When I was first becoming deeply interested in birds, many times did I read and reread the fifth edition of the A.O.U. Checklist of North American Birds and Grinnell and Miller’s Distribution of the Birds of California. Anyone familiar with these works knows that subspecies figure prominently in them. What are these subspecies, I wondered, and how could I learn to identify them? The A.O.U. Checklist and Grinnell and Miller were long on the where, said a little about the when, but were almost silent on the what. Field guides taught me how to distinguish species, but they tiptoed gingerly around—or ran in horror from—the identification of subspecies.