In this case, do your cockatiel and rosella is also a tame birds? If they are handfed and tame, this will help your parakeet to trust you and will make the process easier. However, if his neighbour is not tame...I think it will take longer time for you...compared to seperate them into different cage.

Btw, seem like it is good progress.

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Best regards,
Tanin Eh Boon
Malaysia Bird Community