I have 10 red sex link chickens. Just recently I have been noticing more cracked shells when I gather the eggs. They aren't cracked open. Just seems like they may have been pecked or something. I've also noticed a few eggs that seem to feel sort of like it has a thinner shell. I am currently feeding them Dumor Layer Crumble 16%. My dad keeps saying I need to get them some oyster shell. But, then I've read that they only need the layer feed. So, do I need to add something?