I have four birds, two budgies and two cockatiels. Recently i noticed that my older cockatiel, Sid, has a large bald spot on the back of his head (see images), so i did a bit of research and most of the stuff was saying that it's probably the other birds plucking feathers, but if that was the case then i would expect him to be more cautious of the other birds and he's not at all. I have separated the budgies just in case but I would rather not take his buddy away.
When I first acquired Sid in 2005 he was thirteen and my other cockatiel (Coo) was about three, but now seven years later Sid is twenty
and Coo ten, I mean they have their moments, but they almost always get along. The only time I've seen any feathers plucked was when Sid was trying to preen Coo and accidentally pulled too hard on his crest feathers.
One other thing I found while researching was information about lutino cockatiels losing feathers because of a mutation, but Sid is not a lutino.