If a statement begins with the term itself and not with all or some then we make the assumption that it refers to all. Crows are birds has the same meaning as All crows are birds. The term itself we call distributed or undistributed. It is distributed when it refers to the entire class and undistributed when it only refers to part of the class. As we might expect, each of the four classes of statement have a different pattern.
All crows are birds.
Some birds are flightless birds. No elephants are birds. Remember that the defining characteristic is whether or not we are dealing with the entire class. If we say All crows are birds then we are speaking about every crow, but not about every bird. So crows is Dsitributed and birds is Undistributed.