My Isa reds are about 18 months old. Recently some of their shells have become thinner. Today one broke in the nest. They eat 20 percent lay pellets combined with 22 percent crumbles,free choice. Oyster shells are available at all times. They free range 4 to 5 hours a day when l'm home. I think they may be getting ready for their first moult. I read that Isas need more calcium because of how much they lay. Anyone else experience this?