 Suppose first all we know is bird. The latter two rules will not be triggered. The first rule will be applicable:  bird  is the case and we have no proof of penguin whence not penguin is assumed. This allows us to infer flies. Now suppose we know penguin. In this case the first rule is not applicable since the default negation of penguin is false, but the latter two rules are triggered and we derive bird and not-flies. 