When you have a bird and think it might be a Ferruginous, the upper wing surface at a distance will put the ID to rest. The large square pattern at the base of the primaries extends onto the coverts and out to the leading edge of the wing; this group of coverts is dark on Ferruginous Hawks and the white extends well out onto the base of the primaries.

See the upper wing pattern of my Mississippi Ferruginous at: http://www.pbase.com/ol_coot/image/38374855