The Northern Harrier can be monogamous or polygamous, with the male having two to seven mates. Polygamy occurs in dense populations of harriers where voles are abundant. Polygamous males tend to favour certain females with more food and more vigorous nest defense. Whether monogamous or polygamous, the harrier tends to nest semi-colonially, in favored locations with, for example two or three nests per 400 yards (364 meters). Birds show little mate fidelity at their next mating. Nests are located in a variety of site types, ranging from dry habitats such as cultivated and uncultivated fields, pastures and grasslands (about 40 percent), freshwater marshes (18 percent), marsh meadows (17 percent), bogs, swamps brackish marshes, and dried wetlands (7 percent), to miscellaneous sites such as sage rangelands, brushlands, deciduous woods, stubble, and brushy rangelands (20 percent). Nests are built on the ground in a range of cover often including grasses, sedges, willows, spirea, or other vegetation often next to a bush or tree, and often near water, or over water on a muskrat house or floating mat of cattails. The male may initiate construction by building a platform or 'cock nest' with the female finishing the job, or she may build a new nest herself. Occasionally, the same nest is reused when new materials are added. The nest is usually a hollow lined with grasses, or in damp situations, sticks or weed stalks. The nest's structure differs from dry to wet sites. A dry site nest may be only one to two inches (2.5 to 5.1 centimeters) high and 15 to 18 inches (38 to 46 centimeters) across with a shallow or nearly flat bowl. A wet site nest may be 15 to 18 inches (38 to 46 centimeters) high and be a floating raft. Green grass and other leaves are often added to the nest during incubation and while the young are in the nest.
Clutch size ranges from two to 10 eggs, but averages four to six eggs. Larger clutches are laid during times of high vole populations. The egg is an average of 1.8 inches (46.3 millimeters) in length and 1.4 inches (36.3 millimeters) in breadth with a smooth, non glossy shell and a shape between elliptical and short subelliptical. A fresh egg is a very pale bluish but very quickly fades to soiled white during incubation. About 10 percent of clutches show scattered spotting of very pale brown.
The female incubates the eggs for 29 to 39 days per clutch or 29 to 31 days per egg. Young birds are fairly precocious, very active and noisy by two days of age. They vocalize a high "cheek-cheek-cheek". The female will shade young by standing over them. Females retrieve young accidentally knocked from the nest. When frightened, young over five days hide up to 20 feet 6.2 meters) or more away from the nest, returning when the female returns with food. Young will also shade themselves in cover away from the nest. At three weeks the young begin to beat their wings. The male does all hunting and provisioning of the female and young until the female resumes hunting (close to the nest) when the young are over five days. The male usually transfers prey to the female in flight. The young start to fly at 30 to 35 days. By the time the young disperse from the nest the female is providing most of the food. Monogamous males tend to provide young with food longer than polygamous males do. The young remain together near the nest area for 21 to 50 days after fledging. During this period they hunt invertebrates and snakes and are also fed by their parents. Dispersal from the home area is apparently solitary. Each breeding female has a single brood. Replacement clutches are rarely laid.
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