Ferruginous Hawk (Buteo regalis)

Prepared by:  Kevin Hunting, California Department of Fish and Game

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MANAGEMENT STATUS: federal Non-game Migratory Bird Species of Management Concern; California Species of Special Concern


I. Historical distribution and abundance: Grinnel and Miller (1944) describe the California range as "south from the Oregon line, east of the humid coast belt, to Mexican line chiefly west of Colorado desert". Little data are available on this species historical distribution in California.

Grinnel and Miller (1944) considered the Ferruginous Hawk "formerly abundant; still more or less common locally" adding that a few individuals occur in summer and may nest in the extreme northeast corner of California. Based on this description, the species had apparently declined in California by the 1940's.

Current distribution: Bechard and Schmutz (1995) depict the range as the entire state except for roughly triangular polygons in the northeast (from about Lake Tahoe northwest to Tule Lake) and northwest (from about Napa northeast to Tule Lake) portions. The range in Bechard and Schmutz (1995) excludes the northeast portion of California based, in part, on an apparently erroneous interpretation of Small (1994). With respect to this portion of the species range, Bechard and Schmutz (1995) cite Small (1994) as "except in northeast" while the account in Small (1994) states "very common [wintering] in the open country of the extreme north and northeast". Analysis of Christmas Bird Count (CBC) data for the period 1980-1997 reveals detections of this species in the northeast (Honey Lake) circle.

Bechard and Schmutz (1995) offer no explanation for the exclusion of northwest California from this species range. Ferruginous Hawks were detected nearly every year between 1980 and 1997 on the Arcata, Centerville, and Willow Springs circles (Humboldt County); regularly during this period on the Del Norte and Klamath circles (Del Norte County); and every year during the period on the Mendocino Coast circle (Mendocino County). Harris (1991) considers the species a "locally rare winter visitor" in northwestern California. During the winters (November - February) of 1993 through 1998, 15-25 Ferruginous Hawks per day were detected on road transects run three times per week from Arcata to Cape Mendocino (Jeff Kidd, pers. comm.). Ferruginous Hawks clearly occupy both northeastern and northwestern California where they should be considered migrants and uncommon local winter residents.

Distribution of the Ferruginous Hawk in the remainder of California is defined primarily by observations from the CBC, a few published accounts of communal roost sites (Antelope Valley and Cuyama Valley [Bloom, cited in Olendorff, 1993], Harper Dry Lake, Helendale, Lake Henshaw [Grindrod 1997], and monitoring data collected incidental to breeding bird atlas projects. Rosenberg et al (1991) cite the Ferruginous Hawk as an uncommon transient or winter resident from mid-October to mid-March along the Colorado River. It is considered common in grasslands and agricultural regions in southern California from mid-September to early April (Garrett and Dunn, 1981). With the exception of the CBC, little data exist on distribution in the central valley or Sierra Nevada. The species has been detected regularly on 97% of all central valley CBC routes between 1980 and 1997 (kh). Garrison (1990) indicates winter range extends southwest from the breeding range throughout most of California.

Geographic extent of range probably unchanged from historic times except for local extirpation from the southern California coastal plain and valleys due to habitat loss. In the early 1900's, California supported approximately 8,900,000 ha (22 million acres) of grasslands with about 20 percent occurring in the central valley (Moore et al 1990). Currently, grassland habitat has been nearly extirpated in the San Joaquin valley with less than 60,700 ha (150,000 acres) remaining. In the intervening period, conversion of grassland habitats to urban and agricultural uses proportionately exceeded conversion of any other habitat type (Ewing et al 1988, Moore et al 1990).

Analysis of CBC data comparing the period 1980-1989 to 1990-1997 (Table 1) show that both mean Ferruginous detections per count circle and the mean number of count circles detecting this species increased in 6 of 7 California physiographic regions with a decrease noted only in South Coast region (see Carter et al 1998 for a description of physiographic regions). Garrison (1990) also reported a general increase in abundance of Ferruginous Hawks.

Table 1.
Mean Ferruginous Hawk Observations (Circles Detecting Ferruginous Hawks) for each of Seven California Physiographic Regions

Physiographic Region
Mean Count Circle Observations (Circles)


Mean Count Circle Observations (Circles)


Numeric (percent) change
North Coast (93)
2.43 (4.8)
2.76 (7.5)
0.33 (12.1)
Sierra Nevada, Cascades and Inner Coast Range (66)
3.53 (2.2)
8.14 (3.8)
4.61 (56.7)
Great Central Valley (91)
2.73 (11.6)
4.43 (14.7)
1.7 (38.5)
Central Coast (92)
2.70 (20.4)
4.63 (27.0)
1.93 (41.7)
South Coast (90)
5.06 (13.6)
4.77 (15.8)
-0.29 (-6.1)
Mojave and Colorado Deserts (82, 83)
3.17 (4.2)
3.83 (4.7)
0.66 (17.4)
High Desert, Mono and Inyo Counties (80)
0.33 (0.2)
1.30 (0.8)
0.97 (77.5)
Mean Observations1
3.27 (±0.96)
5.22 (±2.88)

1 Due to small sample size, values for High Desert, Mono and Inyo Counties (physiographic region 80) omitted from calculation

Increases in the mean number of count circles detecting this species can be attributed, in part, to an increase in the absolute number of California count circles. An increase in the mean number of observations per count circle can be attributed to a number of factors including observer hours and the number of observers per circle, observer skill level, and location of newer routes in areas supporting more Ferruginous Hawks and should not necessarily be interpreted as an increase in abundance in California (Garrison 1990). Additional analysis is needed to standardize count circle data to remove known biases (Raynor 1975). The data and analysis presented here are intended to impart relative distribution and abundance from a physiographic regional standpoint.

Outside of California. The following description of Ferruginous Hawk distribution in the United States is modified from Grindrod (1997): winters in grassland and shrub-steppe habitat from northern Baja and irregularly to Baja California Sur (Unitt et al., 1992); south to southern Arizona, New Mexico, west Texas, and into northern Chihuahua and the central states of northern Mexico; and southeast to western Kansas, Oklahoma, and central Texas (Bent 1937; Olendorff 1993; Bechard and Schmutz 1995).

II. Current breeding distribution:

ECOLOGY: Please be as specific as possible in regards to bioregion (when possible).

I. Average territory size:

Winter - Bechard and Schmutz (1995) indicate this species may defend a winter territory. Smith and Murphy (1978) estimated a winter density of 1 per 932 km2 in central Utah in the only published account of winter density. Roosts communally (Steenhof 1984, Bechard and Schmutz 1995).

Nesting - Apparently defends a nesting territory against conspecifics, aggressively when nesting at high densities (Smith and Murphy 1973). No data on nesting territory size.

II. Time of occurrence and seasonal movements.

Northern populations completely migratory (Bechard and Schmutz 1995), but little is known of migration of southern breeding populations.

A. Arrival date on breeding grounds: Late February to early April depending on latitude (Schmutz and Fyfe 1987). In South Dakota, first arrivals were observed on 24 March, 1976, and 27 March 1977. Late March in North Dakota (Gilmer and Stewart 1983). Last week in February or first week in March (Smith and Murphy 1973).

B. Departure date from breeding grounds: August (young) and late September to early October (adults) (Schmutz and Fyfe 1987). In South Dakota, first week in September (Blair and Schitoskey 1982).

C. Spring migration period: Unknown

D. Fall migration period: Ferruginous Hawks are rarely detected in fall migration in California away from the breeding grounds.

E. Extent of wintering in California: Winters extensively in California. Between 1980 and 1997, detected on 134 of 161 (83.2%) CBC routes, in every physiographic region, and in every California county.

III. Migration stop-over needs/characteristics: Ferruginous hawks breeding in the north and east great plains apparently migrate east of the Continental Divide following grasslands feeding on ground squirrels (Spermophilus spp.) and prairie dogs (Cynomys sp.) while western birds follow desert habitats and feed on lagomorphs (Lepus spp., Sylvilagus spp.) (Bechard and Schmutz 1995).

IV. Nest type: Open platform constructed of sticks, sagebrush stems, twigs and debris from ground (Bechard and Schmutz 1995). Prior to 1900, nests in N. Dakota were often partially constructed of bleached bison ribs (Houston and Bechard 1984).

V. Foraging strategy: Bechard and Schmutz (1995) describe four types of foraging pursuit patterns: still hunting from perches, short-distance strikes originating from the ground (on ground squirrels and gophers), aerial hunting (infrequent), and hovering. Typically forage from the ground or a low perch, over a short distance, targeting ground squirrel colonies. May employ longer and steeper flights when chasing lagomorphs.

VI. Displays: Prior to breeding, adults may engage in aerial displays involving diving, screaming, and spiraling toward the ground with interlocked talons (Powers 1981). Exhibits aggressive displays towards conspecifics in defending breeding territories or prey (Smith and Murphy 1973).

VII. Social Organization: Wintering grounds - Solitary and apparently widely spaced during foraging.

A. Typical breeding densities: Blair and Schitoskey (1982) reported densities of one pair per 292 km2 and 412 km2 for two consecutive study years in South Dakota. Reported densities in other states include one pair per 17.4 km2 ([north-central South Dakota] Lokemoen and Duebbert 1976), one pair per 39.9 km2 ([Utah] Weston 1969), one pair per 99.9 km2 ([Colorado] Olendorff 1973 inBechard and Schmutz 1995), approximately one pair per 33 km2 ([North Dakota] Gilmer and Stewart 1983). Breeding densities are apparently limited by suitable nest sites. In Alberta, artificial nest platforms were readily used and increased densities from "already high" to approximately one pair per 4 km2 to 15 km2 (Schmutz and Moore 1984).

B. Mating system: Apparently monogamous but 3 adults observed at some nests (Houston in Bechard and Schmutz 1995) where breeding status is unknown. Some pair bonds may be maintained year-round (Bechard and Schmutz 1995).

C. Delayed breeding: No information

D. Post fledging biology of offspring: Altricial young. Young first leave nest at 38 to 50 days with males leaving as much as 10 days before females (Powers 1981). Despite hunting attempts in which success was never observed, fledglings dependent on nest site for food for approximately two weeks post-fledging (Blair and Schitoskey 1982). In South Dakota, maximum area (ha) used by foraging juveniles (n=6) increased from 60 (±46.2) during the first post-fledging week to 488 (±318) during the fourth post-fledging week (Blair and Schitoskey 1982).

E. Post breeding social behavior: No information.

XII. Clutch size: In United States, reported clutch sizes vary from 4.3 (n=21) in South Dakota (Lokemoen and Duebbert 1976) to 2.8 (n=65) in Idaho (Howard and Wolfe 1976).

VIII. Incubating sex (female/male): Primarily female. Male may incubate but with decreasing frequency as incubation proceeds (Powers 1981). Occasionally both male and female incubate although males primary role is hunting and nest guarding (Bechard and Schmutz 1995).

IX. Incubation period: Estimated at 28 days (Bent 1937), 32 days (Palmer in Bechard and Schmutz 1995), and 35 days (Olendorff 1973 in Bechard and Schmutz 1995). Young hatch over a period of 2-4 days (Bechard and Schmutz 1995).

X. Nestling period: Reported as 2 months (Bent 1937), and 38 to 50 days (Bechard and Schmutz 1995). In South Dakota, median egg laying dates, assuming a 35 day incubation period, were 21 April 1976, and 19 April 1977. Therefore, in South Dakota, nestling period would be approximately May 26 to about July 7.

XI. Development at hatching: The following account of hatchling development is modified from Bechard and Schmutz (1995). Young lie or sit for first 2 weeks but can seek shade or shelter after 5-7 days . Nestlings stand at 18-20 days, attempt self-feeding at about 11-12 days, and begin casting pellets at 16-18 days, and seize food at 22-23 days. Capable of flapping and jumping at 33-34 days.

XII. Number of broods: No reports of multiple broods.

XIII. Who tends the young: Primarily female although male will tend young for brief periods later in the nestling period (Bechard and Schmutz 1995). One incidence of the female discouraging the males presence at the nest was documented (Schmutz 1995).

XIV. Diet:

A. Major food items: See Table 2.                     B. Drinking: Drinks in captivity.

XV.   Site Fidelity: Uses 1 nest per year. Nests may be used in ³ 1 year even when vacated for the intervening year (Bechard and Schmutz 1995, Bent 1937).

XVI.  Timing of Breeding: This category seems redundant.

Table 2.
Ferruginous hawk prey from 20 studies throughout the species range (except California). Adapted from Bechard and Schmutz (1995).

Sample Size
Percent Occurrence
Percent Biomass
squirrels and prairie dogs
pocket gophers
kangaroo rats
Amphibians and Reptiles


HABITAT: Variables to be considered when evaluating habitat characteristics. This section is broken into three subsections; at the nest, vegetation surrounding the nest, and larger landscape factors . (This section should be revised to remove obvious bias for riparian and tree nesting species and species which breed in CA. Also, in most veg. classification systems, canopy refers to the uppermost vegetative layer which, in some cases, is not trees. Appears here canopy refers only to trees. I assumed uppermost layer regardless of form. Also, I vote for considered grass or herbaceous cover as a distinct layer instead of including it as a component of ground cover. - kh)

I. Nest Site

A. Substrate (species): Nests most frequently (49%, n=2119) in trees (often Juniperus sp.), or shrubs followed by cliffs (21%) utility structures, and ground outcrops (10%) (Olendorff 1993). Eastern cottonwoods (Populus deltoides), peachleaf willow (Salix amygdaloides), box elder (Acer negundo) and green ash (Fraxinus pennsylvanica) were common nest trees in North Dakota (Gilmer and Stewart 1983). B. Height of nest: May be on ground, in small tree, or on elevated utility structure. Thirty five ground nests in South Dakota ranged from 0-25 m above the surrounding prairie (Blair and Schitoskey 1982). In Washington, Bowles and Decker (1931) observed a nest at about 9m in a Juniper tree, and at about 10m in a locust (Locust sp.) tree. Also in Washington, Fitzner et al (1977) measured 65 nests ranging between 3.5 and 7.5 m above surrounding ground. Artificial nests successfully used by Ferruginous hawks in Alberta, Canada were between 1.9 and 4.3 m above ground level (Schmutz and Moore1984).

C. Height of plant: No information.

D. Objects/Plants concealing nest: Usually nests in solitary trees or remote cliff sites (Bechard and Schmutz 1995). Isolated tree nests usually in transition zone between shrub-steppe and forested habitat types (Snow 1974).

E. Percent nest cover: Nests usually relatively exposed. Apparently favors unconcealed nests as some pairs have abandoned traditional tree nests in favor of artificial, exposed nests stands (Bechard and Schmutz 1995).

F. Average nest tree DBH: No information.

II. Vegetation surrounding the nest: In central Idaho, Wakeley (1979) lumped vegetation cover surrounding nests into one of four categories: dense (>75% cover), moderate (20-75% cover), sparse (5-20% cover), and absent (<5% cover). Ferruginous Hawks selected the dense cover class for foraging more frequently than predicted by availability. Also in Idaho, Howard and Wolfe (1976) noted desert shrub-steppe with an understory of crested wheatgrass (Agropyron cristatum) was the dominant vegetation type surrounding nests. Gilmer and Stewart (1983) noted that in North Dakota, pasture and haylands comprised 94.8% of land surrounding tree nests and 76.5% of land surrounding ground nests (n=340). In South Dakota, Blair and Schitoskey (1982) found 55% of nests in barren types and the remaining in ungrazed or lightly grazed prairie (n=35).

III.. Average top canopy height: No information

IV. Dominant plant species in canopy: see Nest Site, Substrate above.

V. Average shrub cover: Nesting habitat described in general terms as grasslands, sagebrush (Artemesia spp.), saltbush (Atriplex)-greasewood (Sacobatus vermiculatus) shrub lands, and the periphery of western pinyon (Pinus sp.)-juniper woodlands and other forests (Bechard and Schmutz 1995). No information available on co-dominant or associated species cover or composition.

VI. Co-dominant plant species in canopy: See D above.

VII. Dominant shrub species: See D above.

VIII.. Co-dominant shrub species: See D above.

IX. Average forb cover: See D above.

X. Dominant forb species: See D above.

XI.. Co-dominant forb species: See D above.

XII. Ground cover:

1. logs: No information
2. grass/sedge: Predominantly wheatgrass (Agropyron sp.) and needle grass (Stipa sp.) in eastern range. No information in west and inter-mountain west.
3. water: No information. Reports of nesting in riparian zones (Bechard and Schmutz 1995, Gilmer and Stewart 1983, Restani 1991).
4. leaf litter: No data
5. rock: No information
6. bare ground: See II above
7. other: No information
XIII.. Slope: Nests usually located in valley bottoms, shrub-steppe, or desert scrub habitats with little relief. However, ground nests are usually located at prominent relief points such as small cliffs or abrupt knolls. Restani (1991) demonstrated no preference for slope in a study of nesting birds in Montana.

XIV.  Aspect: Restani (1991) demonstrated no preference for aspect in a study of nesting birds in Montana. No information for other areas.

XV.  Tree DBH: No information

XVI..  Snags: n/a

XVII.  Distance to water: No specific information but will nest in riparian areas (see 2. Grass/sedge above).

III. Landscape factors

A. Elevation: Breeding - Breeding elevations of 1,974 to 2,544 m reported for Montana (Restani 1991), 1,460 -2,440 m in Utah (Smith and Murphy 1973). No information for other locations.

B. Fragmentation: Continuing loss of sparse grasslands in both breeding and wintering grounds has resulted in fragmented distribution.

C. Patch size: No information

D. Disturbance (natural or managed): No information

E. Adjacent land use: see II above.

F. Climate: Breeding - Hot and dry with average annual precipitation ranging from 30 to 38 cm and mean maximum temperatures of 16°C (April), 21°C (May), 27.2 °C (June), and 30 °C (July). Wintering - Cool and moist. Climates typical of North American great plains and inter-mountain west and Mediterranean conditions in the far west.

G. Other:

IV. Notes

SPECIAL FACTORS: Factors influencing occurrence and viability.

I. Brood parasitism: Not applicable.

II. Dietary: Western and some inter-mountain west populations trends correlated with abundance of jackrabbits (Lepus sp.) [density-dependent]. In Utah, Woffinden and Murphy (1977) documented the relationship and dynamics of dependence and subsequently confirmed the relationship over a 20 year period (Woffinden and Murphy 1989).

III. Sensitivity to human-induced disturbance: Breeding - Very sensitive to disturbance during incubation (Bechard and Schmutz 1995) and nest abandonment from human disturbance documented in several areas (e.g., Fitzner et al 1977, Smith and Murphy 1973, Smith and Murphy 1978). In Idaho, White and Thurow (1985) found a significant difference in nest desertion between nests with created disturbance designed to simulate human activities and control, undisturbed nests (n=62). The Bureau of Land Management has documented nest abandonment after a single visit by researchers and consider nest abandonment a potentially "severe population limiting factor" (Snow 1974). Productivity is apparently influenced by nesting substrate with lower productivity documented in human-created substrates (Snow 1974), but, in some areas, a higher incidence of use of these substrates (Gilmer and Stewart 1983). Olendorff (1973 inBechard and Schmutz 1995) calculated a 24.5% greater probability of fledging from a remote nest site compared to nest sites with public access (n=43). These data suggest human-created nest substrates may be a population sink. Wintering - Little information. In one instance, 4 Ferruginous Hawks congregated within 10 m of a vehicle and collected prairie dogs as they were shot (Chesser 1979).

IV. Pesticide use: Little data. Bechard and Schmutz (1995) suggest pesticides are little threat although impacts from strychnine poisoning of ground squirrels (Spermophilus sp.) are possible.

V. Predators: No data for California. Tree nests are predated by Great-Horned Owls (Bubo virginianus) and Corvids (Corvus sp.) while ground nests may be predated by badgers (Taxidea taxus), foxes (Vulpes sp.), and coyotes (Canis latrans) (Bechard and Schmutz 1995).

VI. Exotic species invasion/encroachment: No data

VII. Other/Notes:

POPULATION TREND: Declining in some areas, but decline factors and effects of these factors poorly understood (Bechard and Schmutz 1995). Trend correlated with prey abundance in some areas (see Dietary factor above) and populations presumably experiencing effects of several threat factors during low prey abundance may be extirpated (Woffinden and Murphy 1989). Severe declines, with local and regional extirpations, documented in Saskatchewan (Houston and Bechard 1984). In California, population apparently stable despite continuing significant loss of grassland habitats (Garrison 1990) although caution should be exercised in interpreting CBC data (see Status above for more information on current trend).

DEMOGRAPHICS: Evidence of two year-old breeding but no record of first year (HY) breeding. Productivity - Between 72% and 82% nest success in South Dakota (Blair and Schitoskey 1982); 64% to 75.9% in North Dakota (Gilmer and Stewart 1983); 45.6% in Idaho (Steenhof 1995 in Bechard and Schmutz 1995). Reproductive output (fledglings per breeding pair per year) - 2.9 in Idaho (Thurow et al 1980 in Bechard and Schmutz 1995); 2.2 in North Dakota (Gilmer and Stewart 1983); 2.1 in South Dakota (Lokemoen and Duebbert 1976); 2.0 in Utah (Smith and Murphy 1978); 2.5 in Washington (Fitzner et al 1977). Survivorship - Banding data suggest 65% first year mortality rate (Schmutz and Fyfe 1987) and Woffinden and Murphy (1989) estimate overall annual mortality at 25%. Sources of Mortality - Predation by Great-horned Owls (Bechard and Schmutz 1995), exposure (Snow 1974, Tomback and Murphy 1981), shooting (Smith and Murphy 1973), and other sources.


HABITAT AND POPULATION OBJECTIVES: No habitat or population objectives determined for this species. Olendorff (1993) estimated the global (North American) population at between 5,842-11,300 individuals although Schmutz et al (1992 in Bechard and Schmutz 1995) estimated the population in the great plains grasslands at 14,000 individuals. Assuming a conservative 2.5 individuals per count circle detected on CBC routes (mean for all CA circles detecting FEHA 1990-97), the average wintering California population could be between 400 and 500 individuals.

Although data are lacking for California, Ferruginous Hawks clearly can forage in former grasslands converted to certain types of cover crops (Gilmer and Stewart 1983, Bechard and Schmutz 1995). However, given the lack of data on winter territory size, winter habitat requirements, and the role of lagomorph abundance to winter distribution in California, it is premature to rely on agricultural lands for long-term viability of winter Ferruginous Hawk populations. While no habitat objectives have been described for this species, sufficient grasslands, grassland/agricultural matrices or desert scrub should be conserved to support current population levels. Using breeding ground home ranges, ranging from 5.9 km2 per bird in Utah (Smith and Murphy 1973) to 7.6 km2 per bird (males) in Idaho (Bechard and Schmutz 1995), as a coarse guide of winter habitat requirements, between 2,360 and 3800 km2 (583,000 - 939,000) acres of habitat would be required to sustain estimated population levels. Using the Utah and Idaho home range estimates, the estimated 607 km2 (150,000 acres) of remaining grasslands in the central valley would support between 80 and 103 birds (approximately 21% of the estimated average annual wintering population).

Ferruginous Hawks often use communal roosts on wintering grounds (Bechard and Schmutz 1995, Steenhof 1984) presumably to facilitate hunting. Communal roosts are typical in areas of abundant food supply but not necessarily limited roost sites (Steenhof 1984). Important communal roost sites in California include the Cuyama Valley (San Bernardino County), Harper Dry Lake, (Los Angeles County), and Lake Henshaw (Riverside County). These areas support extensive non-native grasslands and desert scrub habitats around which Ferruginous Hawk conservation areas could be developed.

The Bureau of Land Management (BLM) developed a system of identifying important raptor areas (Key Raptor Areas [KRA]) based on criteria including number of raptor species, importance to populations, and presence of special habitat features. Harper Dry Lake has been designated a KRA and identified as an area of Critical Environmental Concern based partly on communal roosting by Ferruginous Hawks (Grindrod 1997). Similar Ferruginous Hawk conservation focus areas should be developed in the central valley and might include the Los Banos, Mendota, and Grasslands complex of southern San Joaquin valley Wildlife Areas.

River bottoms along the north coast in Humboldt and Mendocino appear important to wintering and migratory Ferruginous Hawks. This species regularly winters in these habitats and is often observed at surprisingly high densities of 8.2 birds/day in the Eel River bottoms (Kidd, pers./ comm.). North coast grasslands may offer an opportunity for conservation and research of Ferruginous Hawk foraging ecology and habitat use.

MONITORING METHODS AND RESEARCH NEEDS: Monitoring - Ferruginous Hawk populations in California are probably well monitored by the CBC as sample sizes, abundance at circles with detections, and distribution of circles with detections are sufficient for most analyses. However, CBC data should be standardized by number of observers, observer hours expended, or other factors to account for count circle effort (Raynor 1975). Garrison (1990) analyzed CBC data for Ferruginous Hawks in California for the period 1950-1987 compensating for variations in search effort and observer bias by standardizing by 100 party miles and found an increasing trend. Sources of error in estimating trend for Ferruginous Hawks using CBC data include bias of location of old compared to more recent count circles (older circles in or near major cities) and increased awareness of raptors over the CBC census period (Garrison 1990). Despite sampling bias and other error sources, CBC data are probably the best monitoring tool available for detecting gross changes in trends in California Ferruginous Hawk populations.

Communal roosts, large, unfragmented grasslands and desert scrub lands, and other important raptor areas should also be monitored to determine annual use by Ferruginous Hawks. These areas are consistently used by this species and awareness by local Audubon chapters and the birding community is sufficient for annual passive monitoring.

During 1971-1974, the Department of Fish and Game designed and implemented a raptor census utilizing prescribed route and road-side survey techniques. The focus of the census was the southern Sacramento Valley but the methods employed could be applied to other regions. The census established a good baseline data set from which future sampling could be done to determine local and regional trends.

Research - Bechard and Schmutz (1995) establish the following research priorities for Ferruginous Hawk populations:

A) Wintering ecology; especially in Mexico

B) Xeric environment adaptations

C) Dispersal

D) Apparent density-dependent relationship with lagomorphs

E) Factors affecting migration and sub-population taxonomy and morphology

A thorough review of available literature revealed a lack of information on Ferruginous Hawks wintering California. While the CBC is adequate for determining distribution and local abundance patterns, little is known of the species winter ecology. Research priorities in California should include:

1) Winter range and spatial habitat use and requirements

2) Foraging ecology (relationship to CA lagomorph abundance)

3) Winter source populations

4) Population limiting factors (e.g., roost sites)


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