When I had an egg eater, she usually pecked small holes in the egg and only ate a little of it. The egg itself was usually left pretty much intact. One time, the egg was crushed and a lot of egg was left behind in the bedding in the nest. I think that one time, the egg eater weakened the egg and another hen stepped on it when she went into the nest to lay. I'm at home most days and usually check on them several times a day anyway, but I was down there a lot trying to catch her once I saw what was going on. Maybe I got the others before they got crushed? I think some of my others would eat an egg that had been opened, but lucklily she had not taught any of the others to open the egg themselves. Although the damage is not what I usually experienced from an egg eater, that's still my top guess. I still would not rule out rats either, but I have never had rats eat eggs. I don't know what that looks like.
Just because you are offering oyster shell does not mean all the shells are hard. Some hens' shell glands do not work properly and their shells are thin compared to the others. That is a genetic problem, not a problem of not enough calcium being available. With it happening in three different nests the same day, I don't think this is your problem, but I'll mention it. Something else that I don't think is your problem is that some hens lay eggs with lumps of calcium on the shell. If one of those lumps gets accidently knocked off, you have what looks like a puncture in your egg. That can lead to egg eating, but it is not going to happen often. This is way down on my list of possibilities.
I don't think the size of your nest has anything to do with it. I made mine 16" x 16" with the idea that when two hens decide to lay together they would have enough room. A lot of times two do lay together and I've never had a problem with crushed eggs, except that one time when I had an egg eater.
Something I would try is to put something soft in the bottom of the nests that they cannot scratch away under the pine shavings, although you have 4" of pine shavings in there already. I don't think this is your problem but you need to try something. I'd suggest collecting the eggs as often as you can. That may only be possible on weekends, but see if that makes a difference or if you notice something new. I'd leave fake eggs in there all the time. If it is an egg eater, she may get tired of beating her brains out on something that won't crack. And I would put out rat traps, to see if you catch anything. I don't believe in poison except as an absolute desperate last resort, but here you are more looking at seeing if you have rats that might be the problem instead of seeing if the poison disappears. Mice could eat the poison and chickens eat mice.
I don't know if any of these will help you. This can be a really hard one to solve. Good luck!
I put back the golf balls & left them there for weeks. I was finally able to remove them without any egg eating.
