This is a trick an old timer taught me, and it works every time.
I use this on Chickens and pheasants.
First, you have to do what all the others have suggested. Nest box height, etc.
Then you have to leave a few decoy eggs in the nest box.
The eggs you use for decoys are filled with a mixture of cyanne pepper and egg.
Take a couple of the eggs and put a small pin sized hole on each end and blow the egg contents out into a bowl.
Mix the contents (whites and yolks) like scrambled eggs.
Add about 2 TBS cyanne pepper, Mix real good.
Take a syringe. A larger size works best. No needle.
Pull plunger out, and pour mixture into syringe and install plunger.
I put a little bees wax on one of the holes on the end of the egg.
Insert the syringe in the other hole and fill egg with mix.
Plug filled end with bees wax.
Continue till this you have the desired amount of eggs.
Mark the decoys, install in the nest area where the egg eating happened.
After 3 days I have have yet to have any birds interested in eating eggs.
This is the only method that has ever worked for us.
We use about 6 eggs for a day or two. In each flock that started egg eating.
Just my.02