The proposition "All birds are cows" is equivalent to which of the following propositions?

Under the traditional square with existential import, "All birds are cows" yields all four listed propositions.

By subalternation it gives "Some birds are cows", which is Statement 3.

Converting that particular affirmative gives "Some cows are birds", which is Statement 1.

Statement 2 (Some cows are not non-birds) is the obverted form of Statement 1 through double negation, and says the same thing.

Statement 4 merely restates "Some birds are cows" with the word many, so every one of them follows.

The working rests on subalternation, conversion and obversion within traditional immediate inference.