The Northern Harrier is an opportunistic hunter which varies its diet depending upon the local abundance of prey. However, across much of its range it depends heavily upon its primary prey, voles (Microtus speciies); in fact, vole populations can have a strong effect upon the harrier's mating systems and productivity. During nesting, many harrier populations also prey upon fledging grassland and marsh songbirds. Harriers will also exploit other temporarily abundant and vulnerable prey like young ground squirrels and young cottontail rabbits. Reptiles, amphibians and invertebrates make up a minor part of its diet. Carrion is eaten, and can be an important food source in hard winters. Rarely, fish are taken.
Mammals taken include voles, cotton rats, shrews, skunks, muskrats, brush rabbits, marsh rabbits, snowshoe hares, cottontails, and jackrabbits. The larger species taken are young, injured, or dead animals. In Canada and the United States, small rodent prey predominates from late fall until spring. Birds taken belong to at least 80 species including Clapper Rails (an important food species in some southeastern marshes), young Bobolinks, Mourning Doves, blackbirds, House Finches, meadowlarks, flickers, ducklings, and sparrows. Young birds mostly appear in the diet in spring. Larger birds are occasionally attacked, for example American Wigeons, and several other duck species which are often found crippled by hunters. Young upland gamebirds, and occasionally small poultry, is also taken.
Other food includes rattlesnakes, garter snakes, racers, Leopard Lizards, fence lizards, whip-tailed lizards, horned toads, toads, and frogs. Invertebrates include spiders, crickets, beetles, and grasshoppers, sometimes a common prey item.
PELLETS
The pellets of Northern Harriers are usually about 1.4 to 2.0 inches (35 to 50 millimeters) long and 0.8 to 1.0 inches (20 to 25 millimeters) in diameter (sometimes up to 2.6 inches [65 millimeters] in length) and are most likely to be confused with those of Short-eared Owls. However, because harriers break up field mice bones more than owls do, the harrier pellets are spongy compared to the owls' firm pellets. Fresh harrier pellets are also less coated in mucus.
In one study, 85 percent of prey were eaten on the ground and 15 percent while perched, mostly on fence posts. Prey is eaten piecemeal beginning with the head. The gastrointestinal tract is often discarded. Avian prey is usually plucked first. The legs, sternum, and sometimes the wings usually are not eaten. Large avian prey is not always plucked. Harriers have been seen to neatly skin frogs and strip fur from voles.
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