Darkness (Motif)
Darkness in "Mad Girl's Love Song" is an active, dynamic force rather than a mere absence of light, suggesting how loneliness, sadness, and isolation are loaded with intensity and sensation for the speaker. Therefore, even when she closes her eyes and the world "drops dead," she can't seem to find peace. Early in the poem, when darkness replaces starlight, it "gallops in." The verb "gallop," with its connotations of athleticism and even militarism, shows that the speaker's feelings of lonely emptiness are far from passive and relaxing. Later on, when "hell's fires fade," the use of the word "fade" doesn't lessen the intense imagery of the phrase "hell's fires." The darkness left behind only evokes the hellish, frightening light that was there in the first place. Essentially, darkness and light are equally upsetting for the speaker (as are presence and absence, noise and silence, madness and sanity)—each one alternates with and evokes the other, leaving her doubly overwhelmed.
Thunderbird (Symbol)
The thunderbird symbolizes reliability and consistency here. Comparing her lover to a thunderbird, the speaker argues that a thunderbird, at the very least, would "roar back again" in spring. Thunderbirds are a mythical figure in some Native American traditions. According to legend, the enormous animal is able to produce thunder by flapping its wings and shooting lightning from its eyes. It's a bit strange, on the surface, that Plath would so explicitly link a noisy, huge, frightening creature to the value of consistency, which is after all perhaps the thunderbird's least interesting attribute. But the speaker is already surrounded by sensation and unable to understand which of the things she sees around her are even real. Therefore, the thunderbird's at-least-somewhat predictable chaos is preferable to what she faces.
