Why does the caged bird sing???

Maya Angelou - I know why the caged bird sings
A free bird leaps on the back of the wind
and floats downstream till the current ends
and dips his wing in the orange suns rays and dares to claim the sky.

But a bird that stalks down his narrow cage
can seldom see through his bars of rage
his wings are clipped and his feet are tied so he opens his throat to sing.

The caged bird sings with a fearful trill
of things unknown but longed for still
and his tune is heard on the distant hill
for the caged bird sings of freedom.

The free bird thinks of another breeze
and the trade winds soft through the sighing trees
and the fat worms waiting on a dawn-bright lawn and he names the sky his own.

But a caged bird stands on the grave of dreams
his shadow shouts on a nightmare scream
his wings are clipped and his feet are tied so he opens his throat to sing.

The caged bird sings with a fearful trill
of things unknown but longed for still
and his tune is heard on the distant hill
for the caged bird sings of freedom


The other day David caught a bird, and kept it in his room for a few days.  He fed it, and cleaned up after it, and then he made a cage out of paper for it, and hung it in the middle of his room.  He released it back outside in a couple days, but he did enjoy the company of the bird.    As he was telling me this story of his bird this poem by maya angelou came to my mind.  I remember learning about it back in high school.
Now, years later as I read it, it takes on a different meaning.
Why does a caged bird sing?       Is it because it has HOPE and Faith of things unseen?