They are one of my favorite hawks. Perhaps it is because they spend North America's winter in Argentina where I lived and served for two years. Perhaps it is also because a pair nested here in my neighborhood of Avimor, but the nest failed and I pity them. I hope they come back to Avimor this year! UPDATE: I just found them this afternoon building a nest at Foothills Heritage Park!
I don't know what it is, but most hawks seem take a little extra effort to identify. Here are a few tips from my last couple of years of birding in Idaho that help me pick out a Swainson's Hawk:
1. In flight, look for the lighter leading edge (shoulder) on the wing, with darker flight feathers. (see photo below) This feature seems to hold true for all the other color variations among the same species.
2. While perched, you should see a darker tail - helps to eliminate all the Red-tailed Hawk morphs.
3. Swainson's are often seen perching on the ground or on low fence posts. During seasons where they overlap with Rough-legged Hawks, this might not be as helpful, but during the summer it seems to be a trait that makes Swaison's different. Red-tails may be on the ground when feeding, and Northern Harriers will sometimes perch low, but when I see a big dark hawk on the ground, it is most often a Swainson's.
4. Swainson's do seem to have some variable plumages. Compare how dark the breast and belly of the Star Swainson's versus the light-bellied and hooded pair that nested at Avimor last year.