Though it still could sing.
The last of the light of the sun
That had died in the west
Still lived for one song more
In a thrush's breast.
Far in the pillared dark
Thrush music went --
Almost like a call to come in
To the dark and lament.
But no, I was out for stars;
I would not come in.
I meant not even if asked;
And I hadn't been.
In the last line, the speaker repudiates the idea that the world is just a shadow of some perfect world that we’ve lost. This idea has a long tradition in the West, at least going back to Plato. See:  The Allegory of the Cave . 
 The birdsong was just birdsong. It wasn’t trying to beckon the speaker to a deeper meaning. 
 Instead of lamenting the beauty that we’ve lost (the sun), we should appreciate the beauty we have, no matter how fragmented and dispersed it happens to be (the stars). 
 See  Mediations at Lagunitas , a poem in which Robert Hass explores the same theme.
Come In