In Shelley's "To a Skylark," the poet contrasts man and the skylark.
It should be noted that Shelley was considered one of the three major poets leading the Romantic Movement of literature, along with Keats and Lord Byron. Two earlier poets who brought these sensibilities into the 19th Century were Coleridge and Wordsworth.
There had been a growing interest in different types of poetry to do with three topics: nature, heightened feelings, and primitive cultures.
A great deal of their work reflected the movement's love and respect for nature, especially as the Industrial Revolution in Europe began to change the pastoral landscape.
Shelley's admiration of nature is evident in this poem. Some examples of the contrasts he draws between man and bird, follow.
Consider the first stanza:
That from Heaven, or near it,
Pourest thy full heart
In profuse strains of unpremeditated art. (3-5)
Shelley notes that the creature must never have been a bird, but a "Spirit" that close to heaven can sing forth unscripted art so beautifully in the form of music. He writes that while man is earth-bound, the skylark can move closer to heaven to sing its praises.
Teach us, Sprite or Bird,
What sweet thoughts are thine:
I have never heard
Praise of love or wine
That panted forth a flood of rapture so divine. (61-65)
He looks to the skylark (whether it be "Bird" or "Spirit") to teach him the bird's thoughts. He infers that man is not capable of such sounds of praise for he says that he has never heard anything before (either in praise of wine or love) that is equal to what the bird has shared in song.
In lines 76-80, Shelley describes emotional responses that humans have, saying that the bird would never express the same negative reactions:
With thy clear keen joyance
Languor cannot be:
Shadow of annoyance
Never came near thee:
Thou lovest: but ne'er knew love's sad satiety.
"Languor" is listlessness or stagnation. This would be seen as a shortcoming in man, as would "annoyance." While he writes that the skylark must know of love, the bird's joy indicates that it has never experienced "satiety," which is defined as having been supplied with so much excess as to experience disgust or weariness. Obviously, Shelley sees the bird as a creature that could never tire of joy, while humans, even with the best of things, often grow tired of what they have. In this case, he speaks of the sadness one might feel having grown disinterested in love once it has been acquired. This gives the reader the idea that Shelley sees people as creatures that become bored with love.
While Shelley contemplates the differences between the bird and man, he more often seems disappointed with the shortcomings he attributes to people...but finds nothing that is not praiseworthy in the skylark.