Well now I have a bit of time to reply properly, I will demonstrate that hens ARE being fed a vast amount of calcium, a vast unnatural and cruel amount in fact.
So lets calculate...
A hens eggshell weighs about 6g, 2.5g of that egg is calcium, but only about 50% of that calcium gets absorbed by the bird.
Therefore to sustain a hen to produce an egg per day you need to feed the bird about 12g of limestone everyday. But a hen doesn't produce an egg everyday but about 300 times a year so lets approximate to 10g of limestone per day.
Now a natural hen ovulates 12 times per year that is 12 eggs per year but 12X12g spread over a year is 0.4g per day is attributable to egg production.
That is 25 times less calcium in its diet than its battery hen counterpart.
If I was to scale that up in human terms then the numbers become massive. A layer hen should weigh about 2.7kg. Now I weigh 80kg, so proportionally if I was a hen then I would need to eat 80kg/2.7kg X 10g = a massive ~300g of limestone per day!!
That is the equivalent of eating the weight of 3.5 average-sized apples per day of calcium carbonate or limestone. In anybody's opinion apart your own, how is this not a vast amount of limestone?
In other words, you have just trivialise the amount of limestone or oystershell a bird needs to have as a supplement in their diet.
EDIT: On another point about the grit, you are deliberately confusing insoluble grit that is needed to digest food for a natural hen with limestone needed as supplement, two very different things.
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