I agree with Corinna, but I will phrase it a little differently. It is a lack of training. You can't just keep telling a dog "no" and expect him to suddenly know how to behave. In addition to teaching Sammy not to chase the chickens he has to show him how to behave when the chickens are around. This might mean that Sammy will only be allowed out on leash until he is taught how your father wants him to behave. It might mean putting up different fencing to keep the birds out of Sammy's part of the yard and vice versa. But there are definitely ways that they can all learn to live together peacefully again. Yes, chasing them is instinct, but I have seen other PTers who have somehow managed to teach their dogs to live with birds.