-The birds, upset about the constant violence between men (having witnessed World War II) decide to punish mankind. This is later supported by Jim and Trigg’s decision to go shoot a bunch of birds.

-The “east wind” that Nat assumes is bringing the birds is, in fact, Communism coming over from the USSR. Communism, assumed to be the greatest threat to the Western world, would have been perceived as just as scary as all birds suddenly freaking out and attacking people.

-30 miles away, in a small bird-filled town, the woman who normally feeds all the birds suddenly died of a stroke, leaving no food for the birds. Crazed with hunger and grief over the loss of their dear friend, the birds lost their minds and began taking out their frustration on humans. [This one, as you may have guessed, has nothing to do with anything.]

I hope at least one of those works for you.