Bird migration: getting to the point

It was fun listening to Melvyn Bragg trying to get to grips with the ineffable mysteries of bird migration. The facts don't seem to have changed much since I taught the subject, decades ago,¹ although there are now many more of them. And almost all of those facts are clearly enough to make even Lord Bragg's mind boggle, as we heard throughout the show. But I kept waiting for one of the guests to make one key point.

All the remarkable narratives of bird migration, every one of them, is told by an animal that has survived to tell the tale.

What we observe is the end result of who knows how many rounds of natural selection. It may seem amazing, and unfathomable, and all that, but the only reason we even know anything about animal migration is because it does actually work. And yet, even in the bonus material, none of the guests made this point.

They came close, alluding to the death of maybe 90% of all juvenile birds making a particular trip. That's quite some selective force, generation after generation. No wonder the survivors are so amazing.

As for mechanisms -- how do they do it, rather than what do they do -- from what I heard, we're still no closer to a clear understanding.