If you feed their eggshells back to your hens you are in effect throwing out those eggshells. Because of the nature of a chickens' digestive system, eggshell particles don't remain in the hens' body long enough for the small amount of calcium they
contain to be utilized by the hen. However most of the really bad chicken diseases are transmitted through the egg. Many of these really bad chicken diseases are seldom seen in the USA today, mainly because of the efforts of the commercial
poultry industry. So eating uncooked eggshells can be a route for infection in your flock and any laying pellets or laying mash worth the name already has sufficient calcium in it. Do whatever it is that you want to do anyway but remember that
feeding their eggshells back to them will not get you or your flock past Goal nor will it mean that you get to collect $200 from the bank.