Thanks for your reply. We have never given them grit (our assumption was that they would pick it up from being outside). I will try that. I also like the grass idea. My parents don't want them on our grass, so that might be a way to bring grass to them).
My three chickens seem to have developed a bad habit of eating their pine shavings and also dirt; I noticed today that they had kicked a bunch of straw and mulch aside and were digging with their beaks in the dirt). Also, I noticed that one of them seemed to have a more pronounced breast bone...