The different coloration of the island birds is likely a successful mutation that arose among the birds once they were stranded from the mainland, probably by means of natural forces such as long-term tectonic changes or even the short-term shifting of a sand bar that previously allowed passage between the island and the mainland. Were it not for the genetic data confirming that these two bird populations are related through molecular means in terms of their DNA, it might be possible to posit convergent evolution—the idea that two essentially unrelated species can very closely resemble one another as a result of inhabiting similar environments and filling similar roles in those environments—as a force at hand. However, given the genetic data, this is obviously not the case. Instead, what likely happened was that the population’s coloration changed as a result of multiple factors.
Small islands have many unique characteristics that influence the course of evolution. Isolation is one such factor already mentioned. In addition, there is island dwarfism, which is the tendency of island-dwelling populations to evolve to be smaller than mainland counterparts over time, perhaps due to a relative shortage of food. Though this component is not directly at play here, it is still related because this same geographically limited supply of food also limits the number of predators a small island can sustain. Given that as nutrients move up the food chain, much energy is lost, a small island may simply not be able to sustain enough plant life to ultimately support any top-level predator species already on the island. These then become extinct. This can explain the island birds’ gaudier plumage; in the absence of predators—including humans—the cost of such showiness is diminished. The upsides, on the other hand, remain the same, for the more garish coloration an animal has, the easier it is to find mates. With Darwin's legacy of natural selection dictating that those most suited to survive and reproduce will be the ones to pass on their traits, if the downside to a given characteristic is reduced but the upside remains the same, over time, the population tends to carry that trait in greater numbers. It is also possible, however, that the coloration is a recessive trait that was overrepresented among the island population. This is another hypothesis that could be tested.