Why dont the birds kill the livestock in The Birds?
Based upon critical analysis of the story, there is much that is not understood with regard to the birds' violent behavior. It is suggested that it may be tied to the East wind, which could be symbolically referring to fear people in America and Europe experienced in response to Communism and the Cold War.
It is especially frightening to the people living in the Cornish coastal village in England, that creatures so often associated with beauty and peace should become brutal and murderous. Nat Hocken and his family are the first to notice the unexplained change in the birds. They soon discover that attacks are occurring all over England, even in London.
Theories to explain the birds' uncharacteristic, violent behaviors are seen as rationalizations in face of the fear of the unknown, not observations based on facts: there are no facts.
Similarly, when the townspeople decide to shoot the birds, they are acting upon a fear, reacting to something they don't understand, similar to the way people will kill any kind of bug from fear, even it the bug is harmless, like a lightning bug.
No explanation is offered as to why the birds attack the people of this town or why they don't attack animals. At the end of the story, the birds have yet again attacked Nat Hocken's house. It seems that everyone else in the small town may be dead, but there is no indication whether or not Nat and his family will survive.
The main theme is that not everything in the story can be explained.