Bird thou never wert, (2)
Essentially, the speaker is telling us that this skylark is actually a supernatural being and never was just a bird. This is kind of an unexpected way to start off a poem that's supposed to be about a bird. It tips us off, right from the beginning, that this isn't really a poem about nature, at least not in the literal sense. It's about emotion and spirit and purity and a whole list of other big fancy ideas. In a way, this little brown bird is just a jumping-off place.
In the golden lightning Of the sunken sun, (11-12)
This poem is just packed with intense images of nature. Phrases like "golden lightning" communicate the kind of extreme, almost overwhelming imagery that Shelley and his fellow Romantic poets were famous for. Lines like this make us feel like we're being washed over with a sea of color. Shelley turns the sort of simple, peaceful idea of sunset into a symphony of passion and joy and excitement.
Like a glow-worm golden (46)
We think there's something really cute about putting the glow-worm in this poem. Fireflies are great, but there's nothing huge or exciting or splashy about them. In fact, they seem kind of simple and quiet, although they are definitely magical in their own way. Our speaker's passion for nature runs from the really big stuff (the sun and moon and stars) all the way down to a little bug hidden away in a damp little valley.