Treats formulated for a cockatiel and millet are sure to be a feathered fan favorite, but should not make up more than 10% of their total diet. Fresh fruits and vegetables such as bananas, zucchini, bell pepper, mango, peas and corn are a nice addition to their daily diet. Birds should never be fed avocado, fruit seeds, chocolate, caffeine or alcohol as these can cause serious medical conditions. Sugar and high fat treats should also be avoided for the health of your pet. Consult your veterinarian for a complete list of foods your cockatiel can and cannot eat.