posted a question
Two populations of birds with somewhat different coloration live on opposite sides of a peninsula. The habitat between the populations is not suitable for these birds. When birds from the two populations are brought together, they produce young whose appearance is intermediate between the two parents. These offspring will breed with each other or with birds from either parent population, and all offspring of these pairings appear intermediate to various degrees.
What keeps the two populations separate?
They are already separate species.
lack of hybrid viability
behavioral reproductive isolation
spatial reproductive isolation
temporal reproductive isolation