ok, after watching the full video I think it can be expressed in more than one way. The birds who do not return in the spring (who also migrate south) are a subset of the birds who migrate south. However, since we aren't supposed to incorporate real world knowlegde into these arguments, I guess it can't be assumed that all birds who do not return actually flew south. In this hypothetical there may be birds who flew in other directions.
ok, after watching the full video I think it can be expressed in more than one way. The birds who do not return in the spring (who also migrate south) are a subset of the birds who migrate south. However, since we aren't supposed to incorporate real world knowlegde into these arguments, I guess it can't be assumed that all birds who do not return actually flew south. In this hypothetical there may be birds who flew in other directions.