Many Songbirds Breed in North America in the Spring and Summer

Many songbirds breed in North America in the spring and summer and then migrate to Central and South America in the fall. They spend the winter in these warmer areas, where they feed and prepare for the spring migration north and another breeding season. Two hypothetical species of sparrow, A and B, overwinter together in mixed flocks in Costa Rica. In spring, species A goes to the east coast of North America, and species B goes to the west coast. What can you say about the isolating mechanisms of these two species?

A) The two species do not breed in the same area, so they are reproductively isolated by allopatry.
B) One of the species is probably polyploid, so they are reproductively isolated by genetic incompatibility.
C) They must have strong prezygotic or postzygotic isolating mechanisms in order to spend winter in such close proximity.
D) Reinforcement must be occurring when they winter together.