The Bald Eagle is a large, dark brown to blackish-bodied raptor that attains a white head and tail when it is about 4.5 years old. Up until that time, the plumage varies from an all brown juvenile stage through a series of brown bodied plumages that show varying degrees of white on the belly, on the underwings and on the back, with dark or mottled heads and tails. The white patches on the underwings of subadults always occur in the inner linings and "axillars" with some light areas shown in parts of the primaries. By contrast, subadult Golden Eagles always have dark inner wing linings. In flight, they are impressively large with wingspans that range from 6 to 8 feet (1.9 to 2.5 meters) with the smaller birds in the southern populations.
The in-flight profile is of a wide-winged bird whose head extends almost as far ahead of the body as does the tail behind. The wings are held flat when soaring and the wing beat is slow and fluid, rising higher on the upstroke than deeper on the downstroke. Although resident over its entire range (except for the colder parts of the northern interior of the continent), birds are highly mobile as they move from nesting areas to abundant food sources. Migration is well-developed.
SIZE
The Bald Eagle, along with the Golden Eagle, is the larger, regularly-occurring raptors in North America. Its size range is very close to that of the Golden Eagle with the upper extreme measurements of length and wingspan being only slightly longer. Females average slightly larger than males. Length measurements range from 28 to 38 inches (71 to 97 centimeters) and the average is closer to 31 inches (79 centimeters). The wingspan ranges from 66 to 96 inches (168 to 234 centimeters) with an average of about 80 inches (203 centimeters). Weights range from 6.5 to 14 pounds (2.9 to 6.3 kilograms) and average 9.5 pounds (4.3 kilograms). Northern birds are larger than southern birds with the largest breeding in Alaska and the smallest breeding in Florida. The variation in between is apparently gradual.
MORPHS AND MOLT
There are no color morphs of the Bald Eagle, but individuals may be in any one of six stages of molt. The first full plumage is the juvenile, followed by five basic plumages. The definitive or "adult" plumage is attained at the 5th basic stage. The sexes are similar looking and either one plumage per year is acquired or some molting is occurring all year except in winter months in northern birds. Each basic plumage is acquired by molts that begin during the spring and are substantially finished or completed by late fall of the same year. The new plumage is worn throughout the winter until the pre-basic molt begins the next spring.
The juvenile plumage is acquired during the 1st calendar year of life and is retained until the spring of the second calendar year when the first pre-basic molt begins. Thus the birds are about 5.5 years old (6th calendar year) by the time that they acquire the adult plumage. Adult-like plumage may first be observed when birds are 4.5 years old and they are all but indistinguishable from 5.5 year old birds.
Some field guides assign names to the various molt stages as follows:
Age Plumage Field Name
1st year Juvenile immature
2nd year Basic 1 white-belly 1
3rd year Basic 2 white-belly 2
4th year Basic 3 adult transition
5th year Basic 4 adult
6th year Basic 5 adult
Variations in the adult plumage are known and commonly consist of degrees of dark feathering through the eye and crown of the head set against an otherwise white head. Entire dark feathers have been seen in white tails.
SPECIFIC DESCRIPTION
Adult - Flight
Note: Both sexes very similar, but female may appear larger.
- a large raptor with a dark brown body with a white head and tail
- the underwings are all dark as are the uppersides of the wings
- the head appears to extend as far in front of the bird as the tail extends behind
- in soaring birds, the wings appear very long and uniformly wide throughout their length although they are narrower than the wings of first-year birds
- the wings are held flat when soaring with no hint of a dihedral as in a Golden Eagle and they are held at right angles to the body
- the wing beat is slow and describes a large arc, rising higher on the upstroke than they plunge on the downstroke
Immature (Juvenile - First year) - Flight
- overall, the appearance is of a very large dark raptor with a dark head
- the back and upper wing coverts are tawny brown and contrast with the dark flight feathers
- the dark brown breast contrasts with the tawny belly and the undersides may have some white streaking in places
- the tail may be noticeably longer than in older plumages and it may be all dark or generally dark with a pronounced central whitish band. It can be broadly rounded
- the axillaries and most underwing coverts have whitish patches
- the longer secondaries with toothed points help to give the trailing edge of the wing a serrated look
Subadult (White-belly 1 - Second year) - Flight
- the breast is dark and contrasts with the variably whitish belly
- the wing linings are variably mottled white contrasting with the darker extremities of the flight feathers
- all birds retain some juvenile secondary feathers that produce an uneven trailing edge to the wing
- the tail has a variable amount of white with a dark tip on both surfaces
- the upper surface of the birds are generally dark but the interscapular area is a mottled white, or tawny, with an inverted white triangle
Subadult (White-belly 2 - Third year) - Flight
- the breast is dark and contrasts with the variably whitish belly
- the wing linings are variably mottled white contrasting with the darker extremities of the flight feathers
- all juvenile secondary and primary feathers are replaced, except one or two at most
- the tail has a variable amount of white with a dark tip on both surfaces
- the upper surface of the birds are generally dark but the interscapular area is a mottled white, or tawny, with an inverted white triangle
- these birds appear more adult-like in that they have whitish heads and tails that contrast with the darker bodies, but they lack the hard line of definition
- the underparts are dark brown with varying amounts of white flecking
- the upperparts are dark but there may be a hint of lightness in the upper back on some birds
- the tail is more extensively white with a brown terminal band and some brown flecking throughout
Subadult (Adult - Fifth year) - Flight
- this is essentially a very adult-like plumage with a white head and tail contrasting with the overall dark body
- the body contour feathers are dark brown with buffy edges
- has irregular dark band on tail
- at any distance, this plumage may be impossible to distinguish from the Adult - Sixth year or full adult plumage
Adult (Adult - Sixth year) - Flight
- this is the adult plumage and is as described above
SIMILAR SPECIES
In terms of regularly occurring species within continental North America, no other species resembles the distinctive adults. The White-tailed Eagle has a creamier head, a shorter, wedge-shaped tail and brown undertail coverts. The Steller's Sea-Eagle is larger and has huge white shoulder patches and a longer, more wedge-shaped white tail than the Bald Eagle.
The juvenile Bald Eagle is more likely to be mistaken for a Golden Eagle, but the white wing linings (axillars) and more prominent head in flight distinguish all subadult Bald Eagles from all plumages of the Golden Eagle. The white marks in the underwings of an immature Golden Eagle is in the primaries not the inner wing linings. The immature White-tailed Eagle is similar in all plumages to an immature Bald Eagle but generally appears darker underwing with less white in the axillars and the tips of the tail feathers have white spikes. The long, wedge-shaped white tail of the immature Steller's Sea-Eagle distinguish the various brown plumaged birds from their similarly-aged counterpart Bald Eagle.
OTHER NAMES
The Bald Eagle is also popularly known as the "American Eagle", "Bald-headed Eagle", "White-headed Eagle", "Brown Eagle" (immature), "Fish Eagle", "Old Patriarch", "Common Eagle", "White-headed Sea Eagle", and "Sea Eagle."
ETYMOLOGY
The genus Haliaeetus is Greek for "sea eagle" and is formed from "halos" meaning salt and "aetos" meaning eagle. The reference is clearly about the fondness of this species for ocean edge habitats. The specific name Leucocephalus is Greek for "white-headed" and is formed from "leukos" meaning "white" and "kephale" for "head." Bald is used with its less common definition meaning "marked with white" in obvious reference to the head and tail of the adults and possibly the patchy white markings of the various subadult plumages.
MYTHOLOGY
Eagles have had a long history of symbolic omnipotence and superhuman attributes in many societies. Such characteristics as courage, freedom, independence and truth are common. There are over 300 ways in which various parts of the Bald Eagle's anatomy have been modified in art to impart some meaning, atmosphere or interpretation. The Bald Eagle is an ingrained part of the mythology of many North American Indian peoples and the feathers have been used as part of the head dress for some groups. Carvings and totems are typical amongst coastal communities. As a major recognition of the esteem with which this bird is held, the Bald Eagle was declared the national symbol for the United States of America in 1782, despite the paradoxical opinion that the bird was of questionable moral character, dishonest, cowardly and a feeder upon dead matter and offal.
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