Parrots have diseases, as we do, cancer is common.
Maybe. Maybe not. The rescue boards are flush with stories of birds who are loved and well-taken care of, yet start plucking and self-mutilating.
They all can't be diet or disease-related.
To play devil's advocate, why is Petco selling pets that are prone to cancer?
You're sure doing your best to put forward a worst case scenario for avian ownership, aren't you.