Extended Metaphor

Fingers glide down the smooth, cool wire
As the cage creaks lightly
Its occupant is startled awake
With a soft chirp.
Slowly, black eyes blink in a gray face
Swiveling on a gray neck.
Gray feathers ruffle, gray wings outstretch
Before falling back into stillness.
Fingers poke through the wires
Running over the dull plumage.
The occupant gives no reaction,
Stares blankly into its empty water dish.
Another groan of metal startles it.
Charcoal gaze fixes on the door, now open.
A hand does not enter.
Nothing given, nothing taken.
Just opportunity.
An impulse jerks the creature to its feet.
Wings flutter as if knowing what they'd have to do.
Thin gray legs hop through the opening,
Landing perched on the windowsill.
The air is sweet.
It tastes of bread from the bakery down the street,
Of flowers, crisp and cool.
Adventure.
With one last glance at the only home
The creature had ever known,
It leaped, wings stretched outward
Falling before instinct took over
And wings remembered to flap.
As the creature flew off into the sky,
The sun gleamed off its coat.
A rainbow.

Can you tell what this is a metaphor for?