Birds of prey make their living in different ways; Kestrels are fond of mice, Peregrines are fond of pigeons, Brown Falcons are fond of snakes. Hunting technique are appropriate to the creatures hunted.
Harriers go about their business fairly low over open country with wings upswept. In Australia we have a couple of members of the guild, Swamp Harrier and Spotted Harrier. One prefers wetter habitats the other drier habitats.
The dry plains around my home seem ideal for Spotted Harrier but for all that they are only occasional visitors. I was looking for quail when I encountered this one, so was he probably.
When looking for mice in the grass slow flight is an advantage. Kestrels and Black-shouldered Kites can hover. They are both smaller birds, there is only so much energy in a mouse and hovering is expensive. The equation works for the small birds but hovering is too expensive for harriers to undertake except very briefly.
So slow flight it is. The upswept wings contribute to lateral stability, very helpful when flying close to stall speeds. If a wing stalls it drops relative to the other wing and the bird as a whole side slips to the affected side. Under these circumstances the lower wing develops more lift than the upper wing and tends to restore the bird to level flight (at a slightly lower altitude).
The upward angulation of the wings is called dihedral and it can be seen in this photo of our gliding harrier …
bobmcgee 
Reblogged this on Wolf's Birding and Bonsai Blog.
Whilst I’m no expert on the physics of flying, I have heard there are benefits to flying close to the ground or water. A type of cushion of air sits between wings and the ground which helps with flight.
I have seen Silver Gull and Shearwater glide close to the ocean.
Ground effect increases lift and reduces drag. It begins when an aircraft or bird is about half its wingspan above a solid surface. You see it really well when pelicans come down on smooth water – they reach a point where they seem to have difficulty descending any further. Spotted Harriers cruise well above the height that ground effect occurs.