That they are only glimpsed in silhouette,
And seem something else at first—a swallow—
And move like new tunes, difficult to follow, 
Staggering towards an obstacle they yet
Avoid in a last-minute pirouette,
Somehow telling solid things from hollow, 
Sounding out how high a space, or shallow, 
Revising into deepening violet. 

That they sing—not the way a songbird sings 
(Whose song is rote, to ornament, finesse)— 
But travel by a sort of song that rings
True not in utterance, but harkenings, 
Who find their way by calling into darkness
To hear their voice bounce off the shape of things.