Golden Eagle Aquila chrysaetos
Version: 2.0 — Published September 17, 2020
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A large, heavy-bodied, dark-brown, long-winged raptor (length 70–99 cm, wingspan 185–222 cm; male mass 2,387–4,500 g, female mass 3,048–6,460 g). Measurements and descriptions are from Clark and Wheeler (22), Watson (2), Lish et al. (23), and others as noted. Plumage changes with age. Adults (> 5 years old) are predominantly dark brown but have faint gray bars on the tail and golden, sometimes faded, feathers on the rear of the crown, the nape, and the sides of the neck. For birds older than ~1 year, the leading edge of the wing and upperwing coverts are generally paler than the rest of the feathers. The latter form light/tawny diagonal bars on the upper wing that are visible both on flying and perched eagles. Undertail coverts have been described in different ways, usually as brownish gold to dark rufous, and the feathered tarsi vary from white or light cream color to dark brown (24). The bill is tipped with black, fading to grayish near the base, and the cere is yellow. Sexes are similar in appearance, but females tend to be larger than males, and there may be sex-related differences in the pattern and number of tails bands of adults (see Appearance: Plumages: Definitive Basic Plumage; 25, 26, 2). Plumages are the same throughout the year, although fresh feathers are dark, shiny, and smooth on the edges. Differential fading occurs (27), with old feathers generally appearing faded, more brownish or even yellow white, and frayed on the edges (see Appearance: Molts). Occasionally, some individuals have small white “epaulets”, composed of a variable number of feathers at the upper end of the scapulars (28, 29); the trait appears to be uncommon, but widespread across North America (29), and may be referred to as the barthelemyi or Barthelemyi variant (30, 29, 31); see Appearance: Plumages.
Adult (> 4 yr) plumage differs from juvenile (0–1 yr) and subsequent subadult (1–3 yr) plumages (see Appearance: Plumages). Compared to adults and subadults, juveniles have a uniformly darker color when they first leave the nest. Unlike adults, juveniles usually have white at the proximal base of secondaries and inner primaries (32). These white areas form a white “window” at the carpal joint of the wing, visible in flight from above and below. Occasionally some upperwing coverts are also white (33). The amount of white on the wing varies among individuals, and some juveniles lack it altogether (22). The basal two-thirds of the tail of juveniles is usually white with a wide, dark band at the tip and a narrow white terminal band. Rectrices have some dark flecks, particularly near the dark band (24). The amount of white in the tail and wing varies but tends to diminish gradually with each progressive molt (see Appearance: Molts). However, some older individuals may retain white in the tail (24, EHC; T. Craig, personal communication). Adult plumage is usually acquired in the fifth summer. Physiological condition, stress, or damage to feathers can influence the rate and timing of molt of individual feathers and thus the appearance of the bird (24, 34, 35; see Appearance: Molts). It has recently been speculated that the bold contrasting plumage of young Golden Eagles may be a signal that these younger birds may be more aggressive, and thus willing to fight over a carcass meal (36).
Distinguished from most other raptors in North America by a combination of large size and mostly dark-brown color. The wings of Golden Eagle are longer and narrower than those of many smaller Buteo hawks. Similar in size and color to juvenile and subadult Bald Eagle (Haliaeetus leucocephalus) that have not acquired the distinctive adult white head and tail. The most striking differences involve distribution of white in the plumage and the size and shape of the head (37). Bald Eagle has a more elongated head profile, with a larger and more protruding head and bill, whereas Golden Eagle has a smaller head and bill (38, 39). When perched, Bald Eagle often holds its head feathers semi-erect, making the head appear more 'blocky' than that of Golden Eagle. Young Bald Eagle has a darker (blackish) bill and cere than Golden Eagle, which has a yellow cere. Bald Eagle juveniles and subadults have irregular patches of white or tawny coloration on their bodies, especially the underparts and under the wing (coverts, flight feathers, axillaries) and tail. Golden Eagle lacks extensive white on the body and the white on the wing and tail is not patchy. White on the undersurface of the wing in Golden Eagle is restricted to the base of flight feathers and white on the undersurface of the tail is sharply divided from the wide, dark, terminal band. Individual flight feathers of both species have white and brown patterns. However, the patterns on Golden Eagle are marbled, whereas feathers are spotted on Bald Eagle (40). Structural and behavioral differences are also useful in distinguishing these species. In flight, the head of a Golden Eagle does not project more than half the length of the tail, whereas the head of a Bald Eagle projects more than half the length of the tail (38, 41). In addition, the Golden Eagle has long outer secondaries that produce a noticeable round bulge, or shallow ‘S’ shape, on the trailing edge of the wing. The trailing edge of the wing is straighter for Bald Eagle. At close range, the Bald Eagle has naked tarsi, in contrast to the feathered tarsi of the Golden Eagle (22). Golden Eagle generally has shallower wing beats than Bald Eagle, and when soaring, does not hold its wings as flat as Bald Eagle (38, 39).
Soaring Golden Eagle could be confused with Turkey Vulture (Cathartes aura) , Black Vulture (Coragyps atratus), or California Condor (Gymnogyps californianus) . Turkey Vulture has a small head and is smaller overall; soars with a rocking motion and wings are held in a more pronounced dihedral; underwings are black in front and silver on the trailing edge. In contrast, Golden Eagle soars with wings in a much shallower dihedral and does not rock while soaring, except in very high winds (38, 41, TAM). Black Vulture has a very short tail, and the outermost primaries are all white. California Condor is larger than Golden Eagle but has a relatively short tail, a head that does not extend as far forward in flight, and unlike Golden Eagle, has an anterior white (adults) or white or mottled brown (pre-adult: any bird that has not attained adult plumage) underwing lining that forms a long narrow triangle (42).
In Eurasia and Africa, Golden Eagle can be confused with a wide variety of species. These include several other species of eagles in the genera Aquila, Clanga (formerly Hieraaetus), and Haliaeetus, as well as, other predatory raptors in the genus Buteo, Milvus, Pandion, Pernis, and Circaetus. Probably the most confusing species are Imperial Eagle (Aquila heliaca; 43), Steppe Eagle (A. nipalensis) , Greater Spotted Eagle (Clanga clanga) and Lesser Spotted Eagle (C. pomarina), and the many "brown" eagles of Africa (Wahlberg's Eagle [Hieraaetus wahlbergi], Tawny Eagle [Aquila rapax], Brown-Snake Eagle [Circaetus cinereus]) At a distance they may also be confused with vultures in the genera Gyps , Gypaetus, Neophron, and Aegypius. For details on identification in Europe, see Forsman (43). Confusion with other species is possible in Africa and Asia. In all regions, habitat associations may clarify identification, with the Golden Eagle more likely in mountainous terrain than Imperial or Steppe Eagles, and less likely around water than the Haliaeetus eagles.
Golden Eagles have 10 full-length primaries, numbered p1 (proximal) to p10 (distal); 14–15 secondaries, numbered s1 (distal) to s14 or s15 (proximal); 3 tertials, numbered t1 (proximal) to t3 (distal); and 12 rectrices, numbered r1 (central) to r6 (outer) on each side of the tail. Accipitrid hawks, a group that includes the Golden Eagle, are diastataxic (see 44), indicating that through evolution, a secondary has been lost between what we now term s4 and s5. Wings are long and rounded on the tips. Among the primaries, p6 and/or p7 are longest, the outer webs of p4–p8 are emarginated, and the inner webs of several primaries, usually p5–p10, are notched.
The following plumage and molt descriptions pertain to the North American subspecies A. c. canadensis and are taken primarily from detailed plumage descriptions in Friedmann (45), Spofford (28), Oberholser (46), Cramp and Simmons (47), Clark and Wheeler (22), Palmer (29), Johnsgard (33), Forsman (43), Wheeler (48, 49), and Watson (2). For specific age-related criteria on plumages, see Jollie (24), Bortolotti (50), Tjernberg (51), Baker (52), Bloom and Clark (53), Liguori (54), Liguori et al. (55), Ellis (27), Ellis and Lish (56) and Pyle (26). Plumages are similar for each sex, with sometimes indistinct gender differences in the tails of adult male and female eagles (see Definitive Basic Plumage; 25, 26, 2). Definitive (adult) appearance usually is assumed at the Third or Fourth Basic Plumage. Occasionally, some upperwing coverts are white, and scapulars may have white coloration (i.e., epaulets; 28, 33); see Other Plumages, below.
Prepennae down is short, pale gray with dark tips (24) or white (57) and present on nestlings at hatching. Patches of skin are exposed along the vertebrae and ventral surface (24). Down is darker on the nape, back, and upper surface of the wings. Down around the eye and edge of the eyelid is dark. Short, thick, white down rims the ears (24, 58). Long, white, preplumulae down emerges at about day 6, progressively obscuring the prepennae down by about day 15 (24). Preplumulae down grows for about 30 days, forming a dense, nearly waterproof covering (29).
Juvenile (First Basic) Plumage (HY/2Y)
All North American Golden Eagles have juvenile plumage from May to December of their first year, and most retain this plumage through March–May in the year after hatching. The duration with which this plumage is retained depends on the occurrence and timing of Preformative Molt (see Molts: Preformative Molt). The overall color of recently fledged eagles is dark brown, sometimes tending toward rufous brown, except for the lanceolate feathers on the head and nape, which are golden brown. Exposure to the sun and elements causes the color of feathers to fade as the year progresses (TAM, EHC). The inner secondaries and primaries usually have white bases that show in flight as white underwing patches, although some juveniles lack white entirely in the wing (22).
Juvenile rectrices appear predominantly white with a dark terminal band of varying width and usually a distinct border between the white and dark part of the feather. Occasionally, the tail is mostly dark with minimal white at the base and the tip (24). The white terminal band can wear as feathers age, but remnants usually are visible for the life of many juvenile rectrices (24, 56). Dark flecks, blotches, grayish washes, or lines are sometimes present in the white area of the rectrices, particularly near the border between white and dark (56; see Figure 340 in 26). Some juvenile rectrices exhibit gray marbling in the dark subterminal band. Similarly, some juvenile secondaries and occasionally primaries may have gray marbling in the dark terminal area (27). The exposed portions of the central and outermost rectrices may exhibit darker pigmentation and may be more worn and faded than rectrices that are not exposed when perched (27, 56).
The undertail coverts are lighter than the other underparts but this difference is not always obvious. Juvenile plumage is distinctive in that all feathers are of the same age, show the same amount of wear, and generally are uniform in color, shape, and length. Exceptions to this pattern is replacement of accidentally lost feathers, which are not usually at molt centers, nor are they molted symmetrically (53, 27).
Formative Plumage (HY/2Y)
The Formative Plumage is equivalent to the "First Basic" or "Basic I" plumage of Humphrey and Parkes (59) and later authors; see revision by Howell et al. (60). This plumage appears to be absent or rare in eastern North America (TAM, M. Lanzone, unpublished data) but may be present from December–March or April in some individuals (see Preformative Molt). It is similar to Juvenile Plumage, but with up to 5% or more scattered, replaced feathers on the dorsal and ventral surface of the body. These feathers appear newer and darker than the remaining older and faded juvenile feathers.
Second Basic Plumage (2Y/3Y)
Present primarily October–May in North American populations. Transitional (molting) birds are seen in March–September leading up to this period, but molt may continue through February. This plumage is similar to First Basic Plumage, but the crown and nape feathers are more elongated with paler buff tips. Some juvenile body feathers are retained, including body (especially rump) feathers and wing coverts. Individuals typically retain 3–7 outer primaries, and up to 14 secondaries (generally those between s2–s4 and s7–s15). Retained primaries and secondaries are longer than replaced feathers and narrower, brownish, and worn; replaced secondaries are darker, wider, and have more blunt tips and varying amounts of grayish marbling (less white) toward the base than juvenile feathers (Figure 340 in 26). Up to 8 juvenile rectrices (usually including r2, r5 or both) are retained. Some juvenile rectrices have grayish marbling in the dark tip (27). Central rectrices of similar ages and the portion of outer rectrices that are exposed when perching generally appear more worn and faded than the rest of the tail (56). Body plumage is mixed with various-aged feathers creating a mottled appearance, as opposed to the more uniformly worn feathers often seen in Juvenile Plumage.
Third Basic Plumage (3Y/4Y)
Present year-round in North American populations. It is similar to the First Basic Plumage but is usually characterized by three generations of remiges and rectrices. Retained juvenile feathers can include 1–3 juvenile outer primaries, usually p8, p9 or p10, primary coverts, and 1–5 juvenile secondaries, usually s3, s4, s8, s9, s10, or s11. The retained juvenile secondaries are narrow and faded, often having distinctively whiter bases than the adjacent replaced secondaries. One to four juvenile rectrices (usually r2, r3, r4 or r5) are occasionally retained. They have the remnant white or buff tip and are narrower and more tapered with whiter bases than the replaced rectrices. Two generations of rectrices may be present (Figure 340 in 26). The extent of white at the bases of secondaries and rectrices can be highly variable among individuals (51) and, as such, age cannot be determined solely by the amount of white in the secondaries (22).
Fourth Basic and Fifth Basic plumages (4Y–5Y)
Present year-round in North American populations and not easily distinguishable on most free-flying eagles but may be distinguishable in the hand or in photographs (53, TAM). Some individuals may be assigned to at least one of these two plumages by retained Second or Third Basic rectrices showing more white at the bases than in older birds (51, 26). All remiges usually are replaced, with at most three generations of basic feathers.
Definitive Basic Plumage (A4Y/A5Y)
Present year-round in North American populations. The entire plumage is dark brown except the area encompassing the rear crown, postocular region, nape (extending forward to the rear border of the auriculars), and the sides of the lower neck, which are golden brown. Upperwing coverts, especially the greater, lesser and median coverts, as well as the innermost 1 or 2 greater coverts, often are paler and more buffy, and they form a diagonal bar on the wing that is visible in flight and on perched birds. The rear underparts (mainly undertail coverts) are also slightly paler but this difference is not always evident on flying birds. All flight feathers are marbled with dark tips and usually lack white areas at the bases of the secondaries. The feather tips form a dark band on the trailing edge of the wings. The rectrices are marbled with a wide, black terminal band and may have a uniform gray base or dark bars on dark gray or less often dark bars on light gray or white (Figure 340 in 26, EHC). Rarely adult remiges may lack or show indistinct marbling (M. Lockhart, unpublished data). A few individuals retain, for many years, some white at the base of the tail. As individuals age, feathers on the occipital ridge may become sparse or may be lacking (EHC, T. Craig, personal communication).
Remiges show 2–4 sets of basic feathers that turn over in Staffelmauser (stepwise) patterns (61). Replacement sets of primaries are identified by the presence of a worn feather immediately distal to a fresh proximal feather. Secondaries show mixed generations of feathers in various sequences (62, 63, 64, 26).
Sexes are similar in coloration except that there may be differences in the pattern and number of dark bands on the tails of eagles in Definitive Basic Plumage (25, 2). When distinguishable, males have multiple alternating light and dark narrow, irregular, bands, but females have fewer or a single proximal narrow band and a wide irregular distal band (22; Figure 341 in 26). Using these criteria, sex was correctly assigned ~70% of the time (n = 114) for adults wintering in east-central Idaho, with males correctly identified more frequently than were females (T. Craig, personal communication, H. McFarland, unpublished data, EHC). This method may be useful for estimating sex of photographed eagles but whenever possible should be used in combination with more reliable measurements (e.g., footpad; 50, 65, 26, 66).
There are few examples of aberrant plumages reported for North America. A few cases of partial albinism have been reported (22). A very pale (white and cream-colored markings) individual was captured during winter in east-central Idaho (EHC, T. Craig, unpublished data), and a melanistic taxidermy mount of a Golden Eagle has been described (67). A juvenile was recorded at a camera trap in Arizona with brown and white barred underwing primary coverts (D. Driscoll, personal communication). An adult with little to no barring on flight feathers was captured in Wyoming (M. Lockhart, personal communication).
Occasionally, some individuals have small white “epaulets”, composed of a variable number of feathers at the upper end of the scapulars (28, 29, 33) ; a trait that may be referred to as the barthelemyi or Barthelemyi variant (30, 29, 31). This trait appears to be uncommon, but widespread across North America (29), and has been reported in the French Alps (30). The trait may be heritable; Spofford (28) observed a nest in the northeastern United States where both parents had epaulets, and in two different years they produced young with epaulets.
Molt and plumage terminology used here follows Humphrey and Parkes (59), as modified by Howell and Corben (68), Howell et al. (60, 69), and Pyle (70), unless otherwise noted. Under this terminology a Formative Plumage is recognized, and the first complete molt at roughly 1 year of age is considered the Second Prebasic Molt (71). The Golden Eagle exhibits a Modified Basic Strategy (60, 72), including incomplete prebasic molts by most individuals and a limited preformative molt by some individuals (70, TAM; also see Appearance: Plumages) but no prealternate molts (73, 46, 47, 29, 43, 26). Physiological condition, stress, or damage to feathers can influence the rate and timing of molt of individual feathers (24, 34, 35). There is little or no known geographic or sex-specific variation in molt strategies reported in North American populations (50; see Systematics: Geographic Variation for additional details on geographic variation in appearance).
Prebasic molts exhibit a Staffelmauser (stepwise) sequence and pattern of flight-feather replacement (74, 24, 61, 43, 53, 62, 64, 26, 2). Replacement proceeds from proximal to distal among the primaries from p1 to p10, and distally to proximally from s5 and s1, and in both directions from t2. Except for the prejuvenile molt, which is completed in the nest, 2–3 years are required to replace a complete set of feathers. Replacement of all juvenile feathers is usually complete by the end of the Fourth Prebasic Molt, after which molt proceeds in a continuous cycle and 2–3 generations of basic feathers are usually present. Molt of rectrices proceeds most often in the sequence r1–r6–r3–r4–r2–r5, as in accipitrid hawks, but variation in the order of rectrix molt is common (75, 56). Rectrices are replaced at some loci more often than others, with the the central decks (r1) and the outermost rectrices (r6) replaced most often (75, 56). As many as 9 rectrices may be replaced annually, but usually individual rectrices are replaced every second or third year (cf. 75, 56). Molt out of normal sequence may be related to a variety of factors including mechanical damage, stress, wear from exposure to sunlight and weather (35), or diet and nutrition (76).
Molt occurs during winter more frequently than has generally been recognized (EHC, TEK, TAM, M. Lanzone, personal communication). Molt during winter may be related to migratory status; in eastern North America, 40% (13 of 32) of individuals > 2 years old and captured during winter exhibited non-adventitious flight-feather molt (TAM, M. Lanzone, unpublished data). In contrast, no individuals in formative or second-basic plumage were molting flight feathers.
Prejuvenile (First Prebasic) Molt (HY)
This is the only complete molt of the Golden Eagle (all feathers replaced). This molt occurs in the nest prior to fledging, primarily from April–July in North America. Primaries break the skin at around 15 days, followed by secondaries, scapulars, and rectrices (77, 78, 2). Primaries rupture their sheaths at about 21 days followed by secondaries, scapulars, and rectrices. Upper-wing greater coverts break the skin between 22 and 25 days and burst their sheaths around 27 days (MNK). Feathers in dorsal and ventral tracts emerge through the skin between 22 and 28 days and break their sheaths at 29–35 days. Feathers in capital, femoral, and crural tracts break the skin between 29 to 35 days and rupture their sheaths between 36 and 42 days (79). Capital feathers continue to rupture their sheaths until about 49 days. At about 56 days, preplumulae down begins to be replaced by plumules of juvenile plumage (24). Juvenile feathering is essentially complete at about 60 days (2), but some preplumulae down may still be evident on the head and crop after this time (EHC, TEK). Full feather growth is complete in 80 to 105 days (24, 58).
Previously termed "First Prebasic" or "Prebasic I" molt in some accounts; see Howell and Corben (68), Howell et al. (60) and Second Prebasic Molt (below) for revised terminology used here. This molt is often limited or even absent. When it occurs (primarily from November–March in North American populations) it includes up to 5% of body feathers (possibly more in some birds) and is scattered over the head and dorsal and ventral parts of the body (70). This molt is sometimes considered early commencement of Second Prebasic Molt, but appears to be a separate inserted first-cycle molt (70).
Second Prebasic Molt (1–2 years of age) (2Y/3Y)
This molt is considered the "First Prebasic" or "Prebasic I" molt of previous authors. This is an incomplete molt (only some feathers replaced) that occurs primarily from March–October in North American populations (Figure 1). It includes some to most body feathers, upper-wing secondary coverts, and inner rectrices (4–10 outer rectrices in variable sequences can be retained). Some rump feathers and greater coverts are often retained. During the Second Prebasic Molt, 3–7 inner primaries and 1–9 secondaries and tertials, from among s1–s2, s5–s7, s15, and t1–t3 are typically replaced. The remaining remiges are retained as juvenile feathers until at least the Third Prebasic Molt. Molt of the tertials can begin shortly after p1 is dropped. Molt of the outer secondaries (s1 or s5) can begin sometime in midsummer (May–July), well after molt of primaries has begun.
Third Prebasic Molt (2–3 years of age) (3Y/4Y)
This is an incomplete molt that occurs primarily from April–October in North American populations (Figure 1). However, three of four 3-year-old individuals captured during winter in eastern North America, were actively, and not adventitiously, molting > 1 remex or rectrix during January or February (TAM, M. Lanzone, unpublished data). This molt is similar to the Second Prebasic Molt, but remiges and rectrices continue to be replaced in sequence, commencing where the Second Prebasic Molt terminated. Often a new molt sequence starts among the tertials (t2) and, perhaps rarely, p1. Sometimes all juvenile remiges are replaced by the end of the Third Prebasic Molt, but more commonly, 1–3 juvenile outer primaries (among p8–p10) and 1–6 juvenile secondaries (from among s3–s4 and/or s7–s11) are retained during this molt (26). Occasionally 1–4 juvenile rectrices (from r2–r5) on one or both sides of the tail may be retained. All juvenile body feathers and wing coverts are typically replaced by this molt.
Fourth Prebasic Molt (3–4 years of age) (4Y/5Y)
This is an incomplete molt that occurs primarily from April–November in North American populations (Figure 1); it can commence on breeding grounds and complete on wintering grounds (see Definitive Prebasic Molt, below). Two of four 4-year old individuals captured during winter in eastern North America were molting > 1 remex or rectrix during January or February (TAM, M. Lanzone, unpublished data). This molt is similar to the Definitive Prebasic Molt but can be identified on some birds by the replacement of juvenile feathers among p8–p10, s3–s4, or s7–s11. The second series of remigial replacements usually begins at p1, s1, and s5, by the Fourth Prebasic Molt. All juvenile feathers usually are replaced by this molt, and some birds may not be distinguishable from those undergoing later Definitive Prebasic Molts.
Definitive Prebasic Molt (> 4 years of age) (A4Y/A5Y)
This is an incomplete molt that occurs primarily from April–December in North American breeding populations. Molt may be suspended from January–March, but it is not uncommon for some individuals to molt a few feathers during this period (EHC; Figure 1). Thirty percent (6 of 18) adults captured during January or February in eastern North America were molting > 1 remex or rectrix (TAM, M. Lanzone, unpublished data).
This molt occurs in a predictable pattern, but the rate of molt may vary among individuals. Factors influencing the rate of replacement are not well documented but may include breeding status, environmental factors, physiological conditions, and food availability and quality (29, 76, 35). Molt usually begins while on or near the breeding grounds and it may continue after young have fledged (63, 26). Many accipitrid hawks, particularly breeding males, have impaired or suspended molt during incubation or brood-rearing (80, 63, 26). Golden Eagles, however, may shed many feathers during brood-rearing (TEK), and there is currently no evidence to suggest that they suspend molt during that time.
Definitive Prebasic molts can include many body feathers, with those of the head, neck, back, throat, scapular tracts, and alulae potentially replaced each year. However, wild individuals (> HY/SY) captured during winter clearly display multiple ages of body feathers, suggesting that Golden Eagle does not undergo a complete body molt each year (TAM, TEK, M. Lanzone, unpublished data). Body-feather molt usually begins before flight-feather molt and generally proceeds anterior to posterior, beginning at the head and neck and progressing to the back and belly. Body molt of a captive adult female was not continuous, but occurred in short-term (19 day) cycles that peaked in mid-summer and ended in mid-September (81). Replacement of flight feathers typically proceeds in 2–4 waves through the wing, resulting in 2–4 "sets" of feathers after completion of molt (64, 26).
Bill and Cere
The cere of nestlings is fleshy and often pale yellow (32), but can vary from pale to bright yellow (TEK). The bill and cere of adults are tricolored (22), with the cere yellow, and the bill black at the tip and lightening to grayish at the base. The adult cere also may vary from pale to bright yellow (TEK), and it may lack a fleshy appearance, becoming dry and cracked on some older adults (EHC, TAM).
Iris and Facial Skin
The iris of juveniles is dark brown. The eyes of adults vary from dark brown, hazel, or light yellow, to flecked gold and brown (EHC, T. Craig, personal communication), and even occasionally almost white (D. Bittner, personal communication). Eye color of some individuals may lighten with age but there is currently insufficient information to determine if eye color is a reliable indicator of age (26). The orbital-ring is yellow. The orbital-ring is yellow.
Legs and Feet
The feet vary in color from pale to bright yellow (TEK). Variation in color may depend on diet and individual fitness, as in some species of birds, including other raptors (76). The feet of older individuals may appear paler, drier, scarred, and cracked (TAM, EHC). Talons are black.
Between-sex differences in length of wing chord, tail, culmen, middle toe, foot pad, and hallux claw are substantial, with females being larger, on average, in all respects (Table 1). Wing chord and body mass alone are not reliable predictors of sex (50, 65). Foot pad measurements accurately sexed 100% of 49 eagles found dead and sexed by necropsy in southern Idaho, with males < 138.5 mm and females > 138.5 mm (65). In another study, there was no overlap between the sexes in measurements of hallux claw and head length (66). Finally, a combination of culmen and hallux claw lengths accurately sexed 97–100% of museum specimens from across North America (50).
There are a few age-related differences in morphology. The hallux claws of birds > 1 year of age are longer than those of first-year birds, and tails of first-year birds are generally longer than those of older birds (Table 1). One study of museum specimens from the United States and Canada found few age-related differences among culmen and wing-chord lengths of adult, subadult, and juveniles (50). In a study of birds from Montana and Wyoming (United States), wing lengths of juvenile males, but not females, were longer than those of adults of the same sex (23). Age-class differences of tail and wing measurements of Golden Eagle are not as great as those for Bald Eagle and other sea eagles (82, 50).
There is geographic variation in linear measurements in North America. Migrants from eastern Canada have greater tarsus width (anterior to posterior) than do non-migratory eagles from Wyoming (Table 1; TAM, M. Lanzone, M. Lockhart, unpublished data). Tarsus widths of residents and regional migrants from Idaho were intermediate between the two (Table 1; EHC, T. Craig, unpublished data). Wing chord measurements of migrants from eastern Canada (TAM, M. Lanzone, unpublished data) and migrants captured in Wyoming and Montana (23) were similar.
Data from across the distribution from Eurasia to Africa are published in several compilations. Watson (2) and Cramp and Simmons (47) reported female wing lengths of the 6 different known subspecies of Golden Eagle, with daphanea and kamtschatica said to be the largest (female wing length = 700 mm and 690 mm, respectively), with homeyeri and japonica the smallest (female wing length = 640 mm and 630 mm, respectively). Similarly, museum skins from the chrysaetos subspecies, mostly from Scandinavia and central Europe had the following mean measurements (47) for males: wing = 591 mm (range 565–630, n = 13); tail = 311 mm (range 285–332, n = 7); bill = 41.4 (range 39.5–45.3, n = 9); tarsus = 103 (range 94–114, n = 34); and for females: wing = 661 mm (range 637–685, n = 13); tail = 341 mm (range 325–359, n = 11); bill = 45.3 (range 40.1–50.3, n = 14); tarsus = 109 (range 97–122, n = 30). Forsman (43) reported length of 80–93 cm and wing span of 187–219 cm (n = 14 Finnish migrants). Ferguson-Lees and Christie (83) reported similar measurements with slightly larger ranges, no means, and no data on sample size. Measurements from birds from Kazakhstan were, for the wing, 595–565 mm (males) and 650–741 mm (females; sample size and subspecies not provided; 84).
Females of all age-classes are, on average, heavier than males of the same age-class (Table 1). There are limited data on age-related differences of free-flying birds. In some cases, juveniles tend to weigh less than adults and subadults. In a sample of 33 autumn migrants captured in Montana and Wyoming, adult females were heavier than juvenile females, but there was no similar age-related difference in weights of males (23). See Breeding: Young Birds for mass of nestlings at fledging.
Mean mass of subspecies chrysaetos is reported to be 3,572 g (range 2,840–4,450, n = 15) for males, and 5,194 g (range 3,840–6,665, n = 19) for females (47). That same source suggested weights of 2,900–6,000 g for homeyeri (both sexes, no information on sample size); 4,000–4,100 g (males) and a single 6,380 g (female) for daphanea; and 3,450 (one male) and 4,220 (one female) for kamtschatica. Ferguson-Lees and Christie (83) again reported similar measurements with slightly larger ranges, no means, and no data on sample size. Birds from Kazakhstan were reported to weigh 2,840–4,100 g (males) and 3,840–6,350 g (females; sample size and subspecies not provided; 84)
Wing Area, Wing Aspect Ratio, Wing Loading
Limited information. Lish et al. (23) reported that wing surface area of 33 migrant individuals captured in Montana and Wyoming did not differ by age class (adults vs. juveniles).