Could be many different reasons, is it cold, in a draft? Fluffing up to keep warm.Could be getting ready to sleep, or it can be a sign of a sick bird. We really need more info to say, there several other reasons but if your bird is inactive and puffed up , i'd say sick,sleeping or cold.Maybe a vet visit is in order.
I agree with CaptWest. Briefly puffing up is usually a happy, comfortable thing, or basically reorgainizng feathers, Puffing up and pinning eyes or appearing poised to bite is anger, and sitting quietly puffed up all the time I am told is a sign of a sick bird. It really depends on the context for the behavior.
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