When I was a kid, my older brother was into horticulture and stuff and had converted the shed and the back yard to a chicken coop. When they had a problem with soft egg shells IIRC calcium deficiency was the reason and I believe part of the remedy was to let them eat egg shells. Then he got interested in cars and girls and my mom and dad wound up taking care of the chickens. Then one of the roosters kicked my mom and we ate rubbery chicken for about a week.