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.