"If hens produce eggs with thin shells or shells that are easily cracked, oyster shell supplementation might help."
https://poultry.extension.org/articles/feeds-and-feeding-of-poultry/feeding-chickens-for-egg-production/
"eggshells and oyster shells are cost-effective and proactive methods to ensure our hens stay strong and healthy while producing nutritious, hard-shell eggs.
Most laying hens will benefit from a calcium supplement. For a cost-effective option, feed the chickens dried and crushed eggshells or consider ground-up oyster shells as they are full of calcium carbonate."
https://thefarminguy.com/oyster-shells-egg-shells-chickens/
"A female chicken in her prime needs about four times as much calcium in her diet as a nonlaying hen, chick, pullet, cockerel or rooster. The shell of an egg, like the shell of an oyster, is made mostly of calcium carbonate.
To produce a dozen eggs takes about an ounce of calcium just for the shells. Some calcium comes from the feed, some from foraging (if allowed) and some from being fed spent eggshells. But the rest has to come from crushed oyster shells.
Without enough calcium in her diet, a hen might either lay eggs with soft shells or develop weaker bones by drawing calcium from those bones to make eggshells."
https://www.hobbyfarms.com/chicken-feed-cafe-the-basics-on-grit-oyster-shell/
"Any laying hen needs to have the option of getting extra calcium by consuming either egg shell or oyster shell."
https://thehappydaysfarm.com/egg-shell-oyster-shell-the-great-debate/