This is a fascinating thread. I am a vegetarian (not a vegan) and have spent some time thinking about plant-based nutrition, both human and chicken. However, I have no formal education in this field, so these are just my thoughts.
1. Eggs are protein. In order to lay eggs, which is hard work (somewhat akin to growing a new baby every day or so), chickens need nutritious food, including protein.
2. Corn and the other grains tht make up scratch provide protein as well as other nutrients. However, corn has a lower average protein content than wheat or rye.
2. The commercial chicken feed that I use is 100% plant-based -- no animal byproducts. It has a protein content of around 14%, which may be higher than the protein content of corn and other grains because I'm sure it includes soybeans for added protein.
3. When my chickens free-range, they gobble worms and bugs -- more protein.
4. My girls lay eggs most every day, so whatever protein they are getting from the grains in their scratch, the plant foods in their feed, and the bugs & worms they find outside must be sufficient to allow them to produce eggs.
5. I give my girls only nutritious treats -- seeds and whole grain leftovers -- no white bread! I have read that whole wheat bread has a higher percentage of protein and more minerals than white bread.
I think there's a reason chickens go nuts over protein-rich food, such as cheese, scrambled eggs, yogurt, and worms. I believe they instinctively recognize the protein that their bodies need. So chickens love corn, worms, bread, etc because these foods supply the nutrients their bodies need. I think we are shortchanging them -- and ourselves -- when we feed them non-wholegrain food, because refined grains no longer contain all the protein and other nutrients they need to stay healthy and productive.
Does this make sense?