Caged Bird

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.