Circus hudsonius – hawk that flies in circles.
Wingspan: 3 feet. Weight: .7 – 1.3 pounds
The Northern Harrier is a beautiful, slender, graceful hawk, tilting over fields and marshes as it hunts for rodents and small reptiles and birds. Sometimes as it flies, it gives a high whining cry.
The Circus genus is found worldwide, but the Northern Harrier is the only one in the US. Populations in much of the country migrate, but not the birds in California and the West Coast. It hunts in open pastures, marshes, and fields, flying low over grasses and crops, tilting its body. Its head has a “facial disk” similar to that of owls, indicating that it uses its hearing in hunting. (See below.) Its diet is small rodents, reptiles, insects, and birds.
Unusual among raptors, males and females of this species differ in color: the adult male is gray on the back and white on the undersides, while the female is brown on top, with a deeply striped breast. It may take the male three to four years to develop his complete adult feathering.
Both sexes have a distinctive white rump, which is often visible in flight. These birds are more social than most raptors: they roost on the ground in groups, and the male may have several mates simultaneously. Their calls are alike: a high-pitched whistling whine.
Nesting: Harriers nest on the ground or in low bushes in open fields or pastures. Unusually for raptors, one male harrier commonly will mate with two, sometimes up to five, females, depending on the availability of food. The female incubates the eggs and broods the hatchlings, but the male provides most of the food for the young birds.
Females breed first at one year, males at an older age, but the males’ breeding is dependent on the the rodent population of any given area.
Hunting specialties: Harriers have a modified facial disk of stiff feathers, guiding sounds to their ears, so, like owls they hunt as much by sound as by sight. They fly low over fields and marshlands – indeed, they used to be called “Marsh Hawks.”
Relationship with people: This species was affected by DDT, which interfered with the absorption of calcium in the bird’s body, and caused thinning of the egg shells. This toxin nearly wiped out many bird species, including Bald Eagles and Peregrine Falcons, but it was banned in 1972, and these species have all recovered. The Northern Harrier, like many raptors, is today threatened by habitat destruction. They are vulnerable to many pesticides still in use in agricultural areas, and their low nests make the females, eggs, and young easy prey for dogs, cats, foxes, raccoons, and the like. The nests are often destroyed or disturbed by farming activities.
Predators: Mammal predators at ground nests. Other raptors kill adults.
Life span: 16 years in the wild. Like all raptors, the first year of life takes a hugh toll: 50 to 80 percent of the year’s young die. If they survive that first year, they can learn effective hunting and protective skills, get to know their area and its predators and safe places, and live for a good long while. They can live 20 or more years in captivity. Migration brings unforeseeable dangers, of course, to any birds on the move. But the Harrier is non-migratory on the West Coast.
Status: The Northern Harrier is not endangered, but populations are declining where habitats are being destroyed.
Curious facts: In winter, these birds sometimes roost together in large numbers.
A European species, Montagu’s Harrier, has been found to have ultraviolet components in the male’s cere (the waxy skin area at top of the beak) in breeding season. In studies, the darker the UV component, the more successful the bird was in attracting mates. The females, which lack UV components in their ceres, seem to choose males whose ceres are brightest. This has not yet been studied in our Northern Harrier, but their life styles are similar.