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Scott Wilson

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Threatened and Endangered Species Report - March 1995
Bay Delta and Special Water Projects Division, CA Dept. of Fish and Game

Northern Harrier

Circus cyaneus
California Department of Fish and Game "Species of Special Concern"

Life History

The northern harrier, formerly known as the marsh hawk, is a slim, long-winged, long-tailed, raptor of open country. They measure from 17-1/2 to 24 inches long, with a wingspan of 39 to 48 inches. In all of the various plumages, a white rump patch is exhibited. The males are usually pale gray above and whitish below with black wing tips and a gray hood; females, on the other hand, are brown above, whitish below, with heavy streaks on the breast and flanks; and the immatures are a russet or cinnamon below and streaked only on the breast. An owl type facial disk is distinctive in all ages and both sexes, as is the slender, cross-barred tail. Overhead, the wing tips of the pale male seem to have a "dipped-in-ink" appearance and there is also a black border on the tailing edge of each wing. The wing spread can be up to four feet.

This hawk nests on the ground in shrubby vegetation, usually at the edge of a marsh. The nest is built out of a large mound of sticks in wet areas, and a smaller cup of grasses on dry sites. Most of the nests are found in emergent wetlands or along rivers or lakes, but it may also nest in grasslands, grain fields, or on sage brush flats that are several miles from water. Harriers usually perch on the ground but will use fence posts or other low perches and occasionally trees (Peterson 1990). In the winter, communal ground roosts with numbers from a few to hundreds of birds can be found. These communal ground roosts can include short-eared owls among the northern harriers.

Breeding begins April to September and peak activity appears to be June through July. The most common number of eggs is five, but four or six are frequently found and occasionally as many as seven, eight, or even nine, are seen in a harrier's nest (Bent 1961). Most commonly the female incubates the eggs while the male provides her with food, but it has been noted where both parents shared the duties of incubation. Both adults care for the young. The nesting period lasts approximately 53 days. After the young gain the ability to fly they are often fed by their parents while on the wing. The breeding pair and juveniles may roost communally in the late autumn and winter.

The northern harrier occurs from annual grasslands on the Valley floor up to the lodgepole pine belt and alpine meadow habitats which can be as high as 10,000 feet. It breeds from sea level to 5,700 feet in the Central Valley and both slopes of the Sierra Nevada. In northeastern California, they are found to breed up to 3600 feet. It frequents meadow areas, grasslands, open range lands, the desert sinks, and fresh and saltwater emergent wetlands. They are seldom found in wooded areas. They appear to be permanent residents of the northeastern plateau and coastal areas and a less common resident of the Central Valley.

Northern harriers can be locally abundant where suitable habitat remains free of disturbance, especially that from intensive agriculture. They rely on the use of tall grasses and forbs in wetlands, or at wetland/field borders, for suitable cover. These borders or edges are especially important for nesting, feeding and cover. Their home range usually includes a freshwater site. They are very defensive of their territory and will attack other birds of prey and humans during breeding season.

The northern harrier exhibits year-long, diurnal activity and can be seen skimming close over grassland areas with the wings tilted slightly upward. It tilts and turns as it searches for small prey items and insects. Like owls, they use their sense of hearing as well as sight to capture food but are active during the day and not at night like most owls (Basey 1990). Recent studies have shown that northern harriers can locate prey by sound almost as well as owls can, suggesting an explanation for the facial disk (Peterson 1990).

This bird of prey feeds mainly on voles and other small mammals, birds, frogs, small reptiles, some crustaceans, insects, and, on occasion fish. This hawk is regarded by many as a highly beneficial species, mainly because of the large numbers of mice, rats, and other small mammals that it consumes (Bent 1961).

Findings and Conclusions

California's population has decreased in recent decades (Grinnell and Miller 1944, Remsen 1978). Undisturbed, suitable habitat that remains free from intensive agriculture is one of the keys to prevention of further decline of this species. Breeding populations have been much reduced, especially in the southern coastal district of California. Destruction of wetland habitat, native grassland, and meadows, and burning and plowing of nesting areas are reasons for the decline of this species (Remsen 1978).

The northern harrier is rather common in the vicinity of the lower Stanislaus River using grasslands and open fields for forage. Harriers were not observed along the river, but forage and potentially nest very close to the river in tall grass and other suitable habitat. Any project impacting the grasslands and open areas adjacent to the Stanislaus River could impact the northern harrier and therefore require additional surveys.