So these "new" birds are distinct from all the other birds on the island that they form a separate breeding group. But how distinct are they from the outsider that started it all? Would they breed with birds from that birds home island? If so, I'd say they're not really a new species but a displaced species that survived one generation of interbreeding with the locals.

While certainly interesting, I wouldn't say this is a great example of speciation. One might even argue that it's an invasive species.