Corn is not filler. It is a major and important part of the feed. It is not the whole feed by itself no more than any other ingredient is entire and sufficient unto itself. The miller has a recipe to follow just like making a cake. To get the desired complete protein level add so much of this, so much of that, and this much of this other stuff for the vitamins and minerals that the first two ingredients do not contain. Corn is a filler the same way that flour is a filler when making a cake.
When corn (or any other food for that matter) becomes a filler is when you are feeding a minimally complete ration (a typical layer feed) and you start offering a lot of other feeds such as corn, grapes, bread, whatever in addition to it. The complete feed is just that - complete. When you offer a lot of treats in addition to it you begin to alter the nutrient ratios the birds are taking in. If your layer feed is only in the 15-16% protein range then keep treats such as corn or anything else to a minimum. About half a handful per bird per day.
If your complete ration is higher in protein, especially if your birds have regular access to fresh grass or free range, then you can offer more supplementary feed. All of my birds get a 20% protein complete ration (Purina Flock Raiser) and free choice corn/wheat/alfalfa pellets. They eat roughly half and half of the two which gives them all their necessary nutrients. If I were to do something like offer 28% gamebird starter and grains they'd probably eat something like 30/70 complete ration/grain ratio. If they were free ranging in a good pasture the complete ration consumption might drop even lower as their grain consumption increased.
In a nutshell corn is not filler IF it is properly factored in to the bird's diets and is a filler when it is not.
.....Alan.
