Why is one of my cockatiels skinnier than conventional?

i have two of them. this one has different sized feathers in its tail and it looks thinner than the other one. ive solitary had them for a week
Do you surmise ailing be capable of...
Answers:    One of them might have "wider" looking tails, because they might quality stressed or threatened. My new cockatiel has "wider" tail feathers, so I'm guessing it's just her emotion stressed out in the new environment.
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