Judging by the number of names by which this bird has been known in various parts of the United Kingdom in past times (the male as blue hawk, blue kite, blue-sleeves, white hawk, white aboon gled, grey buzzard, miller; the female as ringtail, brown gled, brown kite; and generally as faller, katabella, moor hawk, dove hawk, dove-coloured falcon, furze kite, vuzz kite, gorse harrier, seagull hawk, hen driver, hen harrow, hen harrower, flapper) it can be assumed that the Hen Harrier was once a very common bird.
Ranging throughout the boreal and temperate zones of North America (where it is known as the Marsh Hawk) and Eurasia, and wintering in temperate to warm temperate zones, it is a bird that has very mixed fortunes at the hand of man.
It is currently in decline or just hanging on in much of its range, where its preferred terrain is open, low-lying country like rough grazing, heath land, moorland, marshes and boggy areas with a high rodent population.

Diet

The Hen Harrier is the most agile of the harriers, and is pretty well unrestricted in its choice of prey (except by size). It is, however, about the fussiest.
It has a definite preference for voles (who'd be a vole - everyone seems to like them!), followed by mice and small birds. In some areas, young rabbits and hares also feature.
The general hunting method is low and slow quartering, systematically checking every tussock until prey is spotted, whereupon the bird will, with outstretched talons and fanned tail, turn on a sixpence and drop like a stone.

Voice

Generally silent except when at or near the nest, or when alarmed, the voice of the Hen Harrier is best described as a rapid chattering, frequently preceeded by a long drawn out "queeow".
Unusually, the voice of the female is at a higher pitch than that of the male.

Status and behaviour in the wild

The colour differences between the sexes of this species (the male being ash grey with black primaries and white upper tail coverts, the female dark brown above and pale brown streaked with dark below) they were, until comparatively recently, thought to be separate species.
In the field, the male is sometimes confused with the male Montagu's Harrier (C. pygargus), but the latter has a longer tail, narrower and longer wings, darker plumage, more mottled above, with two dark bands on the underwing, no white rump, and is generally more slender.
In flight this is a typical harrier, flying close to the ground with wings raised into a shallow 'V', occasionally held level or even slightly drooping. The flight pattern consists of bursts of 5-10 wingbeats with glides between.
In the united Kingdom, the Marsh Harrier, like the Red Kite has suffered mixed fortunes at the hand of man. After extended periods of destruction and persecution, it responded well to protection and habitat changes and numbers recovered. Unhappily, it is now under pressure from sporting interests who charge it with disturbing the game birds (mainly grouse) and killing their young.

Breeding

The Hen Harrier generally starts breeding at 2-3 years, although it is not unknown for one year olds to breed. The pair bond is generally monogamous, although in certain circumstances an older male will mate with multiple females, although when this happens the fledging rate seems to suffer.
At the onset of the breeding season, the pair engage in their spectacular sky dance. This starts with the male climbing steeply to a great height where he performs a few backwards, side-rolls or somersaults before going into a very steep dive from which he seems to recover only fractionally before the ground. The female later joins in, turning over and presenting her talons to him at the base of his dive, as if to ward off an attack
The Hen Harrier is a ground nesting bird, the nest constructed mostly be the female, who is also the sole incubator of the clutch of up to four eggs. Incubation, which may start with any but the first egg, lasts for between 20 and 39 days for the entire clutch.
During incubation the male arrives at the nest area with food in his talons and calls to his mate. She responds by taking to the air with loud begging calls, and flying towards him. Just before meeting, he drops the prey, and she neatly flips over to grasp it from the air with her upturned talons before landing nearby to eat it, or returning to the nest to feed it to her brood.
The young will leave the nest after about two weeks, and settle in nearby cover. Fledging takes between 32 and 42 days, the young achieving full independence within a further 2 to 4 weeks.