Feral chicken are known in several places. They can be pretty successful and have been signaled as threats to ecosystems and crops in archipelagos like Hawaii and Bermuda. But I’ve thinking about Brasil: Given the sheer amount of chicken being bread there, the presence of the Amazon rainforest, which has a similar climate to whence jungle fowls, the chicken’s ancestors come; and its already fragilized ecosystem, isn’t there a specific risk there ? So far, I’ve seen no South American country listed as famous for feral chicken presence . But hypothetically, if a few millions of fowls escaped a massive Brasilian farm and swarmed the Amazon; what could happen ? Would they quickly die off, due to having lost adaptations to wildlife, having an insufficient ratio of roosters and facing many predators ? Would they outcompete one or two local bird species and steal their niche, but otherwise fit fine in the food chain without further disrupting the ecosystem? Or would it spell a great ecological catastrophy ?
In worst case scenario they would overpopulate and consume all natural resources until a majority of their population starved down to an equilibrium. Eventually there would arise some predator that would adapt to take advantage of the new food supply. This would probably take thousands of not millions of years though.
Our limited human lifespans make us succeptable to thinking in short term, in the long term it wouldn’t matter in the least bit if chickens ravaged the amazon ecosystem since it would just adapt over a relatively short time geologically. 50 thousand years is unimaginable to us but peanuts to the planet and environment. Rediculously successful organisms ravaging the environment and killing themselves off is a story as old as biology.