@SmpleJohnIt's misleading to think of evolution as producing things that are "better". Progress in evolution is a misconception that is made quite often.Evolution is not a process that is somehow guided toward perfection, invincibility or intelligence or any such things. Evolution simply results in creatures that are well enough suited to where they live to survive and reproduce. If you were talking about a bird in an arctic climate or some harsh place, then adaptations leading to better survivability in those climates would be likely, because to live in such a place, surviving in a harsh climate is necessary. For a birds that live in temperate climates that do not result in death from exposure, the selection pressure (through the birds dying from exposure) is minimal, and therefore traits that make a bird hardy do not give it any advantage over others.I must stress, as an aside, the evolution does not work toward perfect. It only results in creatures "good enough" to go on surviving. If a population of creatures in some environment is doing well, chances are they will not evolve to do better.In the case of this new species, we're talking about adaptations that make these birds sexually attractive to only those with their new and different song, as compared to the population of birds that they seem to have learned (incorrectly) the song from. The two populations of birds do not appear to interbreed because, due to their differences in sexual selection, they do not take mates with members of the population with the slightly different song.Since mating does not occur between the two populations of birds, they have established two separate gene pools. Over time, the two groups may end up with traits specialized into different environmental niches -- whatever makes them best at surviving and reproducing in their environment. They may also merge back into a single lineage, for example if the song of the new species becomes similar enough to the parent species that sexual selection is no longer a barrier between their populations.