This has always been my perception of bird migration in the fall: The days grow short and cool and then, one day, I notice a V-shaped caravan of Canada geese flying southward. Then another and another. Within a few weeks of that first sighting, I hear their melancholy call one final time for the season. Then they, and all the summer birds, are gone. It’s a mass exodus for warmer climes, over and done in the blink of an eye and long before the snow flies.