red bellied conure

[amriches]

going bald on its back and underneath

[wildwood]

It is almost certainly plucking itself. The reasons can be many, both psychological and dietary.

Without knowing more about the bird or its housing it is impossible to determine.

If in a cage, is it let out regularly for exercise and socialisation?
Cat, dog, kid worrying it when you're not there?
Cage in the sun?
Handreared and now not enough attention?
If caged, try a pelleted diet AND FOLLOW INSTUCTIONS.
Multi vitamin and mineral additive to water.

Even experts can often not diagnose the reason for plucking. You only see/hear of the successes, never all the failures. Plucking becomes a habit which is very hard to break.