What does the bird-seller know of the heart of a bird?

There was a bird in a cage of gold, a small red bird in a cage of gold;
The sun shone through the bars of the cage, out of the wide heaven;
The depths of the sky were soft and blue, greatly to be longed for.
The bird sang for desire of the sky, and her feathers shone redder for sorrow;
And many passed in the street below, and they said one to another:
"Ah, that we had hearts as light as a bird's!"
 
But what does the passer-by know of the heart of a bird?