Just an FYI - my girls are just about ready to start laying so I started them on layer feed about 2 weeks ago (however after reading several posts here on BYC, probably going to switch to all flock feed since I have 2 roosters in the mix that live with my beautiful girls). I actually tried mixing in oyster shell with their feed and putting it in the feeder. Chickens are smart! They pick out the oyster shell and spit it on the ground or just eat around it in the feeder. I can tell this because when I go to fill up the feeder, there is generally a TON of oyster shell left in the feeder and very little feed.

So, if you think your girls need it, put it out, but they won't eat it if they don't need it.
This may change with my girls when they actually give momma some eggs



, but right now they ignore it.