GOOD ADVICE SO FAR AND THIS IS MY TECHNIQUE, MAY OR MAY NOT BE AS SUCCESSFUL AS THE OTHERS POSTED BEFORE ME BUT I WISH TO ONLY INFORM. I HAVE NO INTENTION OF OFFENDING THE PREVIOUS POSTERS BY UNINTENTIONALLY BELITTLING THEIR TECHNIQUES. I WISH TO SHARE MY EXPERIENCE ON THIS TOPIC AND PROVIDE MORE OPTIONS, THEREFORE INCREASING THE OP'S CHANCES OF SUCCESSFULLY STOPPING THIS. I MEAN NO OFFENSE AND POST THIS WITH ONLY HONEST AND GOOD INTENTIONS. EDUCATION IS THE KEY TO SOLVING ANY PROBLEM.
This has worked for me in the past with about a 40% rate of success (approx). In 15 years, I have had a few hens start this and a bit more than half had to find there way to a pot to avoid my flock learning this behavior. Adult chickens tend to be more instinctual creatures rather than inquisitive creatures. Meaning, most of what they know about their needs/wants for a successful life come from natural instinct (and from mama's teachings as a chick). Very few of their behaviors are learned by observing other hens (in adulthood) but this is one that is learned not embedded in their genes.
Find the guilty girl.
Take a hard boiled egg and roll it into the coop. Stay out of sight while they observe the egg. Watch for the hen(s) that pecks at it the hardest with the most determination, that's your girl. Remove her. While following my advice here with the flock, do the same for her.
ON A POSITIVE NOTE:
Get up earlier and feed/water them to ensure you are not missing any eggs or that eggs don't spend un-necessary time in the nest. Try to devote one or two days to observing the flock and find out what time to expect eggs.
Some studies have shown that boredom, thirst ,lack of calcium, vitamin D , and/or Protein Deficiency are the common reasons that this behavior starts. Because it is a learned behavior, prevention is the key to avoiding it completely. This may or may not cure the
Do all of these things
You can prevent this behavior by giving them lots and lots of fresh cool, clear water. Chickens are naturally afraid of water, so they like water that is clear enough that they can see the bottom.
Give them milk with some sugar added to it. This provides both calcium and Vitamin D.
Give them a feed supplement with a protein content higher than 18%. I try to find a feed that I can mix with layers scratch so that they are getting oyster shells as well.
Provide something entertaining for them to do that will occupy their time throughout the day. I usually put a tennis ball in a sock (or nylon) and cover the inside and outside with peanut butter. Then roll it in sunflowers. This gives them about six hours of entertainment.
Hope this helps you.....Best of luck.
Timothy in KY