Does cokatile should be left in pair?

Answers:
If the cockatiel is not a pet bird that is bonded to people, then it really should have another cockatiel there for it to bond with (doesn't matter if it is the same or opposite gender unless you want them to breed).

Also, a pair of cockatiels can have other cockatiels in the aviary as well, but keep a close eye on them to make sure they are all getting along.

Sorry if that wasn't what you were asking about.

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Assuming they are male/female, they can be left as a pair as long as they don't have a nestbox or other place they can nest. The nest will inspire them to make baby cockatiels, which I assume you don't want.

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No, it doesn't need to be that way. I've a single female who is very attatched to me. she loves to snuggle as well. Unless of course the bird is going to be kept alone for long periods of time.. then yeah, a partner would be a good idea.