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Who among us has not marveled at the mystery of bird migration? I know the sights and sounds of vast skeins of geese plying northward, or the kettles of cranes or raptors floating on thermals across the prairie skies, or even the sudden appearance of gaudily clad Baltimore orioles or bobolinks in the spring still quicken my pulse. How do they navigate across the vast expanses of the globe? What cues tell them when it's time to begin their treks? And, more fundamentally, why do birds migrate? These are simply a few of the myriad questions surrounding this natural phenomenon.