When parrots were first kept as house pets, it was assumed they just ate nuts and seeds. Vets began to wonder why many house parrots were prone to sickness. Over time, and with observation of what parrots in the wild eat, more varied and nutritious diets for parrots have emerged. However, there are foods that parrots should never eat. Please do not use this article in the place of your vet's advice.
Alex the parrot image from Wikimedia Commons
eHow Article: Parrot Food Ingredients to Avoid