Go at it with this mindset. "Bending the curve," as U_Stormcrow said.Perhaps the best I can do is supplement.
For instance, in July and August, I can collect hundreds of Japanese Beetles every day, and my chickens love them. I notice that they eat less of their feed when they get "bug snack." I'd guess it's probably 15-20% less, but I can tell.
I can grow/gather other things on my property to feed them during about 8-9 months of the year. Because I don't know the nutritional make up of the weeds/kitchen scraps/bugs I feed them, I don't every intend for it to be all of their feed.
I do not free range my birds because of predators.