A bird sings when it can breath.

That is why they were used in coal mines. When they stop breathing they stop singing.

I think the actual question was why does a caged bird sing.

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. Maya Angelou

But from this I say go outside. They do not need to be caged to sing. Though they might sing more in a cage than we can her them without.