Of course, a whole foods diet must be varied, balanced and from a wide selection of foods. The more diverse your bird's diet is the better. They are isolated synthetic vitamins, minerals, amino acids and so forth along with a few grains, seeds, etc. When you take an incomplete vitamin, that vitamin must take the necessary co-factors out of the body in order to complete its process. So, if your bird is not consuming the proper foods in order to replenish some of that, you create imbalances and deficiencies.
Deficiencies create disease and infection.
But sooner or later the signs and symptoms will appear or develop when the co-factors are depleted. These co-factors are needed to complete the process or activity of these vitamins. As these co-factors become diminished in the body the health of the body will steadily go down hill. The bird gets it from its body. And if its diet is incomplete in a particular area of nutrition, it may very well already be out of it.