BastyPutt
Yes, your Polish is a cockerel...
This is a great post, and 100% correct. Hens are hatched, knowing how to hatch. We NEVER disturb a hatching hen much less her eggs, until we see her walking out and about with the chicks. She knows a million more tips and tricks than we could ever hope to learn.Broody hens don't need to learn anything. They have been hatching chicks using pure instincts since before they were first domesticated thousands of years ago. As with any living animal bad things can happen, life does not come with a guarantee. Sometimes a broody hen might get something wrong because their instinct may not be perfect, but the vast majority of the time the broody hens get it right. I find the more I interfere the more harm I cause.
Not every egg will hatch. That is just the way it is. Many won't develop to start with. Some can die anytime during development, even as they are hatching. It could be something like a birth defect, could be something else.
The way you describe the appearance of those two eggs makes me think of something else though. With the shell crumbled away, one with the membrane intact and one with the membrane broken, it sounds like the hen may have been defending her nest, maybe against a snake, rat, or dog or maybe another hen. I trust you didn't have a kid sticking a hand in there. Rarely but sometimes a hen will kill her own chicks or some of her own chicks, those instincts aren't always perfect. Those would be my top suspects. When a hen goes ballistic defending her babies she can damage eggs or even chicks.
I don't know what happened to that hatch. I have no reason to think you did anything wrong, sometimes things just don't go perfectly. That is part of life.
If there are chicks that fail to hatch, she also knows what to do with those - we usually find them respectfully pushed to the side, while they focus on the new little ones.
Once I attempted to peak under a broody hen and that was the last time I'll ever do that. She attacked me like no rooster ever has. Had me screaming the whole way back to the house.