The facial ruff is easy to overlook in the field but visible at close range: a rim of stiff feathers encircling the face, structurally analogous to an owl's facial disc although the two lineages evolved it independently.
The hunting style explains why. Harriers quarter slowly at a metre or two above moor and marsh, where prey is hidden in dense vegetation and rarely visible until flushed. Work on the related northern harrier demonstrated that birds could locate prey by sound, and their ear structure is enlarged compared with other diurnal raptors of similar size.
Hen harriers are also strongly sexually dimorphic in plumage, males pale grey and females brown, which historically led to the two being described as different species. In the United Kingdom the species remains at the centre of a long conflict over illegal killing on grouse moors, and its national population sits far below what habitat modelling suggests the uplands could support.