Protein, or lack thereof, isnt the culprit. It is often blamed, but a chicken knows nothing of such things. Rather, it's one of those "lore items" we love to repeat, just like using golf balls and giving them hot peppers.
Egg eating is a result of two things rather common things: luck and desire.
Chickens crack or break an egg accidentally (luck) and peck at it like they do everything else. Once they get the taste, they won't quit (desire).
Things to do are...
- Darken the nest.
Darkened nests lead to less moving around, and so less mechanical breakage. Chickens also wont eat what they cannot see well, so they wont peck at eggs that have a spot of poo or clinging feathers... if the light is too dim to see them by. Hang a dark curtain over the nest entrance or use communal box nests.
- Keep a clean nest.
For the above reasons.
- Keep a well padded nest.
A minimum of 2" of soft bedding is recommended, if you're not using a roll away nest design.
- Ensure adequate calcium in the diet, as opposed to protein.
This leads to strong egg shells, and so less breakage.
- Remove eggs often, 2-3 times per day.
If they dont sit there, they cannot tempt them.
- Cull offending hens early
When one learns it, the others are soon to follow. In this case, having 'egg on ones face' is especially bad - for them.
- Use rollaway nests.
These move the eggs out of their reach once laid.
As is true with so many things, the things that work are logical... and boring. Try these.