Splitting this off from the human powered helicopter thread since it's about birds that can hover. Soaring is different than hovering. Hawks can soar very well, but they are poor fliers. I've regular see a local nesting pair of crows / ravens spot a hawk soaring above them and they can quickly fly from tree level to well above the hawk and then take turns diving at the hawk to drive it away. In a no wind condition, it is costly. The local kites I see hovering in a no wind condition only do so for about 30 seconds or so before diving after prey or giving up. If there's a headwind and updraft, there's very little flapping required and they can do this for several minutes. I don't know if the larger osprey can hover in no wind conditions, but their hovering capability is why the twin propeller aircraft capable of hovering choose Osprey for it's name (the V-22 Osprey).