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> How Do They Figure Out Their Impossibly Long Routes Home?

TL:DR they don't know

For those interested in an actual answer, and to absolutely nobody's surprise, it is indeed earth's magnetic field.

http://ketheridge.sites.gettysburg.edu/BIO206/wp-content/upl...

Per the article:

> A long-held view was that birds were aided in their orientation by a cluster of iron neurons in their beaks, which interacted with the earth’s magnetic field. “But, in the past ten years, people have begun to ask if maybe it’s something else.” Gow posits that this something could well be olfaction. “If you have a colony of birds,” she says, “they can find their individual nest among thousands by smell.”

I'm reading the article and it strikes me that the birds might have a very accurate biological clock allowing east/west localization. I sometimes know just when the sun will rise or set, and a few minutes difference from that expected time, let alone a half hour or more, could tell me if I were east or west. Think jet lag, but millions of years of evolution have turned it into something more accurate for migratory animals.

North/south may be temperature or the angle of the sun versus where they expect it. Once again, millions of years may have given birds a more precise feeling for exactly how high the sun should be.

You'd have to watch the birds for better homing ability when the sun is visible vs when completely cloudy. It could be interesting to see how well they home if released at different times of the day, or different seasons.

I'm interested in the actual data that has been gathered.