivering" for a young bird, is due to an immature nervous system, that is quite normal. It will go away, as they mature. Quivering, out of " fear", is something else. We are all intelligent, and can recognize the difference.
If one quivers out of fear.... this birds issues needs to be dealt with. Recognize the difference is very important for the bird, either way!
When I adopted Sophie, she " quivered". I knew it was fear. We all supported her, told her we knew she was scared. She worked it out, with our support. Nancy