Some believe there's somewhere in the brain

    that senses minor¹ fluctuations² in the Earth's

    magnetic field and uses a sort of memory

    of that to travel the same route year after year

    over thousands of miles， over open ocean

    on moonless， clouded nights， and a built-in clock

    that， save for weather's influence， tells

    when it's time to go. But they utter nothing

    of thwarted³ dreams in birds' brains， how

    a few cubic feet near the ground， however

    well-kept and lighted， however large it seems

    around a small bright bird， is like a fist

    closed tight on feather and bone， how， certain times

    of year， the bird's heart races as if to power flight.