We have an egg eater too and it's getting out of hand. I will have to put our fake eggs back in to confuse them, and maybe the mustard trick. I have heard of that before on here, it's just going to be hilarious. First of all my son LOVES mustard, and might get jealous of the hens getting so much. Then imagine the surprised chicken who gets ahold of that treat!
I wish we could pinpoint who was doing it, but with 20 hens it's pretty hard to tell who does what. Especially in this weather. They seem to only dine on eggs when nobody is watching.