I grew up with chickens. Even with everyone on the farmstead watching them carefully predictors were still a problem. Even domestic dogs and pigs may quickly learn to kill chickens and they are predisposed to susceptibility parasites. They don’t accommodate really foul weather without some herding to shelter.
They would talk about doing this but at the moment of truth, they’d all just, y’know, chicken out.
Obligatory evil chicken link.
They would evolve back into dinosaurs.
The chickens that went feral on the Hawaiian islands were not the same breed of chickens we usually get around here. (Here, for me, being the US Midwest.) The heavy types, Orpingtons or Rhode Island Reds, have been bred for meat and have no brains. The Leghorns, skinny white chickens, were bred for putting out eggs like crazy and have no brains. As others have said, I have lost chickens like crazy to coons, minks, and hawks. However, ducks, especially Muscovy ducks that are descended from jungle fowl and are not that domesticated, can take care of themselves much better.
On the other hand, we had a bunch of fairly small chickens when I was growing up, but even when I was a teenager I would be pretty wary of the roosters. Those cocks can be vicious. I would not expect a typical cat to try it more than once. Of course, if they’re fighting for survival, who knows.
Pheasants in the Midwest USA would thrive without humans plowing up their habitat. They are a bit different than a chicken though.
Those pheasants wouldn’t even be there if it weren’t for humans. They’re not native to the Americas.
Grouse, on the other hand, are native.
Regardless, in the OP’s scenario, there would be no more people, not that people never existed. The upland bird population would explode due to increased habitat not seen for a hundred years.
Okay, true.
Present.
Chickens (or at least some varieties of them) could go feral just fine. We used to let our chickens roam free, so they were fully exposed to the possibility of predation from all sorts of wild animals, and they got along just fine. They foraged for bugs to supplement the grain we fed them. They roosted in trees. They would find hidden nooks in the hay loft to lay their eggs.
Those chickens could have gone feral with no sweat. They were half-feral already.
I see no problem with putting feral chickens in your fiction.
Ours also walk around the pens that we keep the other animals in. The goat and donkey share a pen and 2 pigs share another. The chickens come and go as the please. Once in awhile they’ll try the pen with 4 St. Bernard’s. That never ends well.
For the chickens, I assume.
(It must suck when everything your own size or bigger wants to eat you. Chicken–the inter-species choice for meals and snacks.)