Egg shells "work" (sorta) but short term, oyster shell is longer term , unless you like soft egg shells make both available as the others have said. You may not notice them going but I'd wager they are, though my chickens will also eat the egg shells much quicker they certainly eat a bunch of...
About 50, we added the EE last fall late , they've been laying since around Feb,(had 4 hawk got one and a Domique on the same day/time) added the Olive Eggers in winter and at least one has started laying now!
28 today (no green today )
Speaking of "darker" brown eggs we're getting some "off and on" (know it's not the BCM or Welsummers they're too young) sometimes they have "white speckles" LOL