I bought two bantam pullets from a breeder. One is about 16 weeks old, one 14 weeks. Pekin, one Buff, one Speckled. They both have combs starting and the older one is "sits" on the younger, who, in turn scurries to get under the Buff one anytime she feels threatened. They are on Growers Mash at the moment, with fresh greens. When should I put a tray of oyster grit in? And will they eat it too soon and upset their systems? And I assume that it's best to introduce Layers Mash by mixing it gradually over a period of time with the Growers Mash? Thank you for your advice.