Another simile! This one compares the skylark to a hidden natural beauty.
In this case, it's a rose. The flower's loveliness is cradled and covered up by its leaves. We can't see it, but its beauty still finds a way to reach us.
Shelley sets up all kinds of little sound-echoes in this poem. Don't miss the way that the word "embower'd" here connects to the "bower" of the princess just a few lines before (45). For more on form and meter, check out our…"Form and Meter" section.
By warm winds deflower'd, Till the scent it gives
The winds steal away the flower's smell. That "scent" floats away from the secret beauty of the rose, and reaches people who can't see the flower itself.
Again, this is a lot like the skylark, whose beauty reaches the other senses, even when the eyes can't see him.
(There's also a slightly creepy pun here, as if the flower was being sexually violated—"deflower'd"—by the wind. Um, yuck.)
Makes faint with too much sweet those heavy-winged thieves:
We kind of love this image, which also personifies the winds, calling them "thieves." It's as if the rich smell of the flowers was filling the breeze, weighing it down. "[H]eavy-winged thieves" is just kind of fun to say, too.
What about those thieves, though? In this case, even the winds suffer a total sensory overload. Once again, things are left a little dazed by the intensity of their interaction with this bird-maiden-glowworm-flower thingy.
This idea that sound and smell and sight and weight are all interconnected, and potentially overwhelming, is one of the key points in this poem. So it shouldn't surprise us that smells can be sweet and heavy at the same time.