Because of the dogs that are allowed to run loose (despite our leash law) in our area, a dog catcher, a county mayor, and a county sherriff who will do nothing to inforce the leash law, I cannot let my chickens free range. Because of this, they only get the layer pellets, scratch grains, and table scraps that I give them.... and the occasional bug who commits suicide by wondering into the hen yard!! Beforfe I started feeding Co-Op layer pellets to them, I kept oyster shells out for the hens. And, they would eat them. But, since I started feeding the pellets to them they hardly ever touch the oyster shells. I did some reading on the feed bag, and it said that if hens were eating nothing but the pellets there was no need to supplement with calcium. But, if they were eating lots of additional grains and such, then a calcium supplement would be needed. So, I keep the oyster shells out for them. And, I have noticed that the more scratch grain and table scraps I feed them, the more oyster shells they consume.
Robert