When looking at the commercial feeds, know that the least amt of protein is in the feed to get the job done. BECAUSE protein is the most expensive component of feed. Whether grain source or meat source.
IMO I am not sure I buy into there being a limit to how much protein an animal can eat. In that I would look to the normal "wild " animal intake as a basis. What does a wild ground dwelling bird "normally" eat? ( Talking chickens here.) OR if ducks, what a wild duck eats. Some one reminded me that seeds are an important basis of a chicken's diet.
Having said this,in regards to humans, I know the people of the far north traditionally ate a diet primarily of fats and protein ALL the time.Think whale, seal, polar bear, and caribou. The veg was limited to the lichen from the rumen of the caribou. ( No food wasted in a traditional diet.) WHen I am working to shed a few unwanted fat pounds, I switch to a very similar diet. ANd I know of many many people who also follow this eating program safely as well. I would be comfortable eating a diet soley of wild caught meat as this was a documented study of a year done MANY years ago. Most people dont know, or dont question, that there is NO nutritional requirement for carbohydrates in the human diet. What we do gain by eating grains, and other carb sources like potatoes, beets,turnips etc, is a source of calories and wonderful vitamins and minerals. But the carbs themselves are not required for survival. Carbs are not an essential nutrient for humans. Fats and proteins are, as well as a slew of vitamins and minerals.
PS. WHo noticed that a beagle won Westminster. Miss P !!
I think it only fair to point out that the people of the far north were laboring hard each day to hunt and prepare this meat and blubber for consumption, fuel and clothing for the better part of the year and the rest of the time were using it to keep warm, as well as for activity during cold weather...the level of energy expended is important in that equation. And they had more than lichen, they had berries in season. It also mattered what TYPE of animal proteins were consumed...some are more digestible, better utilized and less harmful than others.
Wild birds? Their activity levels to even get the foods they consume and feed their young so far outstrip our chicken's daily activity as to be laughable, so their bodies require a higher intake.
"Switching to that kind of diet" temporarily and maintaining it on a daily basis for the rest of your life are two different things...one is short term, one is long term. There are many studies done on the effects of a long term high protein diet that does not balance out other nutrients...one being ketoacidosis, another being osteoporosis, as well as renal disease.
You can do an experiment on it and start feeding your flock on calf manna, distillers grains or even fish/meat meal only for the next several months and let us know what the effects of it are, good, bad or no effect at all and you could help us all know for sure on the matter. That would be a cool experiment!