As far as I understand ornithological physiology, the power to flap the wings comes from the breast muscles (pectorals) which are relatively large. These large muscles require a deep sternum (breast-bone) to provide sufficient anchorage and leverage. So I think the high-wing config of birds has more to do with muscle and bone layout than aerodynamics.
Apparently, if a man had the same power/weight ratio, he would need a sternum about six foot deep to anchor the huge pecs required!