The question of whether female bald eagles are brown stems from confusion regarding the bird’s appearance throughout its long maturation process. An adult female bald eagle is not entirely brown; she shares the famous dark brown body and brilliant white head and tail of the adult male. This striking two-toned plumage applies equally to both sexes once they reach full maturity. The true source of the “brown eagle” observation lies in the very different look of young bald eagles, which spend the first several years of their lives in various shades of brown.
Adult Coloration and Lack of Sexual Dimorphism
Adult bald eagles, both male and female, are characterized by dark brown body and wing feathers, contrasting sharply with their distinctive white head and tail feathers. This coloration is acquired when the bird reaches sexual maturity, which typically occurs around five years of age. If an eagle possesses this iconic white head and tail, it is a fully mature adult, regardless of its sex.
Bald eagles do not display sexual dimorphism in their plumage, meaning color is not a reliable way to differentiate between a male and a female adult. Both sexes feature the same bright yellow beak, feet, and eyes once they complete their final molt into adult feathers.
The Puzzle of the Brown Eagle: Juvenile Plumage
The many brown eagles seen in the wild are not adult females but rather younger birds still progressing through their long juvenile period. Bald eagles undergo a lengthy transformation that takes about five years, during which their plumage changes dramatically with each annual molt. A newly fledged juvenile, or first-year bird, is almost entirely dark brown, or even blackish-brown, with only a few mottled white patches on the body and tail.
As the eagle enters its second and third years, it is often referred to as a sub-adult, and its appearance becomes highly variable. These younger birds display a mottled mix of dark brown and white streaking across their bodies, wings, and tails. During this phase, their beak remains dark, and their eyes are dark brown, unlike the bright yellow features of a mature bird.
The head feathers begin to show substantial white streaking in the fourth year. It is not until the fifth year that the eagle’s plumage fully converges into the definitive adult pattern.
How Scientists Distinguish Male and Female Bald Eagles
Since adult coloration is identical, scientists rely on physical measurements to accurately determine an eagle’s sex. Bald eagles exhibit reverse sexual dimorphism, meaning the female is visibly larger than the male. Females generally weigh about 25% more than males and possess a noticeably wider wingspan.
This size difference is observable but not absolute due to geographic variation; northern eagles are often larger than their southern counterparts. Researchers must take specific biometric measurements to be certain. They focus on the depth of the beak and the length of the hallux, which is the large rear talon.