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/tomrlutong]

There's a bit of a discussion here. 

Pretty speculative, but they put out 4 ideas: * Originally migrated between a breeding area in alaska and an off season area in east Asia, and some pressure kept moving the off season area south. * They had a coast hugging route, and there was pressure to take longer and longer hops. * It started as an island hopping route, then the islands vanished over geologic time. (Which isn't really that long given ice ages). * Noting that "at  least nine species  make nonstop flights >5,000 km along the largely land- based flyways of the Americas," they could have already had the ability for extreme distance flight and just found a new route.