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Eagle Biology - Development

Adult Eagles

An eagle becomes fully mature at four to five years of age. Gradually over the years leading to maturity, the bald eagle's appearance changes until adulthood is signaled by the characteristic white head feathers, pale yellow eyes, powerful yellow beak, and yellow feet with great black talons. Scientists speculate that the striking appearance of the adult bald eagle serves to warn its enemies of approach, attract and signal bald eagles of the opposite sex, and warn other eagles of territorial rights.

When full-grown, the adult eagle is actually slightly smaller than the juvenile eagle due to the number of feathers lost in the molting process. Even in adulthood, bald eagles are among the largest birds in North America. An adult male bald eagle generally measures 3 feet from head to tail and weight between 7 and 10 pounds. The male's wingspan is about 6 1/2 feet. As with most birds of prey, female adult eagles tend to be larger than males and can weigh as much as 15 pounds, with wingspans up to 8 feet. The scientific term for this size phenomenon has come to be known as reverse sexual size dimorphism.

Juvenile Eagles

Once the bald eagle has reached the stage where its secondary down is beginning to be replaced, it is called a juvenile. From the fourth to the eighth week, the juvenile bald eagle continues to grow at a rapid rate. It continues to molt, losing its secondary down and gaining the feathers or plumage of the juvenile eagle. The plumage of the juvenile eagle is far less striking than that of the adult eagle. It appears to be a dark grayish brown. The coloring of the juvenile bald eagle is very similar to the coloring of the adult golden eagle. In fact, many juvenile bald eagles are mistaken for golden eagles. Even the famous naturalist and artist, John James Audubon, painted a portrait of the "Washington Sea Eagle" that today is recognized as the juvenile bald eagle.

The physical changes that the juvenile eagle undergoes in its growth from youth to adulthood can be described as moving from muted, darker, all-one-color shades to the striking high-contrast colors of the adult.

For example, the juvenile's eyes progress from a dark brown, to a lighter brown, to a cream to its adult coloring of yellowish white. The bald eagle's beak and cere transfrom from a dark black or gray to a mixture of gray and black to a mixed yellow and gray to the adult vibrant yellow. Its head feathers are dark brown to black in the juvenile but get progressively lighter brown and gray until they turn a dirty gray just before achieving the brilliant white head feathers of the adult bald eagle. The lower breast of the juvenile is a dark brown which becomes molted and then returns to a very dark brown in the adult. The tail changes from black with gray near the vane to a mixed gray and black to a final pure white in the adult eagle.

The size of the juvenile bald eagle is remarkable in that it is actually larger than the size of the fully grown adult bald eagle. This is because the plumage of the juvenile bald eagle is actually longer and thicker than that of the adult bald eagle. The adult bald eagle is more streamlined with fewer and shorter feathers than the juvenile. This streamlining contributes to the more graceful flight of the adult bald eagle. The longer feathers tend to make the juvenile eagle a bit clumsy in flight.

Chicks

A newly hatched bald eagle in its first few days of life is called a hatchling. When the hatchling wins its struggle to break out of its egg, it is a tiny, 3 ounce, wet, closed-eyed, cheeping creature. Still wet from the liquid in the yolk sac, or amniotic fluid, the hatchling's fuzzy down is plastered to its pink skin. Only a few hours later, the hatchling's eyes open, its down dries out, and the result is an adorable, large-headed, sharp-beaked, grayish-beige, fluffy bald eagle chick. At this stage, the hatchling is quite helpless, and it is at least a day before it is able to accept food from its parents.

This state of affairs changes rapidly. Soon the hatchling becomes a nestling. The nestling grows extremely fast and within three months becomes a fully-grown fledgling, ready to take off on its first flight. The bald eagle's rate of growth is faster than any other North American bird. From its beginning 100 grams, the bald eagle may gain up to 180 grams a day until it reaches between 4,000 and 5,000 grams, depending on its sex.

By the third week, the hatchling has lost its baby down and has regrown a secondary down which is a much darker, woolly gray. Its legs turn yellow. In two or three more weeks, this nestling begins to acquire the plumage of the juvenile. By the seventh or eighth week, the down has been replaced with feathers and the bald eagle is fully developed by the age of three months.

Male and Female

In terms of coloring, male and female bald eagles are virtually indistinguishable. At all stages of life, the male and female cannot be told apart except generally by size.

An adult male bald eagle generally measures 3 feet from head to tail and weighs between 7 and 10 pounds. The male's wingspan is about 6 1/2 feet. As with most birds of prey, female adult eagles tend to be larger than males and can weigh as much as 15 pounds, with wingspans up to 8 feet. The scientific term for this size phenomenon has come to be known as reverse sexual size dimorphism.

The difference in size is generally accurate for predicting the gender of a bald eagle. After eaglets reach about 12 weeks of age, scientists can tell whether a bald eagle is male or female at well over 90 percent accuracy by measuring the ratio of the bill depth to the length of one of the talons (the "hallux claw"). Especially for younger birds, the only ways to tell for sure are to examine the bird surgically or conduct a DNA analysis.

Southern and Northern Adaptations

At one time, scientists who studied bald eagles believed that there were actually two subspecies of the bald eagle: the southern and the northern bald eagles. The scientific names for these two subspecies were: Southern Bald Eagle: Haliaeetus leucocephalus leucocephalus / Northern Bald Eagle: Haliaeetus leucocephalus alascanus.

The most noticeable difference between the northern bald eagle and the southern bald eagle was that the northern bald eagle was considerably larger. The northern bald eagle was found in colder climates such as in Alaska, Canada, and the northern United States. The southern bald eagle was found in the warmer climates of states like Florida, California, and Arizona.

In 1967, the southern bald eagle was listed as endangered under the Endangered Species Preservation Act of 1966. The southern bald eagle was defined in that document as any bald eagle nesting south of the 40th parallel. The northern bald eagle was not included in this listing because the Alaskan and Canadian populations were not considered endangered at that time.

When scientists first tried to repopulate bald eagles in the southeastern United States, in states such as Florida and Georgia, they were not successful. The Alaskan bald eaglets which were transported to these states carried a disease called avian malaria. The southern bald eagles caught the disease and the bald eagle population in the southeastern states declined rather than increased. These events seem to support the theory that there were two subspecies of the bald eagle.

However, one fact puzzled scientists attempting to describe the essential differences between the two subspecies. The smaller southern bald eagle was found only in the warmer southern states. As one traveled north, the size of bald eagles increased. In the midsection of the United States, the bald eagles were of a moderate weight. Were they southern or northern bald eagles? This was the question scientists attempted to answer.

What they discovered was that the size of the bald eagle grew gradually larger or smaller according to the climate. Bald eagles in the middle of the United States were larger than the largest bald eagles in Florida, but smaller than the smallest bald eagles in Washington state. As they researched further, they found that the only difference between the two subspecies was size. The two types of eagles did not differ in any other way except that the Alaskan bald eagle seemed to have developed an immunity to a disease called avian malaria which it spread to southern bald eagles when it was transported to those states.

Now scientists acknowledge that there is really only one species of bald eagle. Its scientfic name is Haliaeetus (sea eagle) leucocephalus (white head). Although scientists have proven that there is only one species, resource materials may continue to refer to the southern or northern bald eagle to identify the size and location of the eagle.