Vitamin A (Beta Carotene)

Function:
Vitamin A is stored in the liver and is needed for good vision. Without adequate vitamin A, night vision is the first to be affected. It is essential for resistance to infections, particularly in the sinuses. This is one of the most common deficiencies in birds. It belongs to the group of vitamins that are known as carotenoids. It is important for healthy eyes, skin, and mucous membranes.
Symptoms of a Deficiency:
A bird with a vitamin A deficiency lacks energy. The bird can have abnormal feather structure, coloring, and growth. Decreased egg production, longer time between clutches, and poor hatchability are also reported. Males will develop a low sperm count. In extreme cases, the central nervous system is affected. Sometimes there can be lesions around the eyes, mouth, feet, and skin. Bones may not develop properly and even kidney problems may occur. Night blindness is also reported, as well as changes in the cornea. Skin problems evidenced by a dried, thickened appearance, can be seen on the feet, cere, and face. Swelling of the sublingual salivary gland can be present, especially in Amazons, African Greys, and Cockatoos. Renal gout may occur in some cases.
Symptoms of Toxicity:
Decrease in appetite, weight loss, muscle soreness, enlarged liver and spleen, dermatitis, poor growth, diarrhea, and nausea.
Found:
Vitamin A is found in most fruits, and in yellow or green vegetables. These foods include carrots, sweet potatoes, squash, pumpkin, spinach, apricots, cantaloupe, dandelion leaves, red chili peppers, broccoli, peaches, egg yolk, and peas.

Reference: "The Healthy Bird Cookbook" by Robin Deutsch