I just throw them into the coop along with the feed and organic table scraps. The chickens will scratch for what they need whether it be food or minerals. You'd be suprised on how smart these dumb chickens are. They didn't survive extinction for this long being stupid. Even with all of the dozens of preditors that love the taste of a free chicken dinner.
Just throw a few handfulls of the egg shells in and check to see how much they eat. When it seems that they ate most of them, throw some more in. Use your own judgement. I found that chickens that peck and break thier own eggs are usually not looking for extra calcium, They are in need for extra protein in thier diet. The yolk is thier best source of something that they need. Buy a feed that has blood meal or extra soy meal. They will recycle the calcium as needed for the shells.
Just throw a few handfulls of the egg shells in and check to see how much they eat. When it seems that they ate most of them, throw some more in. Use your own judgement. I found that chickens that peck and break thier own eggs are usually not looking for extra calcium, They are in need for extra protein in thier diet. The yolk is thier best source of something that they need. Buy a feed that has blood meal or extra soy meal. They will recycle the calcium as needed for the shells.