Any half decent pasture , scrub, or woods has all the free protein you need spring through fall. You just have to let the birds find it, and find ways to keep them safe in the process. Obviously you need aggressive foragers and you can't overstock the land with too many birds. What they lack in range is carbohydrates, hence the ancient practice of feeding grain. I raise plenty of cockerels and a few cull pullets each season on nothing but a few handfuls of whole grain a day for the whole free range gang. That's after they fully feather out and are large and smart enough to take care of themselves. Trying to raise protein for poultry is working too hard. They can find that themselves easy enough.
I wonder if this hold true for all breeds, or if the game fowl have an advantage in this respect?
I have some young, dual purpose breed birds, that are in poor flesh. They free range every day during daylight hours. The have been fed a 21% protein starter ration that includes fish meal. They've tested free of parasites. These birds are ravenous and will eat anything they can get a hold of.
I wondered if they needed more energy (carbs) feed sources but that would displace the protein level.
I would tend to disagree that they can get all the protein they need by free ranging. Or maybe some breeds - like game fowl- more than others.