If you are thinking about buying an American Bulldog, be very careful whom you buy from. Find a serious breeder who breeds for temperament.
Some American Bulldogs are really dangerous. Some breeders are breeding them to be man fighters, and they are a huge strong dog. If one of them goes after you, you don't stand a chance.
You want a breeder who understands that they were developed to be a livestock catch dog and should be very reliable with people. You don't want a line specially bred to guard marijuana fields.
Because they are supposed to be a livestock catch dog, they wouldn't be my first choice for chicken guarding. However, if you get a well bred one, they are wicked smart and eager to please and you should be able to train him to do whatever you want