When my 7 year old cockatiel started plucking I waited to take her to the vet. Then a friend told me her cockatiel died 2 weeks after she started plucking. So I took her to the vet that day. They put her on antibiotics and I used Pluck-No-More in her water and as a mister. Within 8 days she had quit plucking. She looked just like your bird with all the downey feathers. Its been a month and today I finally felt pin feathers so I'm hoping that soon she will start to look better. I would take your bird to the vet just in case its sick.
