It's usually a combination of bad diet and stress. Unlike dogs, Parrots are very particular about who they bond with. People get a parrot from a breeder or a pet shop and think the bird will be all cute and cuddly like they see on Instagram. Then when the bird doesn't bond with them, and is aggressive and noisy - they get locked in a cage with a bowl of seeds and that's it. That or they're handed from person to person and the birds develop major anxiety. Parrots experience PTSD like humans. These parrots need so much stimulation. And a large parrot like the African Greys, Cockatoos and Macaws live in captivity 40-60 years. If you have kids, think of the terrible twos - that goes on for 40 years.