Read the first stanza of "Auspex."
My heart, I cannot still it,
Nest that had song-birds in it;
And when the last shall go,
The dreary days to fill it,
Instead of lark or linnet,
Shall whirl dead leaves and snow.
The speaker compares his heart to an empty nest in order to:
Emphasize the fact that his love has left.
Describe a physical feeling he is experiencing.