What do the birds symbolize in "The Birds" by Daphne du Maurier?

In "The Birds," the birds symbolize the uninhibited and unbridled power of nature. This is shown most clearly in their violent and repeated attacks on humans. In one incident, the birds attack Nat Hocken's house in the middle of the night, and Nat is forced to fight them off using a blanket:

He felt the thud of bodies, heard the fluttering of wings, but they were not yet defeated, for again and again they returned to the assault, jabbing his hands, his head, the little stabbing beaks sharp as pointed forks.

What is most terrifying about these attacks is how sudden and unexpected they are. Moreover, the human population is completely incapable of fending off the birds. The BBC news broadcasts offer little in the way of practical advice, and the government appears to have no clear strategy. To exacerbate the problem, guns do not provide an effective defense against the birds, as we see through the tragic deaths of Mr. and Mrs. Trigg.
By employing this symbol, Du Maurier sends a very bleak reminder that nature can dominate humans (and their technology) very easily.