Very interesting thread.
I'm curious about what Brandi quoted that "the wingtips of a Harlan's fall short of the tip of the tail" and Robert quoted "Borealis to being accipiter like."
Having measurement and clear conformation differences would help out a lot.
In my limited experience raptors, redtails especially vary greatly in their coloration. Raptors like the ferruginous have color phases that occur in the same nest, frequently. Parrots are much easier to identify LOL. Even their mutations are predictable and easy to identify. The first person to have a black, or even navy blue bluefront appear in a nestbox will make a mint. But these wildly varying redtails are to be found on every other power pole.
My first RT was a fairly normal colored girl till she began to moult her coverts. Looking at pics now I see that she got very dark indeed. But though I read about them and wondered, she was no harlans.
Cool thread.