How To Age A Golden Eagle By Its Tail

It takes five years for a golden eagle (Aquila chrysaetos) to reach maturity and adult plumage. 

Determining the approximate age of a juvenile and immature (young) golden eagle by its plumage can be challenging. Young golden eagles have conspicuous white wing patches, which identify them as birds between 1 and 5 years of age. However, the white wing patches vary in size and shape among individuals and some birds have little to no white wing patches. 

To add to the challenge, young eagles molt their feathers at different speeds, some faster than others. Fortunately, the young golden eagle’s tail changes with age more predictably, making it a good reference to age young birds.

Tail feathers are also called rectrices. Golden eagles have 12 tail feathers.

This article will show you how to use a young golden eagle’s tail color pattern to approximate its age.

Juvenile and immature golden eagles

The terms juvenile and immature are often used interchangeably, but each term refers to different stages of grow.

The term juvenile refers to an eagle (or any bird) in its first plumage. After leaving the nest, golden eagles wear their juvenile plumage for 7 to 9 months.

Immature golden eagles are birds in plumages that follow the juvenile or first year plumage.

Nestling plumage: Natal Down

At hatching, golden eaglets have a fluffy but thin coat of whitish natal down. Part of the eaglet’s body shows bare patches of skink. After about the 6th day and for the next few weeks, the eaglet begins to grow a thick whitish coat of down. 

By day 65, after hatching, eaglets have their juvenile plumage complete and are ready to fledge. 

The following analysis is an oversimplification of the stages golden eagles’ tails go through to acquire their adult plumage. Aging young golden eagles can be challenging. Golden eagle biologists indicate that many field observations go as “young birds of an unidentified age” due to the plumage variation among individuals and difficulty observing enough details for proper age classification. The analysis below uses broad categories to age young golden eagles.

1-year-old golden eagle: Juvenile Plumage

One-year-old or juvenile birds have a black terminal tail band and a broad white base of the tail. Birds retain their juvenile plumage, including the tail, for 7 to 9 months after leaving the nest.

The white base of the tail is broad contrasting with the terminal black band. The bicolor tail of a juvenile golden eagle is noticeable in flight and when the bird is perching. Golden Eagle tail adapted from David H. Ellis, James W. Lish (2006).

The juvenile golden eagle plumage is a uniform dark brown with a distinctive bicolored tail and conspicuous white wing patches.

The wing white patches vary in size and shape and are generally located on the part of the wing where the primary and secondary wing feathers meet. Some birds have small and rounded wing patches, while others have larger and poorly defined ones. Although rare, some juvenile golden eagles have little to no white wing patches.

All juvenile golden eagles have a similar bicolored tail.

 2 to 3.5 Year-old: Immature plumage

2 to 3.5 years. The second-year marks the initiation of the first immature plumage. By the age of 3.5 years, birds grow tail feathers with an additional gray-with-black-streaks area that broadens the initial juvenile’s narrow black tail band and reduces the amount of white on the tail’s base. There are many plumage variations, but a key field mark during this period is a reduction of the white area at the tail’s base.

Notice the additional gray area streaked with black on top of the juvenile black tail band. Golden Eagle tail adapted from David H. Ellis, James W. Lish (2006).

The second-year marks the beginning of the immature or sub-adult stages that last four more years.

During the second year, a juvenile golden eagle initiates a partial molt replacing the juvenile plumage.

Golden eagle biologists estimate that an immature golden eagle replaces only approximately 5 percent of its juvenile feathers during the second year.

The third-year is a continuation of the second year’s partial molt. More fresh darker brown feathers continue to appear. By the end of the third year, an immature golden eagle has replaced most of its body feathers, but some juvenile feathers persist.

A young golden eagle replaces the crown, nape, and lower back and sides of the neck feathers for more ornate and elongated golden feathers during the third year.  

During the third year, young golden eagles initiate a partial replacement of the primary and secondary flight feathers. The third-year wing feathers include two generations of flight feathers; juvenile and newly grown feathers. Fresh flight feathers can be visually separated from juvenile feathers by being slightly shorter.

Tails feathers or Rectrices

Like other eagles, the golden eagle has 12 tail feathers. During the second year, birds began to replace the tail feathers starting with the central pair outwards (see photo). The tail feather replacement continues through the third year. 

The rate at which immature golden eagles replace their tail feathers varies among individuals. Some birds molt two pairs (4 feathers) by the end of the second year, while others have only molted one pair (2 feathers). Some birds replace only a single tail feather during the second year and may look like a juvenile when they are a second-year bird.

Fresh tail feathers have an additional amount of gray-with-black-streaks above the black terminal band. As more tail feathers are replaced, the white base of the tail becomes smaller.

By the end of the third year and the beginning of the fourth, most birds have replaced all the juvenile tail feathers and show a reduced amount of white at the tail’s base.

3.5 to 5 years: Immature plumage 

3.5 to 5-year-old birds show tails that combine immature (still bicolored) and adult (all dark) feathers. The dark adult tail feathers generally appear at the tail’s center. Birds with narrow dark centers of the tails (like that in the photo) are closer to 4 to 4.5 years of age. Birds with mostly dark tails and bicolored outer tail feathers are close to 5 years of age.  

The central pair of fully dark or adult feathers indicates that this bird is at least 3.5 to 4 years of age. Birds with mostly dark tails and bicolored outer tail feathers are close to 5 years of age. Golden Eagle tail adapted from David H. Ellis, James W. Lish (2006).

The fourth year’s plumage can be characterized as a continuation of the plumage molting initiated during the second year. 

The fourth’s year body plumage has three-generation feathers, including those from the second and third and fresh from the fourth year.

Tail Feathers

During the fifth year, the tails continue to darken from the center out as more adult dark feathers replace the bicolored feathers of earlier plumages. Some birds, though, can show some white on the tail’s base even by the fifth year, which is considered the adult plumage.

Adult plumage

Adult birds have a dark tail. Some birds have some white at the tail base after the five fifth year.

Under normal favorable conditions, a golden eagle acquires the adult plumage in the fifth year. In environmental and nutritional stress conditions, golden eagles can retain feathers that are typical of earlier plumages. Golden Eagle tail adapted from David H. Ellis, James W. Lish (2006).

The adult plumage is dark brown, mainly seen from a distance. At close range, an adult golden eagle shows the typical elongated golden-brown feathers of the crown, nape, and neck. Parts of the underwing and undertail coverts have lighter tones, but these colors are noticeable with a bird in the hand.

The adult golden eagles no longer show white wing patches. The wing feathers show streaks of black and have dark tips that form a dark band along the wing’s trailing edge.

The tail looks dark brown from a distance, but a closer look, reveals a dark terminal band preceded by a gray-brown and lighter marbling. 

The folded tail of a perching bird shows light gray bars in a darker gray-brown background.

Studies of golden eagle plumage found that most males have multiple tail bands while most females have fewer or a single bar. Tail bands can be a reference to determine the sex of golden eagles, but some birds do not conform to this rule. 

Conclusions:

Aging golden eagles can be tricky. Birds of all ages can appear similar in the field. While using caution, a golden eagle’s tail appearance can provide clues to place it in one of the broad age categories listed in this article. The more details one can see in the field, the more confident your determination will be. However, bear in mind that some of the birds you see, particularly those gliding overhead, are going to be deemed “birds of unknown age.”

References:

  • Katzner, T. E., M. N. Kochert, K. Steenhof, C. L. McIntyre, E. H. Craig, and T. A. Miller (2020). Golden Eagle (Aquila chrysaetos), version 2.0. In Birds of the World (P. G. Rodewald and B. K. Keeney, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA.
  • Thinking about Feathers: Adaptations of Golden Eagle Rectrices, David H. Ellis, James W. Lish. J. of Raptor Research, 40(1): 1-28 (2006).

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