When Valentine gets kicked out of Milan for trying to elope with the Duke's daughter, he bums around a forest, where he finds a sense of peace bemoaning his sadness in harmony with the mournful sounds of the nightingale:
This shadowy desert, unfrequented woods,
I better brook than flourishing peopled towns:
Here can I sit alone, unseen of any,
And to the nightingale's complaining notes
Tune my distresses and record my woes. (5.4.1)
What's all this nightingale stuff about, you ask? Well, on the one hand, Valentine's reference to the bird's sad music reminds of an earlier moment when he said that, without his beloved Silvia, "There is no music in the nightingale" (3.1.15). OK, we get it. Valentine is sad and lonely without his girlfriend. So much so that not even the otherwise cheerful sound of birdsong can make him happy.
There's also another meaning in Valentine's "nightingale" reference. As literary scholar Jean Howard reminds us, this passage also recalls the mythic story of Philomela, who was raped by Tereus and eventually transformed into a nightingale whose sad tune mourned the loss of Philomela's virginity. (You can read all about Philomela and Tereus in our summary of Ovid's Metamorphoses, Book 6.)
OK, we know what you're thinking. What does the mythic story of Philomela's rape and transformation into a bird have to do with Valentine hanging out in the forest missing his girlfriend? Well, it can't be a coincidence that moments after Valentine mentions the nightingale, his pal Proteus tries to rape Silvia in the very same forest, can it? We know of course that Proteus doesn't succeed (Valentine stops him) but the potential for sexual violence is still there.
In fact, the threat of rape echoes all over the play. It's the reason why Julia disguises herself as "Sebastian" – so she can avoid "loose encounters of lascivious men" (2.7.4). When Valentine makes the outlaws swear to "do no outrages/ On silly women" (4.1.12), it becomes evident that rape is a real possibility for women travelling through the forest. So, it seems like the mournful "nightingale" can function as a symbol of the potential for sexual assault in the play.