According to the PhysOrg article Study catches two bird populations as they split into seperate species,
The question of whether these two populations are on the road to speciation comes down to sex. When two populations stop exchanging genes--that is, stop mating with each other—then they can be considered distinct species.While they can't actually observer every encounter between the two populations to check for mating, male birds are notorious for defending their territory from others males that are potential sexual competitors. The Makira population was seen to not really mind if the brown-bellies came around, which leads the researchers to think they are not seen as much of a threat.
Of course, this doesn't prove anything conclusively...only time can really tell. Over the next hundred years or so, the natural barrier may not be big enough to keep the mutated gene from spreading to all flycatchers, which means no speciation. Still, this is an example of evolution in action, whether or not speciation occurs. Can you guess why?
See Elie Dolgin's post at TheScientist.com for more, including comparison images.