I was paranoid about giving eggshells to chickens because of the dreaded egg eating habit.
But after reading that others do I decided to try. I have been trying to reduce waste in our house and thought this was another way to reduce-reuse-recycle.
So I collect a couple dz. eggshells. Place them on a cookie sheet in the oven for a little while with oven turned low..250 maybe.
Then I put them in a ziplock back and crush them to tiny bits with a rolling pin. I add them to my girls evening bowl of slop/scraps. They have never eaten an egg since I started. I had eggs that appeared to have been eaten in the past but now I think they were accidentally broken in the nest box and no chicken is gonna pass up something yummy. But since I upped the calcium (crushed egg shell, oyster shell, cabbage, spinach, etc) I have never seen a broken egg in the nest boxes.
For the record I would never ever take a freshly broken open egg shell and give it to them. That, I am afraid they might recognize and make the connection to their own eggs.