Northern Harrier (Circus cyaneus)

Prepared by Kristi Cripe, California Department of Fish & Game

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SUBSPECIES STATUS: Northern Harriers (C. c. hudsonius) are year-round residents of California, found from below sea level (Death Valley), through grasslands, alpine meadows and up to 10,000 feet elevation (Martin 1989, Martin 1987, and MacWhirter & Bildstein 1996).

MANAGEMENT STATUS: California Species of Special Concern


I. Historical references:

Historically, harriers were considered abundant during winter and fall migration throughout their range (Grinnel & Miller 1944). Breeding occurred in relatively small numbers in suitable habitat. Breeding sites were documented from the following counties: Siskyou, Marin, Santa Clara, San Mateo, San Luis Obispo, Kern, Ventura, Mono, Los Angeles, Riverside, San Bernadino, Orange, and San Diego (Hoffman 1927, and Grinnel & Miller 1944). II. Current breeding distribution: Breeding range remains similar to historical distribution, however, extensive local population declines continue to occur as habitat is lost (Ramsen 1978, Martin 1989, and MacWhirter & Bildstein 1996). Range includes coastal areas, Central Valley, northeastern California and Sierra Nevada region up to 3,600 feet (Map 1). A. Site Coordinates:

1. Method used to determine breeding status:

Nest searching - Harrier nests were located during concurrent waterfowl production surveys by California Waterfowl Association or Los Banos Wildlife Area staff (Table 1). Study sites were divided into four regions; northeastern California, Sacramento Valley, Suisun Marsh, and San Joaquin Valley.  
Table 1. Northern harrier breeding populations on state and federal lands as found by California Waterfowl Association and
California Department of Fish & Game (Loughman & McLandress, and LBWA).
Region Site Latitude Longitude
Suisun Marsh Grizzly Island WA -121.93 38.17
San Joaquin Valley Mendota WA

Salt Slough WA

Los Banos WA







Sacramento Valley Gray Lodge WA

Sacramento NWR

Delevan NWR

Colusa NWR









Northeastern CA Ash Creek WA

Honey Lake WA





I. Average territory size:

Territory size varies according to habitat type and prey availability (Martin 1987, and Temeles 1987). In Yolo County, California, Temeles (1987) documented that harriers adjusted territory size to maintain a constant prey base.

Female harriers defend territories, thereby excluding nonterritorial males from preferred habitat. Female are about 50% heavier and 12.5 % larger than males and win nearly all aggressive interactions. Male harriers tend to have larger home ranges, and forage more in riparian and open habitats (Temeles 1987, MacWhirter & Bildstein 1996). Wintering females occupied mean territory size of 33.6 ha, ranging from 3.9 ha - 124.9 ha (Temeles 1987). Breeding home ranges averaged 1.13 km2 for females and 15.7 km2 for males (Martin 1987).

II. Time of occurrences and seasonal movements: Many California populations are resident. Migrating individuals may winter in California, others migrate through to Central and South America (MacWhirter & Bildstein 1996).
A. Arrival date on breeding grounds:  In U.S., records indicate harriers begin departing from wintering grounds in late February to early March (MacWhirter & Bildstein 1996).
B. Departure date from breeding grounds: Harriers begin departing from breeding grounds in August but may wait as late as December.
C. Spring migration period: February through May
D. Fall migration period: August through December.
E. Extent of winter in California:
    Winters throughout California, can occur from seal level up to 10,000 ft., rarely found in wooded areas (Map 1).
III. Migration stop-over needs/ characteristics:
A. Stop over period:  Most harriers migrate alone. Harriers hunt on migration. Juveniles may stop and set up temporary home ranges for a few weeks at a time (MacWhirter & Bildstein 1996).
B. Habitat use:  Harriers occupy similar habitats throughout the year. (See Breeding and Wintering Habitat Use)
C. Routes: no data
IV. Nest type:  Ground nester, bowl or platform if nesting in wet areas.

V. Foraging strategy: Hunts on the wing, using low patrol, quartering flights 1-9 m above open ground. Dive from flight or hover (Ehrlich et al. 1988). Harriers have owl like facial ruffs and face structure that aid in prey detection (MacWhirter & Bildstein 1996).

VI. Displays:  Courting male performs a series of dives from near stall, including barrel-rolls in multiple U shaped loops (Ehrlich et al. 1988).
VII. Social Organization:

A. Typical breeding densities:
Breeding densities are determined by prey base and habitat quality. Loughman & McLandress (unpubl. data) reported breeding densities in four physiographic regions of California (Table 2).

Table 2. Harrier Nesting Densities from 4 physiographic regions of California taken from: California Waterfowl Association: Reproductive Success and Nesting Habitats of Northern Harriers in California (Loughman & McLandress unpulb. data).

Nests Located
Suisun Marsh

(n = 150)



















San Joaquin Valley

















Sacramento Valley

(n =24)







Northeastern CA

(n = 45)










* 1987 Suisun Marsh was noted to be a year of high vole abundance.
B. Mating system:  Predominantly monogamous, polygyny also occurs. Number of polygynous males is positively associated with prey abundance. (Simmons et al. 1986, and Ehrlich et al. 1988).

C. Delayed breeding (where are immature birds?):  Sexual maturity in 1-2 years (Martin 1987). Females are more likely to breed during first year than males. First year males are more likely to breed during years of high vole abundance (Table 3) (Hamerstrom et. al 1985).

Table 3. Frequency of first year breeding in Northern Harrier.

Wisconsin female




n = 268

n = 210

New Brunswick female




n = 116

n = 130

(Hamerstrom et al 1985, Simmons et al. 1986, MacWhirter & Bildstein 1996)
  D. Post fledgling biology of offspring (where do they go and when?):  no data, more information is needed.
E. Post breeding social behavior (mixed species flocks, or simply migrate away?):  More information is needed. Juveniles depart from natal grounds separately and migrate individually (MacWhirter & Bildstein 1996).

VIII. Clutch size: 5 (4-9), 2 - 3 days between each egg laid

IX. Incubating sex: female

X. Incubating period: 30-32 days incubation

XI. Nestling period: 30 - 35 days to fledging.  Peak hatching periods are in May, with ranges from April through June.

Growth rates of fledglings are determined by sex and hatching order. Females grow faster but remain in nest longer then males (Bildstein 1992, and Scharf 1992). After fledging, juveniles roost near nest sites and continue to be fed by parents until family unit disperses (MacWhirter & Bildstein 1996).
XII. Development at hatching: semialtricial, immobile, downy, eyes open

XIII. Number of broods: One. Harriers will lay replacement clutches when clutches are disturbed during laying or shortly thereafter (Simmons 1984).

XIV. Who tends the young:

Female lays and incubates, male brings female food during incubation. Both male and female feed young, only female tears food for young (Hamerstrom et al.1985, and Ehrlich et al. 1988). XV. Diet:
A. Major food items (by season):
Harrier ecology is strongly correlated with prey availability. Harriers predominantly feed on small mammal, mainly, microtus species. However, harries are also generalists, diets have been reported to include reptiles, amphibians, birds and invertebrates.

Several studies found small mammals (microtus) to remain the dominate prey during the breeding season. Many microtine species exhibit cyclic populations. During mid and high ranges of microtus cycles harriers exhibited greater nesting densities, clutch size, nest success, and presence of polygyny (Hamerstrom et al. 1985, and Simmons et al. 1986).

Bernard et al. (1987) found that nesting or fledgling passerines became the second most important prey group for nesting harriers. Passerines are higher in crude fat, calcium, iron, and gross energy than voles or grasshoppers (Bird et al. 1982). Harrier nestling stages coincide with passerine nestling stages, providing abundant, easy prey (Bernard et al. 1987).

Martin, (1987) showed that diet shifts were highly correlated with vegetation growth. Harriers hunting alfalfa fields preyed on microtus until the vegetation reached 46 cm, after which time harriers stopped hunting alfalfa fields and shifted diets to reptiles and passerines. Following cutting of alfalfa, diets shifted back to microtus.

B. Drinking:  Drinks in captivity (MacWhirter & Bildstein 1996).  No other data on water requirements found.

XVI. Wintering ground needs and distribution:

Northern harriers winter throughout California where suitable habitat is found (Map1). Wintering habitat includes fresh and saltwater wetlands, coastal dunes, grasslands, deserts, meadows, and crop lands. Harriers are rarely found in forested areas (Grinnel & Miller 1944, Martin 1987, and MacWhirter & Bildstein 1996). BREEDING HABITAT AND NEST SITE CHARACTERISTICS:

I. Overview of breeding habitat

Breeds from below sea level up to 5,700 ft. in the Central Valley and Sierra Nevada, and up to 3,600 ft in northeastern California. Breeding habitat includes fresh water wetlands, coastal brackish wetlands, open wet meadows and grasslands, shrub-steppe, desert sinks, areas along rivers and lakes, and crop fields. (Grinnel & Miller 1944, Martin 1987, and MacWhirter & Bildstein 1996).

Loughman & McLandress (unpubl. data) located harrier nests in upland fields managed for waterfowl nesting on state and federal refuges. Harrier nest sites were described as areas surrounded with erect, annual or perennial grasses, without nest canopy cover.

In Yolo County, California, harrier nests were located in three different upland types, an uncultivated field of grasses and weeds, a cultivated rice field, and a cultivated field of clover (Temeles 1987).

Simmons & Smith (1985) reported that harriers nesting in wet sites (wetland fringe or wet meadows) were more successful then dry sites and wet sites were preferred in relation to their availability. Vegetational differences appeared to be less significant determinants of success then moisture. Nests located in forbs were more successful than those in shrubs.

Hamerstrom & Kopeny (1981) found harriers to be highly adaptive nesters continuing to nest in marsh areas that were drained and either converted to farmland or grasslands. Vegetation consisted of dense tall grasses and forbs. No nesting success estimates were reported.

II. Nest site:
A. substrate: ground, grasses, forbs, small sticks

B. height of nest: on ground, unless built up with vegetation in wet sites

C. height of plant: na

D. nest concealment: open canopy

III. Vegetation surrounding the nest:

Harrier nests in upland fields were predominately surrounded by grasses, and forbs, while harrier nests in wet sites were surrounded by marsh grasses and cattails (Hamerstrom & Kopeny 1981, Simmons & Smith 1985, Loughman & McLandress unpubl. data, and LBWA unpubl. data).

A. Canopy cover:

Most harrier nest canopies are open. Simmons & Smith (1985) found concealed nests to be less successful. Loughman and McLandress (unpubl. data) found 71% of nests at Suisun Marsh and 93% of nests in northeastern California had no canopy cover. Similarly, 78% of nests found at Los Banos and Salt Slough Wildlife Areas had open canopies (LBWA unpubl. data).

B. Dominant plant species in canopy: na

C. Average shrub cover: na

D. Dominant shrub species: na

E. Average forb cover: partial to complete coverage around circumference of nest (Loughman & McLandress unpubl. data, and LBWA unpubl. data)

F. Dominant forb/grass species:

Annual and perennial grasses (Poaceae), sedges (Juncus), rushes (Carex), milk thistle (Silybum), hemlock (Conium), dock (Rumex), mustard (Brassica), clover (Trifolium), and goldenrod (Sildago) have been documented surrounding harrier nests (Simmons & Smith 1985, Loughman & McLandress unpubl. data, and LBWA unpubl. data).

G. Ground cover: annual and perennial grasses, and forbs.

H. Distance to water: no measurements found, however, breed in mesic habitats.

I. Vegetation height around nest:

Loughman & McLandress (unpubl. data) found that vegetation height and status varied between four geographical region studies (see Table 1 for regions). Average live height was greatest in the San Joaquin Valley 59.2 - 96.0 cm and lowest in Suisun marsh 32.0 - 61.2 cm. Northeastern California was the only region where residual vegetation dominated nests ranging 59.1 - 90.2 cm while live height measured 5.8 - 29.5 cm.

IV. Landscape factors

A. Elevation: Found from below sea level (Death Valley) up to 10,00 ft.

B. Fragmentation: no data, research needed

C. Patch size: no data, research needed

D. Disturbance (natural or managed):

Harriers have continued to use disturbed habitat such as, drained wetlands, farmlands, and areas where herbicide has been used to control plant species (Hamerstrom & Kopeny 1981, Simmons & Smith 1985, and Martin 1987). Further study is needed to determine if survival and reproduction differ between disturbed and natural habitats. E. Adjacent land use:

V. Other Roosting: roost on ground in winter often communally. Will perch on trees and stumps.

I. Brood Parasitism: Incidences of brood parasitism have been noted, but are not considered a threat.

II. Dietary: Almost every aspect of harrier ecology is related to prey abundance. (See diet section)

III. Sensitivity to human induced disturbance:

In agricultural areas, nests are destroyed by livestock, haying, and other agricultural practices (MacWhirter & Bildstein 1996). IV. Pesticide use: Historical effects: chemical spraying of DDT caused egg thinning. Populations recovered after regulation of DDT in 1970s (Martin 1987).

The effects of repeated exposure to agrochemical in wintering areas outside the United States needs to be investigated (Martin 1987).

V. Predators: Predation primarily occurs to eggs and nestlings, and occasionally incubating hens. Mammalian predators include; coyotes, foxes, skunks, minks, raccoons, squirrels, as well as deer and livestock (trampling). Avian predators include crows, ravens, and owls. Reptilian predators may include snakes (MacWhirter & Bildstein, LBWA unpubl data). Simmons & Smith (1985) reported only two instance of aerial nest predation over a three year study. VI. Exotic species invasion/encroachment: no data

VII. Other:


Breeding Bird Survey Trend information: (Sauer et al. 1997)

1966 - 1979
1980 - 1996
Trend P N (95%CI) R.A. Trend  P N Trend P N
California 1.8 0.19 53 -6.5 0.2 1.0 -0.1 0.99 22 -4.6 .0.1 50

Christmas Bird Count Trend Data: (Sauer et al. 1996)

1959 - 1988
1.4 5.1

Grassland Breeding Populations Estimates: (Sauer et al. 1995).

1966 - 1979
1980 - 1994
Trend P N (95%CI) R.A. Trend  P N Trend P N
California 2.1 52 -1.6 5.7 0.46 6.8 31 6.0 39

Analysis of Christmas Bird Counts estimated approximately 13,200 wintering harriers in California (Johnsgard 1990).


I. Age and sex ratios:

Sex ratios of females to males averaged 1.1 : 1 with a range of (0.8 - 1.6 : 1) over a 24 year period (1960 - 1983) (Hamerstrom 1985). II. Productivity measures: Loughman & McLandress (unpubl. data) calculated nest success using the Mayfield method. They reported survival rates during laying and incubation to be significantly lower (23 - 37%) than during brood rearing (62 - 80%). Overall success ranged from 16 - 28% (Table 4).

MacWhirter & Bildstein, (1996) combined data from 6 different studies (n = 696) and reported 30% nest success and 44% brood success.

Table 4. Estimated nest successa of northern harriers in California, 1987 - 1992, taken from California Waterfowl Association: Reproductive Success and Nesting Habitats of Northern Harriers in California (Loughman & McLandress unpubl data).
Period Survival


Suisun Marsh
San Joaquin




Northeastern California

95% C.I.



0.25 - 0.44



0.23 - 0.60



0.09 - 0.55



0.13 - 0.49


Brood Rearing

95% C.I.



0.53 - 0.73



0.60 - 0.93



0.58 - 1.0



0.38 - 0.99


Overall Success 95% C.I.



0.15 - 0.28



0.17 - 0.46



0.07 - .44



0.07 - 0.37


a Nest success calculated using Mayfield’s method

b Incubation period survival rate includes laying period.

III. Survivorship: Pre 1950's mortality rates estimated 59% for juveniles and 30% for adults (Bildstein 1988). Balfour, (1957) estimates a 70% mortality rate for juveniles.

Martin (1987) reported 60% mortality in first year hens and 27.6% in adult birds.

Longest life span reported was 16 years 5 months (Bildstein 1988).

IV. Dispersal: Further research is needed.


1) Extensive habitat loss. Protect remaining habitat, address threats of continued habitat loss, and reclaim and restore when possible.

2) Maintaining high prey base in available habitats. Density of prey base highly correlated with density and success of harriers.

3) Providing adequate nesting habitat. Harriers require approximately 75 days to fledge young. Proper nesting habitat must be available to avoid nest depredation and destruction.


Management and protection of habitat for harriers could also benefit a wide variety of species including other ground nesting birds (waterfowl, bitterns, short eared owls, and grassland passerines).

Maintaining grassland and wetland habitats for harriers provides homes for other inhabitants as well.

High prey base will also benefit other raptors. High prey base of microtus could alleviate some nest depredation, by providing enough food for other predators.


1) A detailed extent of current breeding range throughout California. Breeding range has declined in Southern California?

2) Harriers are highly adaptive to disturbed habitats as long as prey is available. However, recruitment rates and survival estimates should be compared between natural and disturbed areas?

3) Habitat loss has left patches and fragments of available habitat scattered throughout the state. Do patch sizes that limit use/ and nesting success?

4) California nesting data were reported for upland areas only due to sampling efforts. Studies in other states have shown nesting success to be higher in wet areas, or wetland fringe. Additional nesting studies are needed to show habitat use and success between habitats?

5) Little information is available on juvenile ecology. Do juveniles migrate? Where do the disperse to?

6) Monitoring effects of environmental contaminants, effects on prey base, as well as overall harrier survival.

Section 2: Action plan summary Summarize the above information into concise statements :

Extensive local population declines throughout state. Breeding range reduced in southern California. Listed as a species of special concern in California.

Predominantly grassland and wetland communities, however, use of variable of habitats. Harriers are ground nesters that require 75 days to fledge young. Nesting sites free from predation with high prey base are very important habitat components.

Continued loss of habitat through conversion to agriculture, development.

Providing adequate nesting areas, secure from predation, with high prey bases.

Determine current breeding range.

Procure, restore historical habitat, protect existing habitat.

Manage for high prey abundance and adequate nesting areas

Consider prey population fluctuations, weather influence, species behavior or land use practices when estimating abundance (Martin 1989).

Grazing in and around marsh boarders should be eliminated in late winter and spring to protect the nest sites of ground nesters (Ramsen 1978).

Manage wetlands to maintain marshes during spring and summer months (Ramsen 1978).


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