North America's Northern Harrier, aka Harrier, aka Marsh Hawk, is one of my favorite birds, because he dwells in my favorite sorts of places. I am happy wherever the Harrier is happy.
He likes vast marshes and fields, where he tilts around very low, hunting small birds and rodents. He is never in the woods.
Many a duck hunter has had one silently swing over their duck blind, six feet over your head, and reflexively raised the gun before making the ID. Never shoot one: forget illegal - it's plain wrong. For fun, you can draw them in by making a squeaking sound - like owls, they can hear well. And while you are busy fooling with the Marsh Hawk, no doubt a flock of Bluebills will pass over your decoys while you are distracted - it never fails.
The males and females are very different in coloration, but the white rump, and the behavior, are the easy diagnostic points.
With draining of marshes, and the reduction of farmlands in the East, their numbers are down. But in Britain, the essentially identical bird was almost extinct, but now happily making a comeback.
More about the Northern Harrier here at CLO. I still call them Marsh Hawks.
Photo courtesy of P. LaTourette.