Typically hunting from a slow, buoyant flight, usually quite low to the ground, the northern harrier takes a variety of prey, ranging from small mammals and birds to reptiles, amphibians, insects, larger mammals and birds, and sometimes carrion (2) (3) (5) (6). Unlike other hawks, it relies heavily on sound as well as vision to detect prey, aided by the facial disc which, as in owls, is thought to help with the location of faint sounds, allowing the harrier to detect prey concealed in vegetation (3) (5) (6) (8).
The northern harrier commonly perches and even roosts on the ground, and outside of the breeding season often gathers in communal roosts, sometimes numbering tens or even, rarely, hundreds of birds. It also often nests in loose colonies, sometimes practicing an unusual degree of polygyny, with males simultaneously raising several broods with as many as seven different females (2) (3) (5) (6). Breeding usually takes place from April to July (2), the male performing a spectacular aerial ‘sky-dance’ at the start of the season, involving a series of steep climbs and near vertical plunges, with twists, rolls, spirals and loops, accompanied by much calling (3) (6).
The nest is built on the ground, usually in dense clumps of vegetation, and comprises a pile of sticks, grasses, sedges and other materials. Around 3 to 6 eggs are laid, and are incubated by the female for 29 to 31 days. During this time, and until the chicks are about two weeks old, the male brings almost all the food, transferring it to the female in impressive aerial passes, in which the food is dropped by the male and then caught by the female in mid-air (2) (3) (5) (6). The young fledge at 29 to 42 days, and are dependent on the adults for several more weeks. Sexual maturity is reached at around 2 to 3 years, and lifespan may be up to 16 years (2) (3) (5).