My green cheek conure on the right has lost adult feathers like the left conure has and only has down feathers remaining; I just noticed this and do not know if this is normal for this kind of bird? I dont remember this happening with the other bird.
If you are keeping them together then watch them closely from across the room to see if the bird on the left is plucking the other. Both of my female crimson bellies will pluck feathers from the male when they are allowed out together so I have to watch them and stop it when I see it. Otherwise they each have their own cages. You can't rule out he is plucking himself either... could be stress or an underlying illness. Do they bath often? Lack of humidity in their area? Diet? Anything that could be causing stress... other animals in the home, loud noises, cages out in the open?
For some reason 2 of mine just did the same thing. The pineapple has mostly grown back after taking more baths. The regular green cheek is about 1/2 way back to normal. In 14 years of having GCC's I have never had that happen. So, try more baths, check humidity and give it a couple weeks to see if it continues.
I would also ask if they are both the same sex. Years ago when Casey- male crimson belly was given to me, he came with a male normal green cheek I had raised from a breeding pair I owned and sold to the friend of the lady who owned Casey. I didn't know those two ladies lived together when they visited my home and bought the green cheek "Joey". They had been keeping the two males together. I wasn't expecting two birds, but just Casey per my phone conversation with the lady and when they showed up with Joey too I was quite surprised but took Joey back and allowed the two males to stay together since they were used to each others company. About a year later I walked into my bird room with the conures and saw a couple of feathers fly. Thought... hmm odd... I better keep an eye on those two for potential problems since Joey was maturing. I think Casey was at least 2 years old then. The next day I walk in and see the cage floor full of feathers. Full of feathers. I investigate the two and see Casey's back is totally naked, no down, just bare skin. I didn't know if he had plucked himself or what. While I was in the room changing papers I witnessed Joey sitting next to Casey and saw him pluck a feather off Casey's back. I set up another cage and separated them permanently. Those two were very bonded and pretty unhappy about being separated but I couldn't allow it to go on or escalate into more destructive behavior. Like I said... just watch them when they are together.