SOUTHERN HOME THEORY--Birds originated in tropics. Population expansion and competition drove those that could survive, north (and south) where, indeed, they did find incredibly rich resources and lots of time to exploit them!
All Northern Wheatears winter in Africa, even those in western Canada cross Asia! Wouldn't it be easier to winter in South America or is it not worth the risk to break with tradition? We do have a couple of Wheatear records in South Dakota.
American Tree Swallow