Hi reeko,

Agree with dogcom, normally feather pluging is resulted boredom if it is a single pet bird. In some case, it is also due to some parasite in that live in their feathers... I think you may try to seek advice from the vet, and if the vet confirm that he didn't need any medical care, you can start to 'cure' from the behaviour perspective...

good luck.

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Best regards,
Tanin Eh Boon
Malaysia Bird Community