the same one described in the Bible and in ancient stories, as if the bird itself was beyond earthly suffering and death, which is what Keats is so desperately trying write or dream his way out of. 

Shelley's choice of a skylark is quite useful because it is a bird that only sings while flying, and it usually does so when it is too high to see. So, when Shelley hears the skylark singing, the bird is out of sight. This creates the sensation that the bird really is immaterial, metaphysical, a spiritual voice. 

The pale purple even
Melts around thy flight,
Like a star of Heaven
In the broad day-light
Thou art unseen,--but yet I hear thy shrill delight, (16-20). 

I think Keats' ode is more full of angst. He was always bothered by thoughts of death and his brother, Tom, had just died of tuberculosis shortly before he wrote this poem. Shelley is plaintive but calm, although he also embraces the spiritual romanticism of the skylark, hoping that it can lead to some higher happiness. He blatantly asks the skylark, "Teach us, Sprite or Bird," (61). You can see this distinction between Keats' paradoxically hopeful angst and Shelley's calm request in the last stanza of each ode. 

From Shelley's "To a Skylark," he sounds polite and humble in asking:

Teach me half the gladness
That thy brain must know, 
Such harmonious madness 
From my lips would flow
The world should listen then-as I am listening now. (101-105)

In the last stanza of Keats' "Ode to a Nightingale," the music has faded. He is dejected, but like Shelley, he believes he might hear the music again and be uplifted if he can just find the right time and/or state of mind in which to have this experience. Keats ends with a much more philosophical question about the experience itself. 

Was it a vision, or a waking dream?
Fled is that music:-Do I wake or sleep? (79-80)

John Keats was a Romantic poet: and of all the characteristics for which the Romantics were known, nature is one of the most (if not the most) prominent themes of their poetry.

Along with Byron and Shelley, Keats (though he died young) had developed early a mastery of placing images of nature on paper. In "Ode to a Nightingale," he writes profusely in praise of nature, of all things born of nature, and his deepest desire to be carried away by nature:

O, for a draught of vintage! that hath been

Cool'd a long age in the deep-delved earth,

Tasting of Flora and the country green,

Dance, and Provençal song, and sunburnt mirth!

O for a beaker full of the warm South...

That I might drink, and leave the world unseen,

And with thee fade away into the forest dim...

One is swept away by the pure bliss Keats imagines in drinking of water long cooled beneath the surface of the earth—that tastes even of the plants growing nearby, so pure it would be. He wishes for the joy of nature, and that he might disappear from the world into the "forest dim."

The speaker talks to the nightingale, wishing he could fly away with it—not by use of intoxication (Bacchus), but through the verses of poetry (Poesy). This poem praises the bird, and the speaker wishes he could soar to the heights as the bird does, free from the worries and heartaches of the day—Keats "diagnosed his own tuberculosis." So it is easy to understand that...

As he listens to a bird’s song, the speaker becomes more and more enraptured by it, and increasingly disgruntled with the mortal world of pain and death.

The poem is made up of eight ten-line stanzas. The pattern of rhyme (found with the last word of each lines) is ababcdecde.

In "To Autumn," there are three stanzas, each with eleven lines. Keats uses personification to heighten the effectiveness of his imagery. The poem concentrates on the later part of the fall, when crops are nearing their harvest and winter is fast approaching.

The work has been interpreted as a meditation on death...

The beauty, warmth and fullness of nature is offered up in the first stanza:

Conspiring with [the sun] how to load and bless

With fruit the vines that round the thatch-eves run;

To bend with apples the moss'd cottage-trees,

And fill all fruit with ripeness to the core;

To swell the gourd, and plump the hazel shells

With a sweet kernel; to set budding more,

And still more, later flowers for the bees,

Until they think warm days will never cease,

For summer has o'er-brimm'd their clammy cells.

"Mists" and "fruitfulness" conspire (personification) with the sun to further ripen plants, such as those on the vines and apples bending the limbs that hold them, filling each with "ripeness to the core." The gourds shall grow rounder, the hazelnut shells "plumper," and the corn sweeter by the day. Flowers will continue to bloom to feed the bees personified to "believe" that these days will never end as they fill the honeycomb to overflowing.

Keats praises autumn. He mentions the spring, but notes that autumn is important now: it "hast thy music too," with the clouds over the "soft-dying day," the mournful "gnats," a light wind, crickets singing and even a robin that still lingers, whistling at the close of the day. 

Keats sees more than grass and trees and sky: he sees it as simple and pure, and praises it—bringing it vibrantly to life for the reader.