1. Itchy, dry skin: Linoleic acid deficiency, poor amino acid profile, food allergies.
2. Squamous metaplasia of the epithelial lining of the upper respiratory tract, gastrointestinal, urogenital tracts, uropygial gland: hypovitaminosis A.
a. Blunting or lack of choanal papillae. Hyperkeratosis of the epithelial surfaces can also occur. It is common on the metatarsal and digital pads of the feet, which can predispose a bird to developing pododermatitis.
b. Clinical signs can also include nasal discharge, swelling around eyes, swollen sinuses, polyuria, polydypsia, dyspnea, or anorexia.
c. Masses (keratin cysts) may be present in oral cavity, white pustules or masses can occur in the oral cavity, crop, or nasal passages. Secondary bacterial infections commonly occur.
d. Treatment for vitamin A deficiency includes parenteral vitamin A weekly, then oral supplementation once improved, usually 2 to 3 weeks. Treatment with antibiotics for secondary infections may be required.
3. Other nutritionally based skin disorders include biotin, niacin, pantothenic acid deficiency, and mycotoxin ingestion.
Reference: "Manual of Avian Medicine" by Glenn H. Olsen and Susan E. Orosz