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Disclaimer: I know nearly nothing about biology.

I was wondering how did the migratory behaviour of birds evolve. Assuming that evolution/natural selection works slowly and gradually (at least most of the time) I don't understand how some birds decide to fly thousands of kilometers once a year and then fly back.

How does a bird species start it's migrational behaviour? The first generation flies close by, then the next a bit further and so on up until the migration distance is thousands of kilometers? Or do they randomly fly in small groups until they find nice land (abundant sunlight and food) and only the small group which find the nicest land reproduces succesfully?

If evolution works gradually and migration distance increases also gradually, how did, for example, the artic tern evolve to migrate from pole to pole? In the gradual increase of distance, some generation of artic tern must have migrated to a tropical latitude and be unable to survive there. Obviously this must be very wrong.

Can someone explain the evolutive mechanism behind migrations?

Thanks!

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    $\begingroup$ This may help. thespruce.com/…. $\endgroup$
    – John
    Commented Oct 18, 2020 at 13:54
  • $\begingroup$ Here is an article discussing bird migration that you might find interesting, although it doesn't really discuss the original evolution (it discusses convergent appearance of migration in many different groups but in ways that suggest it's a latent trait reappearing by mechanisms maybe different from the very first migratory behaviors) academic.oup.com/bioscience/article/57/2/165/228565 $\endgroup$
    – Oosaka
    Commented Oct 19, 2020 at 14:19
  • $\begingroup$ John and @Oosaka maybe post as answers (if this question doesn't just get closed). $\endgroup$
    – Maximilian Press
    Commented Oct 26, 2020 at 19:39

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