Geographically isolated or allopatric species is the answer.
Explanation:
In allopatric speciation the individuals are not able to mate due to changes in the habitat and timing of mating.
when a population is separated by geographical barriers as different island in the question are allopatric species.
The speciation occurs in reproductively isolated birds and they evolve.
Due to the geographical speciation population of birds evolve into separate species and undergo genetic divergence due to natural selection.
