Stanza three begins with a question: "Where are the songs of spring? Ay, Where are they?" which the speaker immediately dismisses. He realizes that it's no use to think of spring when autumn has its own music, even if its tune differs entirely. Night begins to fall, and the speaker describes the "barred clouds" in the sky and the "rosy hue" cast against the emptied plains. As the gnats "mourn" the day in a "wailful choir," lambs bleat in the hills, crickets sing, and swallows gather in the sky, ready to fly to warmer climes.
In the final stanza, the speaker ends with a meditation on time's passing, building upon the highly sensual imagery of stanza one and autumn's personifications in stanza two. Using the metaphor of music to distinguish between autumn and spring, the speaker reconciles the differences between the seasons by refusing to compare them. He sees spring and autumn as two necessary, unavoidable periods of time with their own virtues, shortcomings, and truths.
While stanzas one and two relied upon concrete imagery, Keats turns to another sense in stanza three: sound. Against the backdrop of a rosy sunset, the singing crickets, bleating lambs, and the "wailful choir" of gnats not only signal the end of the day, but the inevitable transition of autumn into winter. This transition thematically corresponds to death, an ongoing preoccupation throughout Keats' poetry. But, through the soft, delicate language Keats' uses in the final stanza, we the see that the death autumn promises is gentle: it's painless, and things don't seem to die as much as they dissolve or disappear, slowly shifting from one state of being to another. The day slowly fades from "rosy hues" into night's darkness, and at the end of the poem, the swallows gather, ready to fly together to some other, warmer place. In the final lines, the soft sounds seem to echo and resound, reminding the reader that all is not lost.
If we remember the poem's context in Keats' life, we know it's the last of the six famous odes he completed in 1819. A bittersweet combination of sorrow and joy permeated these months. Keats had found love in Fanny Brawne, but the couple was unable to marry because of the poet's financial straits. His brother Tom died of tuberculosis in December of 1818, after Keats nursed him through most his illness. In early 1820, Keats would cough blood for the time, confirming he'd fallen ill will the same sickness that killed his mother and brother. In this sense, 1819 was the Autumn of Keats' life—its long, sweet days quickly approached the end.