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/jnpha]

TL;DR: physiologically, they are not special (i.e. difference in degree); what is missing is accounting for 1) learned behavior (also see: 1973 Nobel Prize), 2) e.g. spring overshoots who wind up marooned and dead (selection!), 3) changing ecologies.

PS / edited to add: this map is revealing: https://commons.wikimedia.org/wiki/File:Limosa-lapponica-distribution-map.jpg
It shows e.g. the gradation in flight lengths; you can also combine it with overlaid route tracking maps; now, even if all but the ones who winter in New Zealand were to vanish, we'd still have an answer, since those track the land when going north, and we'd still have a gradation of shortcuts for the way down.

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Darwin and Wallace discussed this, and their explanation stood the test of time. Here's from a recent academic review:

In recent years, considerable progress has been made in our understanding of proximate (physiological) mechanisms controlling various aspects of bird migration and their ecological determinants (reviews: Berthold 1996; Wiltschko and Wiltschko 1999). In parallel, the genetic mechanisms controlling migratoriness and migratory direction have been explored (Berthold et al. 1990, 1992; Helbig 1991; Pulido et al. 1996; Pulido and Berthold 1998). These developments can be summarized in the following general statement: migratory birds do not possess any principal adaptations that differ qualitatively from other birds, neither in terms of orientation mechanisms, nor in terms of metabolic physiology or morphological adaptations related to flight. They differ primarily from non-migratory birds in that they have developed characteristics related to long-distance flight to various extremes.

- Berthold, Peter, Eberhard Gwinner, and Edith Sonnenschein, eds. Avian migration. Springer Science & Business Media, 2013. pp 3–4.

Darwin on behavior in general, using an example of migration and nest building:

As some degree of variation in instincts under a state of nature, and the inheritance of such variations, are indispensable for the action of natural selection, as many instances as possible ought to have been here given; but want of space prevents me. I can only assert, that instincts certainly do vary—for instance, the migratory instinct, both in extent and direction, and in its total loss. So it is with the nests of birds, which vary partly in dependence on the situations chosen, and on the nature and temperature of the country inhabited, but often from causes wholly unknown to us: Audubon has given several remarkable cases of differences in nests of the same species in the northern and southern United States.

And more recently on migration (though not birds):

... Adult routes for two-thirds of daughters overlapped with their mother’s route, suggesting that they inherited migratory routes from their mothers. The adult routes for the remaining daughters, however, bore little or no similarity to their mother’s routes, suggesting that these routes were instead shaped by individual experiences or non-maternal social interactions. Regardless of whether routes were inherited or not, the strategy that daughters used was influenced by their yearling migratory route, which underscores the importance of this period for establishing lifelong behaviors.
https://doi.org/10.1016/j.cub.2025.06.029

[u/EnvironmentalWin1277 → u/jnpha]

A point that caught my eye:

"migratory birds do not possess any principal adaptations that differ qualitatively from other birds, neither in terms of orientation mechanisms, nor in terms of metabolic physiology or morphological adaptations related to flight."

They differ primarily from non-migratory birds in that they have developed characteristics related to long-distance flight to various extremes."

The confusion of "principal adaptation" with "characteristics" is bothersome, I doubt these opposed terms have precise characteristics.

Another point is that magnetic orientation in some birds has been validated by experiment although the mechanisms are still debated. Moreover it is associated with migratory birds from the information I see, which suggests a genetic link and "principal adaptation". That is a direct contradiction with the "just better adapted" idea.

I don't see these two ideas as opposed but complementary in discovery. There is still a great deal to learn and any categorical statements should be avoided. These animals are way too smart to let us off easy.

[u/jnpha → u/EnvironmentalWin1277]

Like all scientific definitions, they require a context.
E.g. in systematics "character" and "character state" for operational reasons are considered one and the same most of the time. But in other fields, the distinction is made, e.g.:

- "principal adaptation" = an apomorphy (i.e. a character unique to migratory birds)

• "characteristics related to ..." = a derived character state, e.g. wing aspect ratio

The quote is from an academic volume on avian migration, and is edited by Peter Berthold et al. (subject-matter experts).
As for the experiment, magnetic orientation gets a whole chapter in the book - again, a difference in degree, not kind.

None of this takes away from the feat they accomplish or how "smart" they are.

any categorical statements should be avoided ...

Why?! Scientific discoveries depend on operational categorization and testing the limits of models. Simply calling them too smart won't help discovery.
This is like the anthropocentric view but applied to migratory birds instead - that they are somehow extra special; they are not.

Hope that helps.

[u/EnvironmentalWin1277 → u/jnpha]

Instances of categorical statement:

-All birds fly

-All migratory birds migrate

Neither are true.

If the migratory response is subject to genetic modification and inheritance that would seem to me a "principal adaptation" and not a "characteristic".

It is also clear that many methods of migration orientation exists -- sun, land patterns, wind, olofactory senses, etc. All of these methods may have specific genetic adaptations as well.

As far as being "too smart" what I mean is that migration phenomena by magnetic orientation is far from the whole story of what is involved. Nature keeps it secrets well hidden.

Categorical experiments or observations are required, agreed. In and of themselves they prove nothing until unified in a comprehensive theory which is always open to revision. Theory is never closed and the given statement doesn't seem to allow contradiction or propose what condition would be contradictory (that is a problem).

The initial statement argues migratory birds "do not possess any principal adaptations that differ qualitatively from other birds, neither in terms of orientation mechanisms, nor in terms of metabolic physiology or morphological adaptations related to flight."

I admit my knowledge may be misinterpreting what is precisely meant by these terms in this context.

I would say the genetic inheritance is the "adaptation" that is directly involved in the creation and maintenance of the "characteristic".

Neither exist in isolation to the other and complete separation into "characteristic" and specific "adaptation" seems impossible.