See the web like cracks? It looks like it started as a thin shell. This sometimes happens with pullets before things get sorted out but can become a problem if they find out how tasty they are and teach others to eat them.
If every one of your chickens are actively laying, they should be on layer feed (4% calcium).
Whether they are all laying or not, you should be providing crushed oyster shell or other good calcium source, free choice, in a separate container.
In addition to calcium, phosphorus and vitamin D need to be in appropriate quantities in the diet. Vitamin D will also come from adequate exposure to sunlight which may be a problem in winter.