Today I was thinking about one of my former green birds. Zip had a sibling, Zap, who plucked out his own tail feathers mercilessly.
The habit started when Zap had an infection in a gland near the base of his tail. I guess it itched. Poor guy. But we never could get him to stop after that. Our vet even tried putting a collar on him. He was such an acrobat that he still managed to pick his tail feathers. And the collar irritated his neck, so he started picking his neck feathers too.
Because he had no tail, Zap wasn't very good at landing after a flight. Usually birds will fan out their tail feathers just before landing to slow down. Without his tail, Zap would crash down -- feet, then chest, then beak, boom, boom, boom. And pick himself up afterwards. I always felt sorry for him.
On the other hand, with no tail in the way, Zap could also hang upside down from the roof of his cage like a bat. Zip always looked at Zap oddly when he did that -- perhaps because Zip couldn't do it himself.