Hen harriers are medium-sized birds of prey that breed in Eurasia. The males are mainly grey above and white below except for the upper breast, which is grey like the upperparts, and the rump, which is white; their wings are grey with black wingtips. The females are brown above with white upper tail coverts, hence females, and the similar juveniles, are often called "ringtails". Their underparts are buff streaked with brown. Immature birds look like females but with less distinct barring, dark brown secondaries dark brown and less-streaked belly.
Hen harriers are found in Europe and Asia and migrate to more southerly areas in winter. Eurasian birds move to southern Europe and southern temperate Asia, in the mildest regions, such as France and Great Britain. Hen harries inhabit moorland, bogs, prairies, farmland coastal prairies, marshes, grasslands, swamps, and other assorted open areas.
Hen harriers are social and in winter they tend to roost communally, often with merlins and marsh harriers. They are active during the day and spend most of their time hunting. Hen harriers hunt by surprising prey while flying low to the ground in open areas, as they drift low over fields and moors. They circle an area several times listening and looking for prey. Hen harriers are territorial and tend to be very vocal while they glide over their hunting ground. In order to communicate with each other, these birds use a variety of calls. The females give a whistled 'piih-eh' when receiving food from the male, and their alarm call is 'chit-it-it-it-it-et-it'. The males call 'chek-chek-chek', with a more bouncing 'chuk-uk-uk-uk' during their display flight.
Hen harriers are carnivores and hunt mainly small mammals and birds. Preferred avian prey includes sparrows, larks, pipits, small shorebirds as well as ducks and their young. Hen harriers will supplement their diet occasionally with amphibians (especially frogs), reptiles and insects (especially orthopterans). These raptors have been also observed to hunt bats if these are available.
Hen harriers are monogamous and form pairs; however, some males may practice polygyny mating system in which they mate with several females. Hen harriers breed in April-July and during this time males perform sky-dancing displays to attract females. These birds build their nests on the ground or on a mound of dirt or vegetation. Nests are made of sticks and are lined inside with grass and leaves. The female lays 4 to 8 (exceptionally 2 to 10) whitish eggs. Incubation is done mostly by the female and lasts for 31-32 days while the male hunts and brings food to her. The male will help feed chicks after they hatch, but does not usually watch them. 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 after hatching. Young females become reproductively mature at 2 years of age while young males are ready to breed for the first time when they are 3 years old.
Hen harriers have a large range and are not considered threatened globally. However, in the United Kingdom, however, Hen harrier populations are in a critical condition, due to habitat loss and illegal killing on grouse moors ( the hunting of the red grouse, a field sport of the United Kingdom.)
According to the IUCN Red List, the global population size of the Hen harrier is around 100,000 to 499,999 mature individuals. The European population consists of 30,000-54,400 breeding females, which equates to 60,000-109,000 mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red List but its numbers today are decreasing.