It's an allopatric population. It's one of the great embarrassments of biology. The question "is this population part of the same 'species' as this other population" is simply not defined in biology when they are allopatric. Look at your country's bird identification guide. If you're in Europe or N. America you'll find lots of "species" that are just allopatric populations accompanied by some mtDNA / sound recording…
This article is a good guide to the topic: https://plato.stanford.edu/entries/species/