I agree with Dena - separate him from the others if they are picking on him. You might want to assess his general condition (eyes, other feathers, eating?) and give him some warmth too - sitting on a
heating pad turned on Low with a towel between it and the cage and put a towel over the cage. I never had a 'tiel whose tail molted all at once like that so I don't know what else to tell you. Just don't hesitate to get him to the vet if you are worried. Birds are great at hiding their symptoms 'cause they have to be strong in the wild to survive.
Are the feathers just lying in the bottom of the cage or is someone trying to make a nest? Do you know the sex of all of your birds? You may just need a next box.
Good luck with him.
Critter
