Caged Bird, by Matthew J. Spireng

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.