When my hens started getting really red in the comb and wattles and I could feel that their pelvic bones were separating close to two fingers width, I put out crushed oyster shell for them. Several more knowledgeable folks on this site have said they'll only eat it if they need it. And that seems to have been true for my hens. I was concerned because my NHR's started laying at 17 weeks and the buffs (who are a week younger) and the EE's have at least another 2-3 weeks before they start laying, but I have only seen those who are ready to lay or are laying to eat it. Follow your instincts - you'll sense when the girls who are ready to start laying are close - mine "assumed the position" about a week before they started laying and love it when I ruffle their rump. I think they think I'm the rooster!
Good luck!
