here is an article regarding sweet potato & dry skin i came across.
SWEET POTATOES--Also popularly called yams, sweet potatoes are a blockbuster source of the antioxidant beta carotene, (one half cup contains 23,000 IUs) linked to preventing heart disease, cataracts, and numerous cancers. When you have reason to be concerned about the health of a parrot's eyes, this is the perfect nutritional boost for eye health. If a parrot has rough or dry skin, the high beta carotene of sweet potatoes will have a positive effect here too. This is considered to be one of the most nutritious foods in nature, and parrots of all ages love the taste, color and texture. It also is an excellent source of fiber.