A new study done on large mammals in zoos found that captivity actually causes brain damage in them. They did not study parrots but they do mention rabbits, cats and rodents as having the same issues and, if you look at the 'constraints' of captivity that cause the brain damage: lack of enrichment, solitude in highly social species, lack of room for proper exercise, etc you have to admit that pet parrots suffer all of them with the added problem of an unnatural diet.
https://theconversation.com/the-neural- ... r%20brains