You take a regular egg, then you poke two holes in the ends and use something like straw to blow out the insides. Then you fill the egg with regular yellow mustard, partway. Put the egg back in the nesting box.
How it works: The Chicken will peck the egg ready for the usual tasty treat inside. Instead, it will taste mustard and I guess chickens don't like mustard so eventually they will give up eating altogether.