It goes back before that. When the Keys were governed by Cuba there was cock fighting. When the Mosquito Fleet took over they released all the chickens. Domestic chickens are still raised but those descendants of the cock fighting were declared wild when they were released. Different breed.
I did read a reference to cockfighting, but that's one of those things I don't like to think about.
I don't like to think about cock fighting but that's why the Key Chickens are wild. You should read the Conch Rebellion story as well. I visited a lot for a month at a time when my daughter lived there. Learned A LOT!