Chickens are big birds - some of them can be bigger than some small cats. I suspect that cats on a farm would probably be cautious about chickens at first, and chickens are able to defend themselves - once those wings get extended they can look really big, they can move quickly, they make a lot of noise. and the beaks can inflict injury. My Mom used to tell a story about a rooster on the farm where she grew up that used to chase the children until he chased them one too many times and ended up in the stewpot. I suspect a cat would probably decide to leave them alone unless well motivated by hunger.