The Northern Harrier (called the Hen Harrier in Europe and Asia) is a slender, white-rumped, medium-sized, and low-flying raptor of upland grasslands and fresh- and saltwater marshes. The only representative in North America of the cosmopolitan genus Circus, the Northern Harrier breeds throughout North America and Eurasia. It is the most northerly breeding and most broadly distributed of all harriers and is a long-distance migrant throughout much of its range. Its degree of sexual dimorphism in plumage and its propensity for polygyny are exceptional among birds of prey.
Like most other harriers, the Northern Harrier nests on the ground, usually in tall, dense clumps of vegetation, either alone or in loose colonies. Most males are monogamous or simultaneously bigamous, although some males pair with up to five mates in a season. In North America, the frequency of polygyny is influenced more strongly by the abundance of food in spring than by a female-biased sex ratio. Females incubate eggs and brood offspring, and males provide the bulk of food for their mates and nestlings.
This raptor forages on the wing, capturing a wide range of prey, mainly small- and medium-sized mammals and birds, while coursing low and buoyantly over the ground. Unlike other hawks, it frequently relies heavily on auditory cues, as well as visual ones, to capture prey. Annual breeding numbers and productivity are strongly influenced by the availability of the species' principal prey in spring, usually microtine voles. In winter, individuals roost communally on the ground.