One of our hens consistently lays eggs so thin that they break - possibly when another hen gets in the box to lay. If the egg does survive, it frequently has an end that is soft to the touch, and more easily cracked open than the rest of our eggs. We have a seperate feeder with oyster shell, we regularly grind up all of our used eggs (from the kitchen) and add them to the regular feeder, so those are 2 sources of calcium. Is there anything else we need to do? I remember some folks adding vinegar to their water. What's that for?