I'm really curious about the evolutionary path these birds may have taken to an adaptation that allows them to fly 8-10 days non-stop. A couple ideas, assuming that any change evolution can keep up with needs to be gradual/incremental:

Could they have been making this journey since the land masses in question were significantly closer together?

Could they have originally migrated between closer locations, but as weather warmed post-ice-age they slowly increased the length of migration between "temperate northern hemisphere" and "temperate southern hemisphere" climates?

Could sea level rises post-ice-age have covered over islands that were formerly stopping points, or pushed back shorelines that effectively increased the distance required?

Curious to hear what other ideas folks might have.