When I think of the Romantic period—that is generally thought to have begun with Coleridge and Wordsworth's Lyrical Ballads in 1798, and thriving most especially from 1800-1836 (with names such as Byron, Shelley and Keats)—nature...or a return to nature...is the first topic that comes to mind. In total, there are about seven characteristics common to Romantic writing; in addition to nature are the supernatural (or the occult), melancholy, imagination, the idealization of women and children, interest in the past (especially the Middle Ages) and individualism.
The popularity of the Romantic motif of nature was in great part a response to the Industrial Revolution (that first arrived in England, later to move to America); while this particular revolution brought about great advancements in science and manufacturing, industry also polluted the environment and took advantage of women and children working long days, sometimes in dangerous conditions. The Romantics dreamed of better days before the Industrial Revolution—for the Romantics found pure pleasure in observing and spending time out in nature. A beautiful example is found in Wordsworth's "Lines Composed a Few Miles above Tintern Abbey, On Revisiting the Banks of the Wye during a Tour. July 13, 1798."
Once again I see
These hedge-rows, hardly hedge-rows, little lines
Of sportive wood run wild: these pastoral farms,
Green to the very door...
Images of hedge-rows, woods running wild, and farms with greenery everywhere bring nature alive to the reader.
Your suggestion of centering on the figure of birds would be very interesting. Both poems mentioned would need to be discussed in terms not only of the Romantics' praise and idealization of nature, but would also have to tie in to what the bird symbolizes in the poem. For example, The Rime of the Ancient Mariner is an allegory—cautionary tale—warning about the effects of abusing nature: and that abuse is the pointless killing of the albatross.
The poem "Ode to a Nightingale" by Keats...
...is a lyric meditation narrated by a poet who is tempted to forsake the real world of human suffering for the ideal world of art.
Birds are symbolic of freedom; they rear their young in the homes they build; they socially interact with other birds; and, they communicate with "talk" and song. However, in both poems we find the narrator is suffering in some way: the birds represent something very different to both speakers. In addressing this along with the characteristic of nature in poetry by the Romantic poets, I think you will be able to provide adequate depth and interest for your reader!
Adventures in English Literature, Harcourt Brace Jovanovich Publishers: Orlando, 1985.
Ferber, Michael. A Dictionary of Literary Symbols. New York: Cambridge University Press, 1999.