The poem begins quietly: the speaker wakes because of a thought, and a "little" one at that. But the effect is out of proportion. The heart is "all astir," as if that tiny thought had set off an alarm. Anyone who has woken in the night with a worry that suddenly seems enormous will recognize the feeling.
Then the poem turns sharply. The speaker compares the stirred-up heart to a "horrible goddamned canary" that "speaks with my mouth." The jolt of rough language breaks the hushed opening, and it shows frustration. A canary is a small, bright, chattering creature, and here it seems to take over the speaker's voice, as though anxiety or restlessness is talking instead of the person. The image is funny and unsettling at once.
The mood is jittery and self-mocking. The speaker seems irritated with their own mind, unable to stay calm or silent once a thought has started singing.