I think that we're well away from the point where we could call these WNW birds a new species. I've seen one of these winter blackcaps and they look, sound and (save for migration) act just like the regular birds.
Fair enough. Who determines how much difference is needed, though? I know the definition of species is up to interpretation, but who is the arbiter?
It certainly is an interesting example of reproductive isolation though.
It certainly is. And Aiki says that the two populations
can breed, but often don't because of the timing of their arrivals. Would a speciation event require them to be unable to interbreed, or is it enough to simply show that they tend not to?
In the Green Warbler example RAZD has brought up, there seems to be multiple species that can interbreed with the ones near them, until the ring closes and the two "ends" can't. Or is there only one species...or is there two, with some undefined line somewhere in the middle? The picture, with the multiple colors shading into each other seems to indicate multiple, interbreeding species.