Answer: Prezygotic barrier
Explanation: If the male sperm never fertilizes the female egg in a mating between two parrots from different populations, a prezygotic reproductive barrier may be operating. Prezygotic barriers prevent fertilization from occurring by blocking mating or fertilization between individuals of different species. In this case, the male and female parrots may be genetically incompatible and unable to produce viable offspring, even though they are able to mate. This type of reproductive barrier can arise as a result of speciation, where two populations evolve into distinct species over time. Other examples of prezygotic barriers include physical differences in reproductive organs and behavioral differences that prevent mating.