Many Songbirds Breed in North America in the Spring and Summer

Multiple Choice

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) They must have strong postzygotic isolating mechanisms to spend winter in such close proximity.
B) They must have strong prezygotic isolating mechanisms to spend winter in such close proximity.
C) These two species mate in different climates.
D) Reinforcement must be occurring when they winter together.