Just posted this in another thread...but here goes:
Lately, seems like I have heard a lot about shell-less and thin shelled.
I had some for awhile. I added vitamin D-3, and tried to boost calcium too. My shells from the particular hen have been good lately, although today's had a puncture, not sure if it was a foot going through or if she is trying to eat eggs.
apple cider vinegar helps the chicken's digestive system have the pH to absorb nutrients, like calcium. 1-tablespoon to 1-gallon of water.
Lots of crushed oyster shell free choice, their own egg shells crushed and fed back to them if you choose to.
I take a D-3 tablet and crush it on the counter, mix it with some feed that I put some water on to make sure that the D-3 powder will stick. Guess I should say vitamin D-3 -- which I got at
Walmart--it helps them with shell formation.
Use your layer ration that contains the most calcium and hold off on treats that don't contain calcium. It takes a little while but that may help the shell problem.