Can someone explain to me how the sentence "Many birds that migrate south in the fall do not return in the spring" differs at all from "Of birds that migrate south in the fall, many do not return in the spring"? To me, these appear to be the same, since both seem to imply that, when talking about birds that return in the spring, we are only talking about the birds that migrated south, which would make it a subset instead of an intersection, no? I don't think the "many" refers to the birds that migrate, rather, the birds that return.

Except in this example I think it can be inferred that all the birds that don't return are birds that migrated south to begin with because it is only talking about the birds that migrated south. Unless I'm wrong, I believe it states that "of birds that migrate south, many don't return".