A harrier is any of the several species of diurnal hawks sometimes placed in the subfamily Circinae of the bird of prey family Accipitridae. Harriers characteristically hunt by flying low over open ground, feeding on small mammals, reptiles, or birds.
There are many subspecies of the bird. For example, the first photograph is possibly a Han Harrier while the second is a Marsh Harrier. I observed some subtle differences in behavior of hunting as well as flight patters between the latter species. In particular, the marsh harrier seemed more shy and took me a lot of time to capture a usable photograph.
More on Wikipedia