They do not *need* 'oyster shell'. If you feed a halfway decent quality feed for layers, they'll get everything they possibly need from that. If you are able to let them free-range, the bugs, grasses, seeds, small critters, and everything else they know to eat will help also, since one cannot get more natural than that. I've raised chickens for 25+ years and have never bought any 'supplement' stuff (oyster shell, grit, ad nausea) and my hens are always strong and healthy and lay fantastic tasting eggs with fantastically strong shells (*easily* twice the strength of store-bought eggs...I've stacked them 6 high/deep in a gallon bowl and never had a crack happen and more than likely could have stacked another layer or two).
IMO, unless you're feeding poor quality feed, all those 'supplements' are nothing but a waste of money. The companies that make that stuff know though that there's plenty of ignorant folk out there who figure 'more is better' all the time and will waste their money *before* actually trying without that stuff first. (by the way, 'ignorant' does *NOT* mean 'stupid', just in case anyone decides to try and rag on me without reading a dictionary first)
