Seeds are notoriously deficient in vitamins, and if your bird is fed a seed-only diet they are likely to be in less than perfect health.
For instance, Parrots with a love of sunflower seeds are especially prone to Vitamin A shortages. This may predispose them to infections, and cause other problems directly.
Bogena Parrot Vitamins supplies Vitamins A, D3, E, C, B1, B2, B6, B12, K and is highly beneficial. Add to water or mix with food.