animal(sparrow)        has_feathers(sparrow)                                   ∀ X. ( animal(X) ∧ has_feathers(X) → bird(X) )
  -------------------------------------------------------- (∧-I)         ---------------------------------------------------------- (∀-E)
                animal(sparrow) ∧ has_feather(sparrow)                    animal(sparrow) ∧ has_feathers(sparrow) → bird(sparrow)
              ------------------------------------------------------------------------------------------------------------------------ (MP)
                                                            bird(sparrow)
                                                      ---------------------- (∃-I)
                                                            ∃X. bird(X)