What does the sea symbolize in the story "The Birds?"

In the beginning of "The Birds," the sea symbolizes routine: both the routine of the birds, as they progress through the seasons, and the routine of Nat's life, as he spends his lunch breaks on the cliff's edge.

As the story progresses, however, the sea's symbolic significance changes. It suddenly transforms into a "fierce" and "vicious" power and represents the darker side of nature. As Nat observes the attacking birds, he concludes that the sea holds some sort of power over them:

There was some law the birds obeyed, and it was all to do with the east wind and the tide. 

In addition, it is also important to note that Du Maurier repeatedly employs the image of gulls riding the sea. This not only functions as a practical reminder of an impending attack but also suggests a conflict between humans and nature. By using the sea in this manner, then, Du Maurier reinforces the idea that nature will always prevail over human technology and culture.