For the example, "Many birds that migrate south in the fall do not return in the spring." And you make the two sets. Can someone explain why the set of "birds that do not return in the spring" is only an intersection and not fully encapsulated by the first set "birds that migrate south in fall."

At first I thought, how can you return in the spring if you don't at first migrate in the fall? But I guess, there may be birds who return in the spring who migrated in the winter or 10 years ago?