I have been extremely lucky so far out of the 50 chicks I got lost 2. One to a rattlesnake and one that was bitten by something and the bite got infected and I finally had to dispatch her. I have been getting plenty of eggs but lately, the yolks have been so weak that they break when I crack then into a pan or a bowl. Is there a missing mineral or something that they need. The shells are ok, I have oystershell that I mix a little in with their feed. They run free all day so they should be getting bugs. What could it be? Thank you for any ideas.