This is a very good quiz, Bob. I have been working hard on learning my kingbirds. Back home in Ontario, Canada, we only have the Eastern Kingbird. But in the west.......oh my!!!!! I see you did not include Thick-billed Kingbird. I guess that is different enough that quiz-takers would eliminate that one right away. I eliminated Couch's because that is more of a southern Texas bird. And I eliminated Cassin's because the gray is darker. Your picture shows a lighter gray. The Tropical Kingbird shows a darker ear patch. The bird in your picture does not have the darker ear patch. The Tropical has more of a notched tail. The bill, however, looks a little longer than I would have thought you would see in a Western. But my guess is for the Western.