I literally just found this. We got a new rooster last night AND I've been gathering eggs for the hatch a long. So I'm pretty quick to get them. This one was kind of hiding, but I'm not sure who the culprit could be.
This could be from a toenail or other cause. The fact that the chicken didn't go ahead and finish breaking the egg to eat it indicates to me you don't have an egg eater, at least at this point. Egg eating usually occurs in older flocks, and is really not that common. It isn't caused by their eating a single raw egg. I promise you that if an egg is broken in front of them they will eat it. Some older chicken keepers will even throw their eggs on the coop floor that are very dirty or otherwise unacceptable, for the chickens to eat, and never have their flock turn into egg eaters. I don't think anyone knows what causes that change in the flock that makes them real egg eaters.
It could've been a toenail, or it could be mice. When I had a mouse infestation, it was very common for me to find eggs that looked just like yours and others that had been perfectly split in half and licked clean. It was so bad I'd even find fresh eggs that had been gotten during the day. A week or so of setting a bucket trap and I was back to my normal, intact eggs. I'm one of those people who throw soiled eggs back to the chickens, I've never had an egg eater. So, I'm thinking yours was either an accident or some other kind of critter tried to have a go at it (mice, rats, wild birds). Good luck!
