Oyster shell is pretty much limestone, so it doesn’t go bad if in a container. My hens have always preferred to eat chopped egg shells, but I still keep the oyster shell out for them. Even with all that, there may be a couple who still will have the thin shells.
I had a hen a couple of years ago who laid an XL hard egg every day. She was only 1 1/2, and had raised a clutch of chicks. She had never molted, but was attacked in out driveway by a neighbor’s friendly dog who was not used to chickens. She was nearly dead, lost most feathers, was cut and scraped, and could not stand for a day or two. Afterward, she recovered, molted, but never again laid an egg with a hard shell. Most were shell-less or very thin shelled. Because of her thin eggs, I feel like she developed egg yolk peritonitis and died last year, possibly from some breaking inside.
I had a hen a couple of years ago who laid an XL hard egg every day. She was only 1 1/2, and had raised a clutch of chicks. She had never molted, but was attacked in out driveway by a neighbor’s friendly dog who was not used to chickens. She was nearly dead, lost most feathers, was cut and scraped, and could not stand for a day or two. Afterward, she recovered, molted, but never again laid an egg with a hard shell. Most were shell-less or very thin shelled. Because of her thin eggs, I feel like she developed egg yolk peritonitis and died last year, possibly from some breaking inside.