The theme of Percy Bysshe Shelley's lyrical ode "To a Skylark" is the power of nature to inspire and delight the human spirit.

While the poet listens to the small night bird sing with delight as it soars to the heavens, he, too, is enraptured with its "rain of melody" that showers down spontaneously:

What thou art we know not;
  What is most like thee? 
From rainbow clouds there flow not
  Drops as bright to see
As from thy presence showers a rain of melody.

In subsequent verses, the poet seeks to compare this delightful bird's song that generates images of maidens, sweet-smelling roses, the gentle sounds of rain, and much more, to the wonders of the heart. And, yet, the poet wonders how this blithe spirit can know such happiness without having experienced sorrow: "Thou lovest—but ne'er knew love's sad satiety." For people's happiness is relative to the sorrow that they have experienced.

Finally, the poet calls upon the skylark to teach him such pure joy as it has because if he were to possess this pure happiness, the world might listen, just as he is listening now to the lovely songs of this bird.

The theme of Shelley's poem "To a Skylark" is the power  of nature to transform men's lives, specifically through the medium of poetry.

The skylark is a tiny bird, so small that when it flies high in the heavens it cannot even be seen by the author, yet its song can still be heard, a song "unbodied joy" (line 15) and "shrill delight" (line 20).  The author hears the skylark and goes on to describe its beauteous song, but it is "a flood of rapture so divine" (line 85) that he cannot fully capture its essence.  The joy expressed by the skylark is beyond that which can be grasped by man, and the author speaks directly to the skylark in the latter stanzas, asking it to reveal to him the secret of its ethereal bliss so that he might then be able to share it with others through his words, and thus transform their lives. 

What is the theme of "To a Skylark" by Percy Shelley?

I believe that the theme of the poem "To a Skylark" by Percy Bysshe Shelly is that humans cannot possibly feel the carefree joy the skylark feels each and every day as we are incapable of staying in the moment.

The poem begins by calling out to the skylark, calling it a "blithe" spirit.  To be carefree, one can have no worries, no concerns.  The speaker in the poem praises the song the skylark sings calling it "unpremeditated art".  Spontaneous song clearly emphasizes the bird's ecstatic joy.

The speaker follows the skylark's journey:  it "springs" from the earth, then flies into the "deep blue" or sky, and as it does this, the bird continues to sing.  This movement again emphasizes the unbridled happiness the skylark embodies.  The speaker continues to track the bird: "In the golden light'ning/Of the sunken sun".  This show how quickly the bird is flying and the next line ""Thou dost float and run" again shows how filled with joy this bird appears.  The bird then continues to fly, and the into the sunrise or sunset (I'm not sure which, but I think it is the sunrise) as the "pale purple even/Melts around" the bird, and while the bird can no longer be seen, the song can still be heard.  This shows once again how the happiness just bursts forth and can't be contained in this little skylark.

The speaker later points out that all the joy the bird experiences is unattainable for human beings - "What thou art we know not" - and then compares the skylark to things who demonstrate some measure of happiness, but still not equal to the joy the bird possesses:  the raindrops from clouds that later produce rainbows.  The bird is like a poet no one pays attention to until people appreciate it after disregarding the real hopes and dreams of the world. The bird is like a princess in a palace who is thinking of her love in her private quarters and like a glowworm secretly spreading its light and joy, and so forth.  These comparison all "shine" with happiness whether anything or anybody else takes notice.  They are "in the moment"

The speaker then ponders what causes this bird to be as happy as it is stating " What objects are the fountains/Of thy happy strain?"  The speaker goes on to say what the bird has never experienced:  laziness, annoyed, heartbreak, even death must not hold the fearsthat it seems to for humans.  The speaker says "We [humans]look before and after" which probably means we are never just in the moment.  We always want what we don't have - "pine for what is not".  Even when we laugh, it is often filled with pain.  Our best songs talk about sad things

The speaker then goes on to say that even if we could eliminate scorn, pride, and fear - all the things that often cause humans sadness, we still would never be able to be as happy as the skylark appears to be, and if this bird could teach humans to be half as happy, it is certain the world would listen to him just as he is listening to the skylark.

What is Percy Bysshe Shelley trying to convey the reader about the skylark (in the poem "To a Skylark")?

Percy Bysshe Shelley was a Romantic poet. The Romantics typically wrote about very similar things. The characteristics of the works typical of the Romantic poet are: love of nature, love of the uncivilized or natural, beauty in nature, the importance of imagination over reason (as a contrast to the Neoclassicists who preceded the Romantics), and individualism.

In Shelley's poem, "To a Skylark," Shelley wants the reader to understand the importance of the skylark. The skylark is a bird which brings beautiful music to the world with its song. Shelley understands the importance of the beauty of nature (animal life included) and the importance of mankind's realization that nature holds a special place in the world.

The comparisons made between the world of man and the world of nature show Shelley's ideology regarding the impact which nature should have on man and the respect that mankind should have, in return, for nature. In the end, Shelley's asking of the skylark to share with him the secrets of nature show that Shelley does, in fact, recognize the importance nature holds in the lives of mankind.