Chickens are not 100% efficient in digesting eggshells. You will find that by looking at their droppings. If their own eggshells are a sole source calcium in their diet, it won't be adequate.
I have read a little about jungle fowl and about those in Sri Lanka, most recently. The reports are that their diets consist a good deal of orchard fruit. I've wondered if this wasn't because the research was conducted because farmers were trying to keep jungle fowl out of their orchards. But, it shows a couple of things. One is that wild chickens may live very close to humans and this may explain how they became domesticated in the first place.
Few fruits have much protein. As has been pointed out by others, eggs are made up of a fair amount of protein. Laying hens must have sufficient amounts beyond what is required by their own bodies.
The Sri Lankan jungle fowl have, what seemed to me, a remarkably small clutch. Typically, they lay only 2 to 4 eggs, twice each year. I suppose it is also remarkable that the species is not considered endangered or threatened. Each hen producing 4 to 8 eggs a year is "working" for these wild chickens.
If we are expecting more from our backyard hens, they are probably going to need a more nutrition-rich diet than their wild relatives. And, more than they can forage on their own.
Steve