Good question. Here is one part of a good answer.
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or Paperback
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Links to books and other items on this page and elsewhere on Greg Ladens' blog may send you to Amazon, where I am a registered affiliate. As an Amazon Associate I earn from qualifying purchases, which helps to fund this site.
I would also add that, when the magnetic field reverses, it’s not as if it suddenly switches overnight. It diminishes, it moves about, it does all sorts of variation, and then, after thousands of years, finally stabilizes in the opposite polarity. So birds have plenty of time to slowly change (evolve!) and keep up with it.
Ahcuah — they probably don’t evenneed to evolve. For those that have a magnetic sense, it doesn’t say “go this way”. It says “there’s stuff that is along these lines” and the birds can use that as a landmark in figuring out where they are and how to get to where they want to be. So the field flipping isn’t that big of a deal. If they can navigate in spite of rivers completely changing courses and humans going and reworking the whole landscape, I’m sure they can recognize and adjust to variations in the magnetic field.
But yooz guys need to go read the post because you are WRONG. (ish)