First of all, back to your original post: Your birds will lay this year regardless of the coming of winter. When they're ready, they will lay maybe not as much or as often but they will start. Second, they need calcium to cover their eggs, whether they get this from their diet or steal it from their bones, they will get it. By adding calcium through layer feed or oyster shells or feeding back their own egg shells, you are protecting the bird structurally and, probably, promoting a longer, healthier life. At the same time too much calcium in the diet of nonlaying birds is unhealthy since it can possibly lead to kidney problems since once their bodies have metabolized what little they need, it has to go someplace. Consequently you want to avoid overdoing it with young birds before they begin laying. Thus it is probably best to hold off feeding layer food to young birds until they are either laying or almost laying (the latter can be hard to judge if you have a flock of mixed varieties or ages so wait until all are laying). If some of the birds are laying, offering oyster shells on a free choice basis allows those that need it to get it without forcing it on the whole flock in their main diet. At the same time, some of the nonlaying birds may sample the shells but, since it is not the main source of food, will soon ignore it until they need it for eggs and before the excess will harm them. At least that is what we, in theory, hope--some chickens are not good dietitians. I might add as well that there are places where birds that free range manage to pick up enough calcium through diet so they get along just fine without the extra supplement.
