The Harpy Eagle is a massive eagle, with a large, strongly hooked bill; very robust tarsi; a divided crest on the central crown; broad, rounded wings; and a relatively long, banded tail. The adult readily is recognized by its very large size, gray head and neck, broad black breast band, dark upperparts, and white lower breast and belly. The Harpy Eagle takes several years to attain the adult plumage. Juvenile and younger immature plumages primarily are light gray and white. Birds in these plumages are more easily confused with comparable plumages of the smaller (but still large!) Crested Eagle (Morphnus guianensis); see Similar Species.
Because of its large size, the Harpy Eagle is likely to be confused in the field only with the Crested Eagle (Morphnus guianensis), the distribution of which broadly overlaps that of the Harpy Eagle. The following discussion of the distinctions between these two species is based on Howell and Webb 1995, Hilty and Brown 1986, Ridgely and Greenfield 2001, and Hilty 2003.
In all plumages, the Crested Eagle is smaller and more lightly built than the Harpy, with a relatively longer tail. Furthermore, the crest of the Crested Eagle is made up of a single tuft of feathers; the crest of the Harpy Eagle is clearly divided into two tufts.
The adult Harpy Eagle further is distinguished from the adult Crested Eagle by the broad black breast band. Also, the lower breast and belly of the Harpy Eagle are white and unmarked; these regions of light morph adult Crested Eagle usually are barred with rufous (although the intensity of the barring varies, and it is not always conspicuous at a distance). The underwing coverts of the adult Harpy Eagle are largely black, but are unmarked white in the aduld light morph Crested Eagle.
Juvenile and younger immature plumages of Harpy Eagle are mostly light gray above and white below, with white underwing coverts. In all these respects, these plumages of Harpy Eagle are very similar to comparable plumages of Crested Eagle. Birds in these plumages can be difficult to distinguish when structural characters (overall size, size of the tarsi, and relative length of the tail) can not be assessed. Another possible distinction is that the tail barring of Harpy Eagle may be coarser than in comparable plumages of Crested Eagle (Howell and Webb 1995).
The plumage of the Harpy Eagle gradually becomes daker with age. By the second basic plumage, the upperparts and the upper breast show at least scattered black feathers.
Sexes similar in plunage, but differ in size; females are significantly larger than males (see Measurements). The following description is based on Bierregaard (1994), Howell and Webb (1995), and Hilty (2003).
Head and neck light gray. Dark (blackish) elongated feathers on the crown form a crest, which usually is raised, and is parted down the middle into two sides. Uppersurface of the wings, the back and the rump are dark gray or blackish. There is a broad black band across the upper breast, separating the gray head from the white belly. The lower breast and the belly are white. The tibial feathers are white, barred with black. The uppersurface of the tail is black, with three broad, ashy gray bands; there also is a narrow, grayish white tip to the tail. The undersurface of the tail is black, with three broad white bands; the basal of these white bands often is largely covered by the undertail coverts. The lesser and median underwing coverts are mostly black, barred or mottled with white. The primary underwing coverts are mostly white, tipped with black. The undersides of the remiges are white, barred with black and with broad black tips; the dark tips are particularly broad at the ends of the outer primaries.
The timing of the molts in the Harpy Eagle has not been studied in detail. Most information in this section is based on Rettig (1978; hatchling) and on Howell and Webb (1995; juvenile and later plumages).
Hatchling: The hatching is covered with dirty white down. The remiges begin to appear by 45 days of age. At 54 days, feathers are apparent on the crown, sides of the head, back, scapulars, and upper wing coverts. The rectrices begin to emerge by 77 days.
Juvenile: The plumage is whitish overall. The upperparts are whitish, with gray vermiculations. There also is a small amount of black mottling on the greater upperwing coverts. The remiges are darker: blackish above, mottled and barred with paler gray. The upper surface of the tail is grayish with seven-eight blackish bars. The under surface of the tail is whitish with six-eight narrow black bars. The underwing coverts are white. The under surfaces of the remiges are white with dark barring, but the barring is less bold than in the adult.
First basic plumage: Similar to the juvenile plumage, but a little darker. The upperparts are mottled with blackish. The crest is tipped dusky. The chest and axillars are pale gray. The pattern of the tail is variable. The upper surface of the tail is gray, with one-five narrow (often broken), black bars near the tip (the pattern more typical of males?) or with six-seven narrow black bars (this pattern more typical of females?). The under surface of the tail is whitish with two-six narrow black bars. The under surface of the wings is more boldly marked than in juveniles.
Second basic plumage: The upperparts show an increased amount of black mottling. The crest is tipped black. Some black mottling is present on the chest. Also, at least some black barring often is present on the tibial feathers. The upper surface of the tail is gray with four-six bold black bars. The undersurface of the tail is whitish with four-five black bars.
Third basic plumage: By this plumage the eagle resembles the adult, but retains some pale mottling on the upperparts. The black chest band also is mottled with pale gray. The upper surface of the tail is black or blackish with three-four gray bars.
The definitive plumage is attained by the fourth prebasic molt, when the bird is about four years old.
Information is taken from Bierregaard (1994), Howell and Webb (1995), and Hilty (2003).
Iris gray or brown. Cere black or blackish. Bill black. Tarsi and toes yellow.
Females are larger than males, although mensural data rarely are reported separately by sex.
Length: 86.5-107 cm (Howell and Webb 1995)
Wingspan: 183-224 cm (Howell and Webb 1995)
Mass, male: 4000-4800 g (Fowler and Cope 1964, Bierregaard 1994)
Mass, female: 7600-9000 g (Fowler and Cope 1964, Bierregaard 1994)