The most important aspects regarding in-flight hawk ID are shape and flight style. Yes, plumage is important, but shape and flight style are more reliable traits when plumage is difficult to judge. I was watching hawks in April 2009 at Gunsight Mountain in Alaska on a day when there were several hundred Rough-legged Hawks and Harlan's Red-tailed Hawks. A distant hawk approached at eye level. In certain postures and conditions, Harlan's can appear quite lanky in shape, and hold their wings in a slight modified dihedral (wings raised at the shoulders and level at the wrists) similar to a Rough-legged Hawk. Anyway, I said, "here comes another Rough-legged Hawk". The observer turned to me and asked in a genuinely curious way "how can you tell?"
Quite frankly, the key is to learn the basics of shape, plumage, and flight style well before tackling the minutiae. These days, I think people concentrate on the finer details of plumage. It's only natural, many guides show pretty pictures, and the Internet is full of beautiful bird portraits that reveal incredible feather detail. I enjoy close-up photos too. However, if you are looking for some obscure plumage detail in the field, chances are you won't see it and may overlook the obvious ID traits such as shape, flight style, or the manner in which a bird holds its wings. You don't need to know the fine details to identify hawks in the field. You need to KNOW THE BASICS WELL to be adept at identifying flying raptors, PERIOD! Besides, it's a whole 'nother ball game to learn and apply the finer details accurately in the field...that takes years and years of practice. Oh yeah, compared to a Red-tailed Hawk, a Rough-legged Hawk is less broad at the chest, and the wings are slimmer, longer-handed, and held in a more pronounced modified dihedral when gliding...every time.
I'm leading a few hawk ID workshops this fall and will stress all aspects of raptor ID, so check HWI.org for a calendar of trips, I hope to see you then. I promise I'll do my best to make them fun and a learning experience. Here's a composite of Goshawk shapes as a refresher, hope you recognize one instantly the next time you see one!