Golden Eagle, Aquila chrysaetos, Bald Eagle
The golden eagle (Aquila chrysaetos) is a large, dark-brown raptor with long, broad wings and a golden nape (Kochert et al. 2002). Total length ranges from 70-84 cm (28-33 inches) and wingspan ranges from 185-220 cm (73-87 inches) (Kochert et al. 2002). Weight of these large raptors ranges from 3,000 to 6,125 g (8.7 – 13.5 lbs), with females approximately 25% larger than males (Kochert et al. 2002). Males and females are otherwise identical. Juvenile birds have tails that are white at the base and dark at the tip, and have white patches on the underside of the wing (Kochert et al. 2002).
Seasonal Patterns
Throughout most of the western United States golden eagles are mostly year-long residents (Polite and Pratt 1999), breeding from late January through August with peak activity in March through July (Polite and Pratt 1999). Migratory patterns are usually fairly local in California where adults are relatively sedentary, but dispersing juveniles sometimes migrate south in the fall (Kaufman 1996). They may move attitudinally in
response to changing weather conditions (USFS 2008). They may also move down-slope for winter or upslope after the breeding season (Polite and Pratt 1999).
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| Golden Eagle-Bad Eagle |
Golden eagle populations in the northern portion of its range are generally migratory with populations in the extreme northern portion being entirely migratory (USFS 2008). Some northern populations winter in California.
Golden eagles construct a large nest of branches, twigs, and stems of any kind (Kochert et al. 2002). Maintenance on the nest can occur at any time of year (Kochert et al. 2002). Golden eagles often maintain alternate nest sites within a breeding territory, and old nests are refurbished and reused (Zeiner et al. 1990). Females lay 1-3 eggs in early February to mid- May (Polite and Pratt 1999). Incubation typically takes 43-45 days (Beebe 1974) and the nestling period usually 65-70 days (Polite and Pratt 1999). The male delivers
food to the female, and the female feeds the young (USFS 2008). The young fly at about 50 days, remaining near the nest site for a few weeks (Baicich and Harrison 1997, Zeiner et al. 1990). Breeding site fidelity in adults is high (USFS 2008).
Golden eagle breeding success is variable and often fluctuates in close correlation with prey population abundance (USFS 2008). Annual reproductive success rates in Oregon were correlated with jackrabbit abundance, with a 15-year mean of 1.08 young fledged per breeding territory, 1.7 young fledged per successful nest, and 51% of the nests successful (Johnsgard 1990). More recently, Hunt et al. (1999) reported natality estimates of 0.64 and 0.58 young per pair for 57 and 59 pairs, respectively, in 1996 and
1997, within a 190-sq km (73 sq mi) study area in the central Coast Ranges. Golden eagles are considered to be long-lived birds. Captive golden eagles have lived more than 40 years, and one captive bird reached 48 years of age (USFS 2008). There is one record of a wild golden eagle living at least 20. Another eagle (a female) was known to live for 30 years (Brown 1977, Roberson and Tenney 1993).
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| Golden Eagle - Aquila chrysaetos |
Home Range/Territory Size
Home ranges are likely the same as territory size (Polite and Pratt 1999). Size of home range related to prey density and availability, and openness of terrain (Polite and Pratt 1999). Territories are well defined and actively defended (USFS 2008). Pairs tend to nest on the periphery of their territories, often near an adjacent pair (USFS 2008). Golden eagle territories in southern California were found to average 93 km2 (36 mi2)
(Dixon 1937), while northern California territories average 124 km2 (48 mi2) (Smith and Murphy 1973). Territories are generally larger in open grassland habitats than in more complex, mountainous terrain (Roberson and Tenney 1993). Territories are defended year-round by use of aerial behaviors such as undulating flight displays, dives, mock attacks, and soaring or "hanging on the wind" over the area (Johnsgard 1990).
Foraging Behavior and Diet
Golden eagles primarily prey on lagomorphs and rodents but will also takes other mammals, birds, reptiles, and some carrion (Polite and Pratt 1999). Studies of golden eagle diet indicate that mammals comprise 82 percent of the diet, supplemented by birds at 12.6 percent, with the remainder consisting of reptiles and fish (USFS 2008). Throughout its range, mammalian prey most commonly taken includes black-tailed jackrabbit, arctic ground squirrel (Spermophilus parryi), white-tailed jackrabbit (Lepus townsendii), and yellow-bellied marmot (Marmota flaviventris) (Johnsgard 1990). Avian prey includes waterfowl and wading birds to the size of Canada goose (Branta canadensis), great blue heron (Ardea herodias), and cranes (Grus spp.) (Dunne et al. 1988).
Lagomorphs make up most of the diet of golden eagles throughout North America (Olendorff 1976). Several studies have suggested a relationship between golden eagle reproductive success and density of black-tailed jackrabbits (Lepus Californicus) (Murphy 1975, Kochert 1980, Thompson et al. 1982). In most areas, however, a wide range of prey species are taken (Thelander 1974, Olendorff 1976, Bloom and Hawks
1982, Collopy 1983). Some studies indicate that golden eagles shift their selection of prey species with changes in prey density (McGahan 1968, Bloom and Hawks 1982, Steenhof and Kochert 1988).
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| Golden Eagle |
In the interior central Coast Ranges of California, golden eagles forage primarily in grazed grasslands, open shrublands, and oak savanna communities supporting large populations of ground squirrels (Spermophilus spp.) (Carnie 1954). Most studies of golden eagle foraging habits have shown that ground squirrels (S. beecheyi) are the principal prey item of golden eagles in the interior central Coast Ranges (Dixon 1937, Carnie 1954, Connelly et al. 1976, Hunt et al. 1999). Golden eagles hunt by soaring high (30-90 m [98-295 feet]) along open ridges; they also make low (7-8 m [23-26 feet]), coursing flights (Polite and Pratt 1999). They will occasionally hunt from an exposed perch, where they initiate directed flights toward prey (Zeiner et al. 1990).
Habitat Requirements and Ecology
Golden eagles nest primarily on cliffs and hunt in nearby open habitats, such as grasslands, oak savannas, and open shrublands (Grinnell and Miller 1944). Rugged, open habitats with canyons and escarpments are used most frequently for nesting (Polite and Pratt 1999). Trees may also be used for nesting and are more commonly used in the interior Coast Ranges where suitable cliff nesting habitat is scarce. Nest trees include
several species of oak (Quercus spp.), foothill pine (Pinus sabianiana and P. coulteri), California bay laurel (Umbellularia californica), eucalyptus (Eucalyptus spp.), and western sycamore (Plantanus racemosa) (Hunt et al. 1999). Nests are typically located 3-30 meters (10-100 ft) above the ground (USFS 2008). They are known to occur from sea level up to 3,833 meters (0-11,500 ft) (Grinnell and Miller 1944).
Important elements of suitable breeding habitat include 1) cliff ledges, rocky outcrops, or large trees for nesting; 2) open foraging terrain such as grassland, desert, savannah, and shrubland; and 3) availability of small to medium-sized mammalian prey, particularly ground squirrels and rabbits (Zeiner et al. 1990, Kochert et al. 2002). In southern California, golden eagles generally avoid heavily forested mountains, the coast, and
urban areas (Ferguson-Lees and Christie 2001, Garrett and Dunn 1981). In central California, golden eagles have nested primarily in open grassland and oak savannah, and occasionally in oak woodland and open shrubland habitats (Hunt et al. 1995, 1999).
Species Distribution and Population Trends
Distribution
In North America, the golden eagle breeds mainly in western portion (west of the 100th meridian) of the continent from Alaska south to central Mexico, with small numbers in eastern Canada and a few isolated pairs in eastern United States (Kochert 2002). In California, the species is an uncommon permanent resident and migrant throughout the state, except for the center of the Central Valley (Polite and Pratt 1999). It is considered more common in southern California than in the northern part of the state (USFS 2008); however, the central interior Coast Ranges support a significant nesting and wintering population (Hunt et al. 1999).
Population
The golden eagle appears to be thriving in North America; however, relatively few reliable population estimates exist (USFS 2008). Approximately 25,000 pairs have been estimated for the New World, and a world population of 50,000-75,000 pairs was conservatively estimated in 2001 (Johnsgard 1990). However, long-term survey data indicate overall population declines in most of the western United States (Kochert andSteenhof 2002). Golden eagle populations have declined, in particular, near human population centers (Thelander 1974, Scott 1985). Additionally, declines have been noted, especially along the southern California coast, as a result of habitat loss (Ferguson-Lees and Christie 2001).
The nesting distribution of golden eagles in Yolo County is restricted to the high elevation mountainous areas on the western side of the county. Like most of the interior Coast Ranges, this area is dominated by a mixture of oak woodland and chaparral communities, steep topography, and rocky ridges. There are potential cliff nesting sites along Blue Ridge and other high elevation ridge lines, some isolated rock outcrop sites that are capable of supporting golden eagle nests, and numerous potential tree-nesting sites. There are few official records of golden eagle nests in the area (CNDDB reports no nesting golden eagles for Yolo County [CNDDB 2008]); however, several have been incidentally reported over the years and are likely extant.
