April 2018

Growing Up Eagle

Marla Mertz, Marion County Conservation

Bald eagles are a most striking and stately bird. The bright white head and tail can show up along tree lines, roosts, and glisten amidst a dense fog. In the winter, Lake Red Rock holds hundreds of eagles within the Des Moines River below the dam and throughout our river communities. Thousands of people will come to the area to see and photograph our National Symbol. Most are casual observers struck by the grace and beauty of roosting eagles and taking flight over the open water to catch a fish.

But, what about those big brownish colored birds? Some are dark, some are speckled, and some just look bigger than an eagle. These birds are considered ‘immature’ or ‘juvenile’ eagles until they have reached their adult coloration or plumage. The layers of feathers that cover a bird, the pattern, color, and arrangement of those feathers are the bird’s ‘plumage.’

The plumages are acquired by feather ‘molts’ that begin each spring and are completed by late fall and will carry the bird through the winter months. Feathers are similar to our own hair and nail structure and are made of a protein keratin. If they become damaged or worn, they need to be replaced. Hormones and seasonal changes trigger feather molts. Also, if a bird loses a complete feather, it will immediately be replaced without waiting for their molting period. Feathers need to be in the best shape possible for them to be in top flying condition. Annual molts must occur over a long period of time and in a pattern so that they can continue to fly and hunt for food. Unlike waterfowl that will molt their feathers all at once and are left flightless for a long period of time. Understanding basic molt patterns assist in identification of a species, as well as determining an age.

See also: Feathers...of course

Can you tell an eagle’s age from their color?

The aging of bald eagles is not a clear-cut science. Even after all the years of study, there is no finite way to get an exact age just by looking at plumage color. Health, diet, and injury can play a strategic part in feathers, as well. Some eagles that receive injury may become a year behind in their plumage changes, may speed up color change, and can look different than their same-age siblings. Once they reach maturity and become an adult, it is almost impossible to attribute an 'age' to a wild bald eagle.

As young, immature eagles age, their eyes (iris) will begin to lighten, going from a dark brown to yellow and their beak changes from a charcoal black to a brilliant yellow. These changes occur over years of time, sometimes we overlook the younger generations until they become mature with their stately appearance.

So, just how can you possibly get an idea of how old those eagles might be?

The first four weeks, young eagles are covered in fluffy white down.

The basic black and brown plumage appears at about five weeks of age.

At ten weeks of age, young eagles are fully feathered. Note the dark eyes and charcoal color beak.

A hatch-year bird (HY) will keep its chocolate brown plumage through the winter until spring. Many times, this age of eagle can easily be confused with the golden eagle.

In their second year, the plumage begins to start mottling with more white speckling appearing throughout the body. The eyes and beak show a very slight lightening of color.

These juvenile eagles (below) are in their 3rd year. They have a noticeable ‘belly band’ and more white mottling. The beak and eyes are beginning to show some notable fading.

As eagles are within their 4th year, the head and tail feathers are showing the distinct color and look of the bald eagle. Note the distinctive eye line. Yellow is beginning to appear on the beak with eyes changing to a much lighter brown to soft yellow.

Most eagles molt into their white head and tail feathers at the age of 5. Some may still show some blotches of brown or grey as they each will molt differently. The beak becomes vibrant yellow and the eyes have changed to a pale yellow.

Eagles are prolific nest builders, you may witness young birds carrying sticks and building numerous ‘practice’ nests. Of course, those ‘practice” nests may just come in handy as a humble abode to other raptors.

See also: Natural Real Estate: A bird’s humble abode

Some immature or juvenile eagles may look somewhat larger than their adult counterparts. They have a little more fat reserve, a few more contour feathers to add a little extra padding assisting them through the 5 years of growing, learning, hunting and surviving. Eagles have approximately 7,000 feathers, and maybe more!

Why does age matter?

Who really needs to know or wants to know the age or progression of changes in our eagle population? This is certainly interesting information for the casual observer. But knowing bald eagle ages is very important with population counts, biologist’s surveys, reported injuries to rehabilitation centers, and all types of research.

Midwest eagle research

Seeing all ages of eagles in our river communities lets us know that these awe-inspiring birds have made a comeback in our lifetime. If you would like to learn more about the history of the bald eagle please check out one of our past articles “Eagles of our time.”