They can die of salt toxicity in a few hours if not given water at all times. They respond well to familiar routines. Producing a huge lump of protein in the form of an egg every day doesn't leave room for erratic feedings or lack thereof. It's also a matter of trust, some of us believe. See the protein section for mixing grains and protein.
Protein needs to be measured.
You don't need to mix them, if birds are free range. Let them have a special protein dish in the morning. Maybe soybeans mixed with instant oats, or some fish you have left over, mashed up. They really love canned fish, but it's too expensive for the usual fare. Unless you have a good source.