What turns on your "time to go" clock

Last year we cheated and drove the SUV down, but prior to that I liked to think of myself as a true snowbird. We would leave with the geese, and come home with the hummingbirds. Geese would leave with the shortening days, and the hummingbirds would come north when there was spring time blooms to feed on. All we had to do was look around and see if we were going to fast or slow.