Where are the songs of Spring? Ay, where are they? Think not of them, thou hast thy music too,--
The final stanza begins with another rhetorical question, which we'll paraphrase as, "Where are your songs at, Spring? Huh? Bring it, if you got it. I can't hear you... Yeah, that's what I thought."
That's the super-aggressive version, at least. But the speaker is definitely needling the season opposite to autumn on the calendar. Spring might be great and all, but it doesn't stick around, so who needs it.
He reassures autumn, who might be feeling a tad inadequate compared to her more celebrated counterpart, that she has her own music.
Keats alludes again to the pastoral tradition in poetry, in which shepherds typically "sing" in springtime, often while playing a lyre.
While barred clouds bloom the soft-dying day, And touch the stubble-plains with rosy hue;
The speaker begins to describe the "song" of autumn. It's more of a metaphorical song, in that the scene begins with light and images.
He describes the patchy clouds, between which patches of sky can be seen, as "barred." These clouds appear to be in "bloom," like flowers, as they light up with the colors of sunset. The use of "bloom" is a direct challenge, again, to springtime.
The day is "dying" at sunset, but it's not a tragic or violent death. It's "soft" and gentle.
The reddish colors of the sunlight "touch" the fields gently. The fields have been harvested, so all that is left is a flat "stubble" of crop.
Then in a wailful choir the small gnats mourn Among the river sallows, borne aloft Or sinking as the light wind lives or dies;
The gnats by the riverside "mourn" the dying day like a choir at a funeral. They are "wailing" as if the daylight had been a favorite grandparent or something.
In fact, they are just doing what gnats do: coming out at evening time. The choir sound is the collective buzzing of their tiny little wings. Some people would have a different word than "choir" to describe this sound: namely, "extremely annoying."
Gnats especially like to hang out in wet areas, near trees, and here we find them near the willow or "sallow" trees down by the river.
Their movement appears to be coordinated with the light. Light gets brighter, gnats go up; light gets dimmer, gnats go down. Keats is having all kinds of fun with movement and directions in this poem.
The speaker continues to paint the sunset as a life-or-death struggle for the light.
The sound of the gnats contributes to the song of autumn.
And full-grown lambs loud bleat from hilly bourn; Hedge-crickets sing; and now with treble soft The redbreast whistles from a garden-croft, And gathering swallows twitter in the skies.
The poem concludes with more animal sounds, but those of a more conventional variety than the buzzing of gnats.
Lambs are bleating near the small stream, or "bourn," that flows down a hill. Notice that the speaker calls them "full-grown lambs," which is like saying, "full-grown child." Wouldn't that just be a sheep? He seems to want to highlight the in-between stage between the glorious ripeness of youth and plain old adulthood.
Crickets are "singing" by rubbing their wings together, otherwise known as "chirping."
With a soft but high ("treble") voice, the redbreast robin is whistling in an enclosed garden, or "garden-croft."
Last but not least, the swallows have taken to the sky at twilight, and they "twitter" joyfully as the sun goes down.
Now, really, what kind of ending is that? We just have a bunch of images of different birds and beasts! If this were a movie, you would probably leave the theater scratching your head. Fortunately, it's a poem, so we can keep asking questions, which is why you should check out the other sections of Shmoop's analysis.