The Northern Harrier ( Circus hudsonius ) is a slender, medium-sized raptor known for its distinctive hunting style and preference for open country. Often called the "Marsh Hawk," "American Harrier," or "Hen Harrier," the adult male is also known as the "Gray Ghost" due to its striking plumage. As the only harrier species in North America, the Northern Harrier is a unique and fascinating bird of prey.
Identification and Appearance
Northern Harriers are easily identified by their slender build, long wings and tails, and a prominent white rump patch visible in all plumages. They also have owl-like faces, an adaptation that aids in their hunting strategy.
Adult males are primarily light gray above and whitish below, with black wingtips and trailing edges on their underwings. Adult females are brown above with buffy underparts heavily streaked with dark brown. Juveniles are dark brown above with cinnamon breasts and bellies. Females are typically 10-20% larger and can weigh approximately 50% more than males. Adult harriers have lemon-yellow eyes, while juveniles have grayish eyes that turn yellow by their first winter.
Habitat and Distribution
Northern Harriers are found throughout much of North America, as well as in Europe, Asia, and Africa. They breed from Canada and Alaska south to California and Texas in the West, and through the Mid-Atlantic States south to Virginia in the East. During winter, some birds migrate as far south as Central America and northern South America.
These harriers prefer open habitats such as tidal marshes, emergent wetlands, fallow fields, grasslands, meadows, airports, and agricultural areas. They typically nest in high marsh areas dominated by salt hay, marsh elder, or reed grass, and may also nest in freshwater tidal marshes. Communal winter roosts are located on the ground within drier portions of marshes or in grasslands.
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Hunting and Feeding Habits
Northern Harriers are diurnal hunters, meaning they hunt during the day. They are known for their unique hunting style, flying low to the ground in buoyant gliding and flapping flight. The species prefers to hunt in open areas with mixed vegetative cover and avoids areas with only short vegetation. Harriers rarely hunt while perched.
These raptors rely heavily on their sense of hearing to capture prey, thanks to their owl-like facial ruff that concentrates sound towards their ears. This adaptation allows them to hear potential prey beneath the vegetation. They circle an area several times, listening and looking for prey before plunging to the ground to capture it.
The Northern Harrier's diet is diverse and opportunistic, varying with location and season. They primarily feed on small mammals, especially voles, cotton rats, and ground squirrels, which can make up to 95% of their diet. They also regularly hunt birds such as sparrows, larks, pipits, small shorebirds, and the young of waterfowl and galliforms. Additionally, they may consume reptiles, amphibians, insects, and carrion.
Large prey is usually torn apart before being eaten, while small prey, including mice and insects, are sometimes swallowed whole. Harriers sometimes hide or cache surplus food, particularly during the breeding season. In winter, individuals sometimes rob prey from each other.
Breeding and Nesting Habits
Northern Harriers exhibit both monogamous and polygamous mating systems. Polygamous males may have two or three mates, and in rare cases, up to five females. In polygamous situations, each female has her own nest, and the male tends to each female individually.
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Courtship begins in late February and peaks in April. Males perform elaborate aerial courtship displays, called "sky-dancing," to announce their presence in their territories and maintain pair bonds with females. This involves a series of U-shaped, undulating flights at heights ranging from 10 to 300 meters above the surrounding countryside.
Nests are built on the ground in dense grassy or shrubby vegetation, often in wet areas to reduce the risk of predation. Either the male or the female selects the site, and both help build the nest. Nest construction takes from several days to several weeks. Pairs continue to add nesting material during incubation and sometimes until the nestlings are three to four weeks old.
Female harriers lay one clutch of four to six eggs annually, with eggs laid in two-day cycles. Replacement clutches are sometimes laid if the first clutch is destroyed. Incubation, primarily by the female, begins after the first egg is laid and lasts for 30 to 32 days. The young hatch asynchronously, giving the older young a better chance at survival if food is scarce.
During the incubation period, the male provides food to the female, passing the prey to her in an aerial transfer. After hatching, the male continues to provide food for the female and nestlings. By the time the nestlings are two weeks old, they begin to walk and hop up to 15 meters from the nest. Young harriers take their first, short flights when they are four to five weeks of age. Parents continue to provide the fledglings with food for two to four weeks after fledging, transferring prey in the air.
Migration
The Northern Harrier is a partial migrant, with harriers breeding in the northern portion of the species’ range tending to be long-distance migrants. Like most raptors, Northern Harriers begin migrating across a broad front. However, they are less likely to concentrate along corridors than other species of raptors.
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Harriers are primarily solitary migrants and the species has a protracted migration, in both autumn and spring. During migration, harriers incorporate a mixture of flapping and gliding flight with little soaring, and they tend to fly closer to the ground than many other migrants. The species also migrates in adverse weather, including light rain and snow.
Conservation Status and Threats
The current North American population of Northern Harriers exceeds 100,000 birds. However, overall, Northern Harriers appear to be decreasing globally, primarily due to habitat loss. Suitable breeding and feeding habitat has decreased in many parts of the species range because of large-scale monotypic farming, farmland reforestation, and the extensive draining of wetlands for agriculture and other development.
Prey availability decreases in areas due to overgrazing, the emergence of large crop fields and fewer fence rows, and with pesticide use. Agricultural machinery, early mowing, and livestock directly destroy nests.
From the mid-1940s to the early 1970s, the widespread use of organochlorine pesticides including DDT increased pesticide levels in prey that harriers feed upon, reducing reproductive success by causing eggshell thinning. Although Northern Harriers have largely escaped direct persecution in most of North America, they have been targets of persecution elsewhere.
The Northern Harrier was listed as threatened in New Jersey in 1979 due to population declines and habitat loss, and was upgraded to an endangered species in 1984. While wintering populations in New Jersey are stable, the breeding population is considered to be endangered.