Modern breeds have lost their instincts, heritage breeds have not.
You do not need to be a scientist to keep chickens!
Everywhere? No, only in areas with high levels of processed foods. Obesity is considered one of the "diseases of civilization." Look it up.
It is the same thing with dogs. You see how sick they get when they are fed dry kibble instead of raw meat.
Yes, they are for many of us. Some breeds have been around for more than a thousand years.
No, you don't need to be a scientist to keep chickens. But you should know what you are doing if you're straying from the recommended path. Pretty much anything that comes from a commerical hatchery is a modern breed; I don't care if you call it a Plymouth Rock or a Rhode Island Red; it's bound to have been crossbred and mixed and selected for maximum egg production and come from birds that were raised indoors for many generations. These birds don't have much instinct left.
Rocks and Delawares and RIRs are no longer heritage breeds excepting those strains and lines that have never been obtained by hatcheries, but have been
kept by breeders for many decades. Those birds are rare and constitute maybe 10% of all backyard chickens out there. These birds may benefit from additions to their diet, but would still do well to have a base of commercial feed.
Of course some breeds have been around for thousands of years. Mostly Asian Hardfeathers. Asian Hardfeathers are not something that thrive on a sole commercial ration, but they are also quite rare and anyone who wishes to keep them should research their requirements. But the idea that it's fine to treat birds like they have been for so many centuries is a notion that badly needs to be dispelled. Birds have evolved more in the past 50 years than they have in the thousands before. Excluding, yes, those rare old-type lines and exotic breeds that hatcheries haven't been able to dilute and change yet.
Get into cornish cross and you may be onto something far as how selective the breeding is and requiring a specific diet to get maximum productivity and even at that I believe maximum productivity is the issue not health of the bird. They are still just chickens.
Even the white leghorn "the" production layer is a breed recognized well back into the 1800s, don't think there was a lot of scientifically designed and widely available feed.
I think you are very confused about the age of chicken breeds and just how recent a development commercial feed is.
Calling Cornish Cross just chickens is like calling a Lamborghini just a car. They're made for one thing and they excel at it, and they require a very specific diet to do it well.
The idea that the White Leghorns one might have found on the coasts of Italy centuries ago shares much of anything with today's Leghorns is certifiably false. I doubt that even the show lines of today resemble very closely those birds. A really, really good egg layer in that day and age might have produced 150 eggs. Might have!
People act like just because commercial feed is a recent (if you can call 50 +/- years "recent") creation it is untrustworthy and unnecessary. Do you know how many millions of dollars have gone into research of the nutritional needs of poultry? Using both white egg and brown egg layers, most of which share much with today's hatchery quality breeds? The creation of commercial feed resulted in a huge jump in the creation of better poultry which resulted in these new birds, created on a higher quality of feed, requiring this new quality of feed to upkeep their hightened production and body maintenance needs.
Edited to add: And once again I am not saying don't mix your own feed! But know that commercial feed is ideal for the vast majority of modern chickens and do tons of research and no what you are doing before home mixing!
