Hedge-crickets sing; and now with treble soft
The red-breast whistles from a garden-croft;
In a traditional, 10-line ode, these would be the culminating lines, and they are (or at least can be read as) cheerful. Ish. Singing and whistling will always connote happiness.
Also, the ‘red-breast’ robin is a very welcome harbinger of winter:
This is all contradicted by what comes next though…
To help improve the meaning of these lyrics, visit "To Autumn" by John Keats and leave a comment on the lyrics box