The Northern Harrier or Marsh Hawk (Circus hudsonius or Circus cyaneus hudsonius) is a bird of prey. It breeds throughout the northern parts of the northern hemisphere in Canada and the northernmost USA. While many taxonomic authorities split the northern harrier and the hen harrier into distinct species, others consider them conspecific. It most distinctive feature is a white rump.
These, are the one of the few raptorial birds known to practice polygyny – one male mates with several females. Up to five females have been known to mate with one male in a season.
It hunts on long wings held in a shallow V in its low flight during which the bird closely hugs the contours of the land below it. Northern or hen harriers hunt primarily small mammals, as do most harriers. Preferred prey species can include voles, cotton rats and ground squirrels. Up to 95% of the diet comprises small mammals. However, birds are hunted with some regularity as well, especially by males. Preferred avian prey include passerines of open country (i.e. sparrows, larks, pipits), small shorebirds and the young of waterfowl and galliforms. Supplementing the diet occasionally are amphibians (especially frogs), reptiles and insects (especially orthopterans). The species has been observed to hunt bats if these are available. Larger prey, such as rabbits and adult ducks are taken sometimes and harriers have been known to subdue these by drowning them in water.
Harriers hunt by surprising prey while flying low to the ground in open areas, as they drift low over fields and moors. The harriers circle an area several times listening and looking for prey. Harriers use hearing regularly to find prey, as they have exceptionally good hearing for diurnal raptors, this being the function of their owl-like facial disc. This harrier tends to be a very vocal bird while it glides over its hunting ground.