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The Hen Harrier (Circus cyaneus) or Northern Harrier (in the Americas) is a bird of prey. It breeds throughout the northern parts of the northern hemisphere in Canada and the northernmost USA, and in northern Eurasia. This species is polytypic, with two subspecies. Marsh Hawk is a historical name for the American form. It migrates to more southerly areas in winter. Eurasian birds move to southern Europe and southern temperate Asia, and American breeders to the southernmost USA, Mexico, and Central America. In the mildest regions, such as France, Great Britain, and the southern US, Hen Harriers may be present all year, but the higher ground is largely deserted in winter.
The Hen Harrier is long with a wingspan. It resembles other harriers in having distinct male and female plumages. The sexes also differ in weight, with males weighing , with an average of , and females weighing , with an average of . Among standard measurements, the wing chord is , the tail is and the tarsus is . It is relatively long winged and long tailed, having the longest wing and tail of any raptor occurring in North America. The male of the nominate race, C. c. cyaneus (Linnaeus, 1766), which breeds in Europe and Asia, is mainly grey above and white below except for the upper breast, which is grey like the upperparts, and the rump, which is white; the wings are grey with black wingtips. The female is brown above with white upper tail coverts, hence females, and the similar juveniles, are often called "ringtails". Their underparts are buff streaked with brown. The race C. c. hudsonius (Linnaeus, 1766) breeds in North America and is sometimes considered a distinct species, C. hudsonius. The male's plumage is darker grey than that of C. c. cyaneus and the female is also darker and more rufous. In both North American and Eurasia, the adult male is sometimes nicknamed the "Grey Ghost", because of his striking plumage and spectral aura. The female gives a whistled piih-eh when receiving food from the male, and her alarm call is chit-it-it-it-it-et-it. The male calls chek-chek-chek, with a more bouncing chuk-uk-uk-uk during his display flight
This medium-sized raptor breeds on moorland, bogs, prairies, and (in Europe) farmland. The nest is built on the ground or on a mound of dirt or vegetation. Nests are made of sticks and are lined inside with grass and leaves. Four to eight (exceptionally 2 to 10) whitish eggs are laid. The eggs measure approximately . These are the only hawk-like bird known to practice polygyny one male mates with several females. When incubating eggs, the female sits on the nest while the male hunts and brings food to her and the chicks. Up to five females have been known to mate with one male in a season. A male will maintain a territory averaging , though male territories have ranged from . The eggs are incubated mostly by the female for 31 to 32 days. The male will help feed chicks after they hatch, but doesn't usually watch them for a greater period of time than around 5 minutes. The male usually passes off food to the female, which she then feeds to the young, although later the female will capture food and simply drop into the nest for her nestlings to eat. The chicks fledge at around 36 days old, though breeding maturity isn't reached until 2 years in females and 3 years in males. In winter, the Hen Harrier is a bird of open country, and will then roost communally, often with Merlins and Marsh Harriers. There is now an accepted record of transatlantic vagrancy by the American subspecies, with a juvenile being recorded in Scilly, Great Britain from October 1982 to June 1983.
This is a typical harrier, with long wings held in a shallow V in its low, contour-hugging, flight. Northern or Hen Harriers hunt primarily small mammals, as do most harriers. Preferred prey include voles, cotton rats and ground squirrels. Up to 95% of the diet is comprised by small mammals. However birds are hunted with some regularity as well, especially by males. Preferred avian prey include passerines of open country (i.e. sparrows, larks, pipits), small shorebirds and the young of waterfowl and galliforms. Supplementing the diet occasionally are amphibians (especially frogs), reptiles and insects (especially orthopterans). Larger prey, such as rabbits and adult ducks are taken sometimes and harriers have been known to subdue these by drowning them underwater. Harriers hunt by surprising prey while flying low to the ground in open areas, as they drift low over fields and moors. The Harriers circle an area several times listening and looking for prey. Harriers use hearing regularly to find prey, as they have exceptionally good hearing for diurnal raptors, this being the function of their owl-like facial disc. This harrier tends to be a very vocal bird while it glides over its hunting ground.
Little information is available on longevity in Hen or Northern Harriers. It is known that the longest lived known bird was 16 years and 5 months. However, even adults rarely live more than 8 years. Mortality in the early stages of life is often related to predation. Predators of eggs and nestlings include raccoons, skunks, badgers, foxes, crows and ravens, dogs and owls. Fledgings are also predated regularly, especially by Great Horned Owls in North America. If a potential predator visits, both parents respond aggressively, issuing alarm calls and striking with talons. Short-eared Owls are natural enemies of this species that favor the same prey and habitat. More often than vica versa, harriers will harass a Short-eared Owl until it drops its prey, a practice known as kleptoparasitism.
This species has a large range, with an estimated global extent 120 million km, and a population estimated at 1.3 million individuals. There is evidence of a population decline, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e., declining more than 30% in ten years or three generations). It is therefore classified as "least concern".
In the UK, the Hen Harrier suffers illegal persecution by gamekeepers and their employers on shooting estates, particularly those managed for Red Grouse shooting, resulting in local and regional extinction in many areas, particularly in England where only 4 breeding pairs survive despite abundant suitable habitat capable of holding several hundred pairs. Because of this they are now very rare in many parts of the UK, and under threat in many more areas. This problem received a high profile in October 2007 when police investigating the killing of two Hen Harriers on the Queen's estate at Sandringham in Norfolk interviewed Prince Harry and a friend during their investigation. No charges were brought as police were unable to obtain sufficient evidence to prosecute. Since the assumed threat to Red Grouse is the main reason for the persecution of this species in the UK, a project funded by Scottish Natural Heritage, the Game & Wildlife Conservation Trust, the RSPB and Natural England was launched at Langholm Moor in Scotland from 2007. The Langholm Moor Demonstration Project (LMDP), a 10-year investigation, costing 3 million, is intended to see whether grouse and raptors can live side-by-side harmoniously. A similar project, the Joint Raptor Study (also referred to as the 'JRS' or 'the Langholm Study') was run on Langholm from 1992 to 1997. The study made many findings and a host of peer reviewed papers were published on the work, in addition to the final report. Among the most often quoted findings were that long term declines in red grouse populations were "extremely unlikely" to be due to raptor predation and were attributed to habitat degradation/loss, and that raptor predation was the most likely explanation for the failure of grouse stocks to recover at Langholm once the population had fallen to a low level. The project ended in 1997, although a follow up supplementary feeding trial was run by the same team in 1998 and 1999. Grouse shooting on the moor was abandoned for the 1998 season onwards.
The Hen Harrier is a bird of open habitats such as heather moorland and extensive agriculture. However, much of its range, particularly in Ireland and parts of western Britain, has been (and continues to be) afforested, predominantly with non-native conifers such as Sitka Spruce (Picea sitchensis) from North America (Barton et al. 2006, Fielding et al. 2010). Hen Harriers nest and forage in commercial forestry when it is young, before the canopy closes (typically at between 912 and years old), but do not make much use of thicket and subsequent growth stages (Madders 2000, O'Donoghue 2004), which typically comprise between two thirds and three quarters of the commercial growth cycle. Where forests replace habitats that were used by Hen Harriers they will therefore tend to reduce overall habitat availability (O'Flynn, 1983). However, where afforestation takes place in areas that were previously underutilised by Hen Harriers, it may increase the value of such areas to this species in the long-term (Wilson et al. 2009, Haworth & Fielding 2009). Areas dominated by forestry may remain suitable to Hen Harriers provided that a mosaic of age classes is maintained within the forest, such that areas of young, pre-thicket forest are always available.