Final answer:
The least likely scenario is that migrant male pied flycatchers would more frequently mate with collared females than with their own species, as birds usually prefer mating within their species due to species-specific mating behaviors and traits.
Step-by-step explanation:
The scenario which is LEAST likely when a group of male pied flycatchers migrates to a region with both pied and collared flycatchers present is that migrant males would mate with collared females more often than with pied females. In most cases, birds tend to prefer mating with their own species to maintain species-specific traits and to ensure the highest compatibility for successful offspring. Hybridization, while it can occur, is typically less common due to differences in mating behavior, calls, or other recognition systems that animals use to identify suitable mates from their own species.
Scenario 1, where migrant pied males produce fewer offspring than resident males, is likely because the migrants may be less adapted to the local conditions or less successful in competing for mates and resources. Scenario 2, where pied females rarely mate with collared males, is also likely due to species recognition behaviors that lead them to prefer their own species.
And scenario 4, where the frequency of hybrid offspring would decrease, is likely if the two species have evolved mechanisms to avoid interbreeding. Overall, habitat isolation due to migration and subsequent differences in reproductive behaviors can lead to genetic divergence over time.