My girls are over 2 years old. I have 5 chickens ....The oyster shell and grit are free choice in separate chicken feeders The problem is one chicken has a really thin shell. I pick it up and it breaks in my hand.This started several months ago.. The other egg shells are just a little thin .Not as bad as the one. The oyster shell and grit never seems to go down in the containers. Is it because of the age of the chickens?The one soft shelled egg doesn't look like the others. It's bigger and has wrinlkles. The other eggs are formed like they should be. I realize they are older. I get 1 to 4 eggs daily. Sometimes 5. But that is probably do to the age and shorter in the season. Less sun.. So any ideas would be helpful. . They get enough feed also. Thanks.