Interesting question on a philosophical level. To me a feral flock would be one that feeds and waters itself and does not get any help from humans for predator or weather protection. They would reproduce on their own. They do not have to be in the middle of a wilderness to be feral. I don't know if you are thinking along these lines or have something else in mind.
To me the biggest challenges would be predators and to a lesser extent food and water. They should be able to handle weather just about anywhere.
I agree that games would be a really good choice. Some bantams might work out pretty well. Stay away from the decorative breeds with fancy headdresses or other handicaps. I'd go for camouflaged colors, reds for sure, maybe black. Avoid white, buff, and other bright colors and patterns. And I'd want hens that often went broody.
Where is the best place? Where are there feral flocks now? Several years back a forum member of great experience and common sense talked about a flock that went feral in the Michigan peninsula one winter. They survived the Michigan winter. He felt they probably ate snow and foraged in farmer's fields. They can do better than many people expect. He mentioned that the next year they caught that feral flock and ate them, did not leave them for years.
But the feral flocks I'm aware of are warmer places. Hawaii has been mentioned. Their predators are pretty limited to mongoose, dogs, and flying predators. Rats could be a danger to eggs. Mike Rowe did a "dirty Jobs" episode on catching feral chickens in Miami. I'd think the best places would be where warmer winters would make food easy. It would probably be easier in cities or towns with parks, some woodlands, and a stream with overgrown banks to give them foraging and hiding places. While they have predators they tend to be suppressed.
How would I establish a feral flock if I found a suitable place? I'd find chickens raised by a free ranging broody hen that raised them without humans feeding them. I grew up on a farm like that, such chickens exist. Then I'd just turn them loose, quit taking care of them. Either they make it or they don't. I'd think predators would be the reason they don't.
Some food for thought. Many pioneers that settled our country had flocks of semi-feral chickens. They'd provide predator protection and not a lot else. There have to have been many times when those pioneers were no longer around to protect those flocks. Maybe they died or just moved on without taking all their chickens. This may not have happened all that often but it had to have happened. I'm not aware of feral flocks of chickens roaming our wildlands like wild turkey or pheasants. Those flocks did not make it, my guess is because of predators.
Leghorns readily roost in trees...very high away from predators...one of the most flighty birds for sure. I have also seen them on a native reservation where they were completely free and saw a hen with a few adolescent chicks following her. God bless everyone
