Some believe there's somewhere in the brain
that senses minor fluctuations 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 thwarted 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.