system
Given the following post, respond as if you're commenting on Reddit:
human
A chicken egg is 40% calcium. How do chickens source enough calcium to make 1-2 eggs per day?

edit- There are differing answers down below, so be careful what info you walk away with. One user down there in tangle pointed out that, for whatever reason, there is massive amounts of misinformation floating around about chickens. Who knew?
gpt
Chickens are omnivores, but free-range chickens eat a great many insects, and insect exoskeletons are similarly rich in calcium.  They'll also eat just about anything else, including small mice, voles, moles, lizards, etc., bones and all.

In a commercial setting, they are supplemented with oyster shells, or whatever source is cheap.  Home chicken keepers sometimes feed old eggshells back to their chickens.

Chickens fed more calcium will lay eggs with thicker shells, and they can accept a fairly wide range, but a deficit will yield fragile eggs.