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?

The two species of sparrows breed only during spring and summer and it has been given that during this time the species live separate from each other. Hence they do not breed with each other.

It is in winter that they live together but this season is not the breeding season. They only feed and prepare for the upcoming breeding season during this time hence the winter habitat has no effect or bearing on their degree of reproductive isolation.

Reproductive isolation only comes into play during summer and spring where they show prezygotic isolating mechanism. They live far from each other thus show habitat isolation due to which they do not mate with each other.