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, species X and species Y, overwinter together in mixed flocks in Costa Rica. In spring, species X goes to the east coast of North America, and species Y goes to the west coast. Which of the following factors most likely keeps these species separate?

A) strong postzygotic isolating mechanisms in the winter
B) strong prezygotic isolating mechanisms in the winter
C) the two species reproduce in different geographic areas
D) reinforcement of isolating mechanisms must be occurring in the spring