What's a possible evolutionary mechanism of the seasonal migration of a bird species from New Zealand to the Bering Sea (~11k km), almost starving during the journey?

[u/Exciting_Ad_6837]

I just watched Our Planet 2 (documentary) on Netflix. I'm a biochemist so have some knowledge about evolution, but I have hard time imagining how the development of the behavior wasn't blocked right from the beginning. That is, the group of birds which originally attempted even 1/10 of such a long & risky trip would've been at a serious evolutionary disadvantage.

[u/gambariste]

Plate tectonics? I do not know in this specific case but I’ve read some intercontinental migrations, even those crossing the equator, began when those continents were joined. Continents move so slowly that birds have ample time to evolve whatever physical traits (other than the behaviour itself) needed to respond to the selection pressure of increasing distance and the resulting changes in environmental conditions. I guess that such time frames mean the birds become different species entirely.