"You are what you eat," is a favorite saying but very true. In order for birds to be healthy, they must consume all of the necessary nutrients (protein, carbohydrates, fats, vitamins, minerals, and water) and do so in the proper proportions. A number of reputable pet food manufacturers have recently developed and are currently marketing specialized processed foods for caged birds. These pelleted foods should be part of your birds diet along with seed and fresh fruits and vegetables.
Vitamin supplementation for birds is also recommended. Powdered vitamins can be sprinkled over table foods, fruits and vegetables to which the powder will adhere. The powder does not adhere to the seed and the bird usually discards the shell anyway so it doesn't get the benefit of the vitamins.
The vitamins and minerals supplements help in case your bird's diet is lacking in one or more types of nutrients. Bottom line, make sure to offer your bird a varied diet.