At what time of year do the events in this story take place?
Yes, the short story "The Birds" takes place after World War II. The year is not certain, but it's probably within a year or two after the war. We know this because the story begins by describing the character Nat Hocken who works part-time at the farm because of a "wartime disability." So he probably returned from World War II not too long ago.
We do know the season, however, because the story begins on 3rd December.