September is a time of transitions, especially here on the Cape. Summer visitors have transitioned back over the bridge. Our kids transition back to school, our beaches and parks transition back to local ownership. And perhaps more than in any other month, transitions in the bird world are everywhere evident, from the exodus of northern songbirds to the arrival of the first winter sea ducks. But few transitions are sadder than the loss of our hummingbirds, those kinetic clowns of flowery suburban backyards.