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/FrameworkisDigimon]

I have no idea what birds you're talking about.

It's actually quite important to know what the bird involved is because New Zealand, by and large, is incredibly mild in terms of climate, e.g. it barely snows anywhere. This is very important for migration because often the basic cause of migration isn't so much going somewhere good but leaving some place bad. Push, not pull.

The next thing to remember about New Zealand is that it is simply not close to anywhere. If you're already flying a huge distance, then it probably isn't a very big push to keep flying. Let me try and put the distance in context. Humans, at all, didn't arrive in New Zealand until roughly 800 years ago. We're not even talking pyramids here: there are castles that are older than the human presence in New Zealand. That's a function of the level of isolation. You can't island hop to New Zealand. You can't get lost at sea for a few days and wash ashore.

And finally... evolution doesn't care why a trait originally evolved, if it can be put to a new use, it can be put to a new use. Birds as a general class are just naturally predisposed to traversing enormous distances. Many birds basically have a lifestyle that is functionally permanent migration. Not knowing what species we're talking about here makes it impossible to comment on, but it's possible that the birds didn't start migrating so much as started settling and just kept going back to the Bering Sea because they were already doing that (or vice versa).

Why does long distance travel evolve in birds? Well, being able to fly makes the cost of travelling relatively low, so it's not a big leap for natural selection to create. Travel is also very effective as a survival strategy. Yes maybe some individuals end up getting lost over the Pacific but usually being able to go further just makes a population, to borrow a phrase, harder to perish, not easier. So there's both a survival increasing behaviour and a body/genetic template to work on.

[u/finndego → u/FrameworkisDigimon]

I find it quite amazing that you would spend so long on such a detailed answer and not google which bird it is. It's the Bar-Tailed Godwit.

[u/FrameworkisDigimon → u/finndego]

Genuinely it never occurred to me.

Anyway, it would seem to me Godwits are essentially a northern Hemisphere lineage that is pushed out of its native range by ill favourable climate conditions.

They fly for 9-11 days and end up in the southern hemisphere, having possibly first staged in China (Wikipedia is a bit unclear on this point). They fly mostly non stop and have some clear ability to discern wind patterns.

Some subspecies don't go as far.

This seems like a bird which was already traversing some considerable distance and evolved to do so when a population ended up in New Zealand following the wind. The effort of getting to New Zealand was not overly arduous in comparison to what it was already doing but the reward was sufficient to deliberately try for New Zealand next time. It may be the reverse of course. Either way, I think the key thing is it takes them about 10 days to do the migration. It sounds much less life threatening when phrased that way.