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All three of Keats' poems, "To Autumn," "Ode to a Grecian Urn" and "Ode to a Nightingale" include references to nature—praise of nature. In "To Autumn," Keats writes:
Where are the songs of Spring? Ay, where are they?
Think not of them, thou hast thy music too,-
While barred clouds bloom the soft-dying day,
And touch the stubble-plains with rosy hue...
In "Ode to a Grecian Urn," Keats refers to nature again:
Ah, happy, happy boughs! that cannot shed
Your leaves, nor ever bid the Spring adieu...
In "Ode to a Nightingale," Keats writes again of nature...
Of beechen green, and shadows numberless,
Singest of summer in full-throated ease.
The author refers to aspects of nature: leaves, green beechen (which is a specific genus of trees), and other growing plants ("the stubble-plains with rosy hue"). Keats was a Romantic poet, and the Romantics of the late eighteenth and early nineteenth centuries, wrote of many things, but one of the most common elements of their poetry was the presence and descriptions in praise of nature.
So in these poems there is not only the description of nature and tribute to its beauty, but also the idea that nature revolves around, and intermingles with, humanity. It is as much a part of life as the human body is.
In "Ode to a Grecian Urn," Keats notes the scenes on the urn...
The first scene depicts musicians and lovers in a setting of rustic beauty.
In "Ode to a Nightingale," Keats...
...can empathize with the bird’s zest for living and procreating at the height of the spring season...
Keats connects nature in the form of a bird to human existence.
In "To Autumn," Keats compares the same things occurring to nature that occur in a human's life:
...its fruitfulness, its labour and its ultimate decline.
So the initial theme all have in common is the presence of nature. The second theme is that nature and human life are intertwined: they cannot be extricated one from the other. This would also support the perceptions of the world through the "lens" of the Romantic writers.