"An apple a day keeps the doctor away." -Benjamin Franklin
Many years ago, when I raised exhibition budgies, pelleted budgie diets were rare, but there were a number of vitamin supplements on the market that could be added to food or water and these supposedly provided the complete balance of vitamins and minerals needed for a bird to live a healthy life.
While these supplements initially contain what the manufacturers claim, the problems with them are many. Once exposed to heat, sunlight, and water, most vitamins begin to degrade. You don't know the storage conditions of the supplements since they left the manufacturer's facility, so by the time the bottle has been shipped to the distributor, store, and finally reaches your home, it may easily have been exposed to temperature extremes in the various vehicles used for transportation and therefore may no longer be what you think you are offering your bird. Your bird may not like the flavor or consistency and may start eating or drinking less food and water than it typically does; as a result you could be creating new problems where none previously existed. Or he may like the supplement more and basically injest too much of a good thing since many of the fat-soluble vitamins can be toxic when consumed in high doses.
Like humans, I believe birds have cravings. When feeding Lely, I saw that one day she might prefer bananas, which are high in potassium, and on another day she would toss them from her dish, opting instead for Romaine lettuce, which is high in folic acid and Vitamin A. Providing a variety of clean, organic fruits and vegetables will give your bird the opportunity choose those containing any nutrient he is lacking on a particular day.
Chop, a supplement of fresh fruits and veggies, can easily be made in your kitchen just prior to feeding your bird. You can learn more in Chop Made Easy. And about those apples--they have both antiviral and antibacterial properties--Franklin had the right idea!
Missed Part 1? Read it here.