In North America, many kinds of birds have closely related eastern and western counterparts (species or subspecies) that sometimes produce hybrid offspring where their ranges overlap

Question

Eastern and western meadowlarks are virtually indistinguishable from each other in appearance, and yet, despite the fact that their ranges overlap throughout much of the Midwest, there are few documented cases of the meadowlark counterparts mating with each other and producing hybrid offspring. Propose mechanisms that would prevent cross-breeding between eastern and western meadowlarks, thus maintaining the counterparts as separate species.

Answer 1

It is possible that the two species sing songs or exhibit some other kind of behavioral differences that prevent their counterparts from recognizing them as potential mates. It is likewise possible that the two species prefer different habitats and would not find themselves mating in the same area, even if their ranges overlap. There may also be genetic incompatibility between the two species, such that an eastern meadowlark mating with its western counterpart would produce eggs that do not hatch or, if they do hatch, that produce offspring that are unable to survive to adulthood and produce offspring of their own.