I recently found a broken egg in a nest box. The shell was so thin that just the weight of the hen was enough to break the eggs. This is not normal. This can indicate there is not enough calcium in the diet. But I give them a high quality feed with plenty of calcium. I also crunch up and return the eggshells to the chickens with their food.
But a weak shell can also be indicative of an older hen. I think that is the case here. I just lost one hen and another may be on her way out.