Description
Bald Eagle, the only eagle solely native to North America, and the national bird of the United States since 1782.
Bald eagles have dark brown feathers on their bodies and wings, and white feathers on their heads and tails. They also have yellow hook-tipped beaks,
yellow claws and sharp curved black talons.
It's a large predatory sea eagle, classified in the bird family as a diurnal (daytime) bird of prey, it's a carnivorous bird that hunts during daylight hours. Birds of prey feed wholly or chiefly on meat taken by hunting or scavenging carrion (dead animals). The victorian scientific name for birds of prey is raptor, before it came to be used for these birds, raptor meant "abductor," from its Latin meaning, "robber, plunderer, or abductor," from rapere, "to seize."
Taxonomy
Haliaeetus leucocephalus, the scientific name of the bald eagle, it literally means sea eagle with a white head.
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Greek Language:
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hali
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aeetus
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leuco
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cephalis
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English Translation:
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sea
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eagle
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white
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head
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The bald eagle has two known North American subspecies and forms a species pair with the white-tailed eagle of Eurasia.
Species Pair:
- Haliaeetus albicilla - The Eurasia white-tailed eagle.
Subspecies:
- H. l. leucocephalus – southern bald eagle. Found in the contiguous United States, Baja California Peninsula and northern Mexico.
- H. l. washingtoniensis – northern bald eagle, larger than the southern leucocephalus. Found in the northern United States, Canada and Alaska.
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Bald Eagle: Common Name and Origin
The Bald Eagle's name comes from the word "piebald", a word used to describe animals with large white patches. Merriam-Webster Dictionary describes the word piebald this way:
The pie of piebald derives from pica (pie is another name for a magpie, magpies are black and white birds of the Corvidae family), pica is Latin for "magpie." The other part of piebald comes from the word "bald", which can mean "marked with white". "Bald" can also be found in skewbald, an adjective used to describe animals marked with patches of white and any OTHER color but black.
Simply put, the shared opinion seems to be that the name "Bald" Eagle comes from the word pie"bald", although skew"bald" seems more fitting.
Range and Habitat
The bald eagle's natural range covers most of Canada and Alaska, all of the continental United States, and parts of northern Mexico. The state of Hawaii doesn't have bald eagles.
They will generally not be found in the northernmost parts of Alaska and Canada. The northern birds are migratory, while southern birds are resident, remaining on their breeding territory all year.
Throughout their range, bald eagles use forested habitats for nesting and roosting, and expanses of shallow fresh or salt water for foraging.
They tend to live on the shoreline or near large bodies of water, such as lakes, marshes, seacoasts, and rivers. When perching, roosting, and nesting,
bald eagles prefer hardwoods, like oak trees, cottonwood trees or hickories, they also use coniferous trees, like white pine trees, spruce and cedar trees.
Eagles may also build nests in snags (dead trees), on top of isolated rock peaks, on man made structures like transmission lines and communication towers.
Food Sources: A Hunter, Scavenger and Thief
Bald eagles are opportunistic carnivores with the capacity to consume a great variety of prey, the type of prey depends on the region. Throughout their range, fish is a favorite prey, often comprising the majority of the eagle's diet. Some of the common fish species are salmon, carp, catfish, shad, herring and trout.
Other birds are the next largest portion of the bald eagle's diet, they will feed on seagulls, herons, ducks, geese, egrets, loons, sandhill cranes, and pelicans.
They will often follow the fall migration of ducks and geese and feed on birds that have been injured by hunters. While hunting waterfowl, bald eagles repeatedly fly at a target and cause it to dive repeatedly, hoping to exhaust the victim so it can be caught.
Bald Eagles also feed on moderately sized wild mammals, some species that are known prey are raccoons, rabbits, squirrels, ground hogs, muskrats, beavers, deer fawns, and harbor seal pups.
The next food source includes invertebrates such as crabs, amphibians and reptiles like turtles and snakes.
They will feed on what is most available, and requiring the least amount of energy to acquire it. They will steal the prey from other animals, a practice known as kleptoparasitism, they'll scavenge carcasses along roadways, forage for leftovers from other predators or garbage at landfills. They do not generally feed on pets, chickens or other domestic livestock.
Hunting Risks
Healthy adult bald eagles are not preyed on in the wild, they occupy a position at the top of the food chain and are thus considered alpha predators or apex predators.
Because bald eagles have a habit of being opportunistic foragers, they become vulnerable to ingesting toxins from poisoned animals or shot carcasses killed with lead ammunition, which causes lead poisoning.
Another cause of death is motorvehicle trauma when feeding on road-killed animals.
Bald eagles are capable swimmers, however, if an eagle falls into the water during an aerial fight over food with another eagle or accidentally falls into the water while hunting,
they could drown during the encounter. If the water is very cold, the bird might be overcome by hypothermia, or if the shore is too far away, they could lose strength and drown. Bald eagles are able to take flight from the water surface, but they must act quickly, if they have enough energy and their wings do not become saturated, they can attempt to take flight.
Hunting Range
The hunting area or home range patrolled by a bald eagle varies from 1,500 acres (2.3 sq. miles) to 10,000 acres (15.6 sq. miles).
Adult eagles with established territories will hunt close to home, roughly 500 acres (about 1 sq. mile) to 4000 acres (about 6 sq. miles). They will stay in the same area year round as long as there's available food or open water nearby where they can hunt. Skilled adults are generally more likely to hunt live prey than immature eagles, which often obtain their food from scavenging.
Bald eagles have excellent eyesight, it's been estimated to be four to eight times that of a human. The eagle can identify a rabbit moving almost a mile away. When flying at an altitude of 1000 feet over open territory, the eagle has a view over an area of almost 3 square miles from a fixed position. Bald eagles are capable of seeing fish in the water from several hundred feet above, this is quite an extraordinary feat, since most fish are counter-shaded, meaning they are darker on top and thus harder to see from above.
Stomach Capacity / Crop
Eagle's have a small stomach, it's about the size of a walnut. The average eagle needs between ½ and 1 pound of food each day.
Because food sources might not be available daily, eagles can gorge on food when it's available, they can store up to 2 pounds of food in their crop.
The crop is a part of a bird's digestive system, it's an expanded storing area in their esophagus, a muscular pouch near the gullet or throat.
Most raptors, including hawks, eagles and vultures have a crop; however, owls do not.
Physical Characteristics: Color Pattern and Maturity
Adult bald eagles have white heads and tails with dark brown bodies and wings, they have piercing eyes and a powerful yellow beak, their legs are bright yellow and their claws have long sharp black talons.
Immature Bald Eagles have brown and white mottled plumage in varying amounts, as juveniles their heads remain dark brown. Their plumage goes through a transition over the first 5 years, each successive molt brings them closer to the classic adult pattern with the white head, they reach maturity by the 5th year. Most bald eagles can breed at 4 or 5 years of age, but many do not start breeding until much older. Bald eagles can live 15-30 years in the wild, the oldest known bald eagle in the wild was at least 38 years old when it was hit by vehicle and died.
Size: Weight, Wingspan, Height and Length
Size
The size of bald eagles varies by location and generally corresponds with Bergmann's rule, the further north of the Equator
the larger the species. Female bald eagles are slightly larger than males, males weigh approximately 25% less than females from the same area.
- Weight
The average weight of a female bald eagle is 10-14 pounds(4.5-6.3 kg), however bald eagles in the northern range tend to be larger than those in the southern range.
For example, in Alaska females can weigh more than 15 pounds (6.8 kg), whereas eagles in Florida can weigh as little as 7-8 pounds (3.1-3.6 kg).
Eagles have hollow bones; the entire skeleton of an adult eagle is generally estimated to be 5-6% of the body weight.
- Wingspan
The average wingspan for bald eagles ranges from 6 to 7.5 feet (182cm-229cm). Alaskan Females may reach a wingspan of 8 feet (2.5 metres).
- Height / Length
Bald eagles can be 29-43 inches (73-107 cm) long from head to tail.
Flight and Speed
Depending on wind speed and directon, bald eagles can fly 20-40 miles per hour (32-64 km) in normal flight and can dive at speeds of 75-100 mph (120-160 km/h), though it seldom dives vertically, if it needs more speed the eagle pulls in its wings to minimize their surface area. They can fly at altitudes of 10,000 feet or more and can soar in the air for hours riding on natural wind currents and thermal updrafts. Their large wingspan enables them to soar without flapping their wings very often and helps conserve energy as they hunt for prey.
Talons
Talons are important tools for hunting and defense. Eagles kill their prey by penetrating its flesh with their talons.
They can open and close their talons at will. If an eagle is dragged into the water by a fish too large for the eagle to lift, it is because the eagle refuses to release it.
Tiny projections on the bottom of eagle's feet called “spicules” help them grasp their prey.
It has been estimated that the gripping power (pounds per square inch) of the bald eagle is ten times greater than that of a human. The lifting power of a bald eagle is estimated at four or five pounds.
The beak, talons, and feathers of an eagle are made of keratin, the same material in our fingernails and hair. Because of this, the beak and talons continuously grow and are worn down through usage.
Eyesight
The bald eagles's eyes are almost as large as a human's, but its sharpness is 4 to 8 times that of a person with perfect vision.
Eagles can rotate its head approximately 180 degrees in each direction, they have 14 cervical vertebrae allowing for greater rotation than humans who have just 7 cervical vertebrae and can typically rotate 70-90 degrees in either direction. Eagles use both monocular and binocular vision, meaning they can use their eyes independently or together depending on what they are looking at. An eagle eye has two focal points (called "fovea" [singular] or "foveae" [plural]) one of which looks forward and the other to the side at about a 45 degree angle.
Eagles have eyelids that close during sleep. For blinking, they also have an inner eyelid called a nictitating membrane. Every three or four seconds, the nictitating membrane slides across the eye from front to back, wiping dirt and dust from the cornea. Because the membrane is translucent, the eagle can see even while it is over the eye.
Vocals
For such a large powerful bird, the bald eagle's voice is surprisingly weak.
Eagles do not have vocal cords. Sound is produced in the syrinx, a bony chamber located where the trachea divides to go to the lungs.
The bald eagle's call has been compared to a snickering laugh, some describe it as a combination of high pitched "whistling" and "piping", or a high piched shrill.
It consists of seven or eight notes sounded quickly, it can be written to sound like ki-ki-ki-ki-ki-ki-ker.
The male has a unique call that serves as a defense mechanism, it's a high-pitched peal that warns other eagles and predators that an area is defended.
Females also have a unique call when they are ready to mate, the sound is soft and high-pitched and repeated multiple times.
After hatching, the tiny nestlings makes a single tonal peep, the cheeping call serves as a way to beg for food and communicate with the adults.
By day thirty of their life their call begins to sound similar to the adults.
Hollywood has used bald eagles in many of it's productions, but instead of using the bald eagle's actual weaker call, they will dub over it with the more powerful screech of the red tailed hawk.
Hearing
Bald eagles have excellent hearing. Their ears are hidden behind feathers on the sides of their head, slightly below their eyes.
They do use their hearing to locate prey or other birds, but since they are diurnal (active by day) hunters, the sharpness of their hearing is not as essential as it would be for some owls, which can locate prey in the dark only by their sound.
Researchers found that eagles can hear over a frequency range of at least four octaves, centered on 2 kHz, which is roughly a “B” note on a piano, three octaves above middle C, with an upper limit between 6 kHz and 10 kHz at 80 decibels, and a lower limit that likely extends below 0.2 kHz.
Body Temperature
The bald eagle's average body temperature is 106 degrees Fahrenheit (41 degrees Celsius).
Eagles do not sweat, so they need to use other cooling methods such as perching in the shade, panting, and holding their wings away from their body.
Since eagles are warm-blooded, they radiate their own heat, their body temperature usually stays the same even during a frigid snowstorm.
Bald eagles have roughly 7,000 feathers to help keep them warm and dry in cold weather. Stiff exterior vane feathers zip together over fluffy down feathers, provide an overcoat that sheds water and helps keep heat from escaping. Bald eagles use countercurrent heat exchange to control core body temperature. Countercurrent heat exchange is an arrangement of blood vessels in which heat flows from warmer to cooler blood, usually reducing heat loss.
Mating, Breeding, Egg Laying, Incubation, Eaglet Feeding, Fledging and Nests
Bald eagles typically mate for life and look for other partners only if their mating partner dies.
When courting, male and female will perform an intense acrobatic display. They fly high into the sky, when they reach a desired altitude, they lock talons and go into a cartwheeling free fall, then break apart when they're nearing the ground. After the spectacular aerial display, courtship continues with nest-building, perching, billing, and preening.
The eagles mate while perched securely on a tree limb or in the nest. The male mounts the female’s back, twisting his tail under hers. The two press their cloacae together and sperm passes from his cloaca to hers. Mating is brief, intense, and takes place fairly frequently while the eagles are in season. Once copulation is complete, egg laying begins roughly 2 weeks later. They can lay 1 to 3 eggs (called a clutch),
incubation can take 34-36 days, slightly longer than a month. Both parents share in the incubation and feeding of the young. The eaglets fledge between 8 and 14 weeks of age, the parents will continue to feed them for another 6 weeks or so.
- Clutch Size: 1-3 eggs
- Number of Broods: 1 brood
- Egg Length: 2.3-3.3 in (5.8-8.4 cm)
- Egg Width: 1.9-2.5 in (4.7-6.3 cm)
- Incubation Period: 34-36 days
- Nestling Period: 56-98 days
- Egg Description: Dull white, usually without markings.
- Condition at Hatching: Covered with light-gray down; eyes brown; gape, legs, and skin pink.
It's unknown whether the male or the female takes the lead in selecting a nest site. Nests (aeries) are typically built in tall trees so the eggs will be safe.
The average size of a bald eagle nest is 4 to 5 feet in diameter and 2 to 4 feet deep, it's one of the largest nests built by any bird.. Each year the adult pair will use the same nest adding new material to it.
Both eagles work on the nest, which can take up to three months to build. They weave together sticks and small branches, making a sturdy base and "railing" around the outer rim.
Ground nests are built with of whatever materials are available, such as kelp and driftwood near coastal shorelines.
In the nesting area, the eagles place softer material such as prairie grass, lichen, or cornstalks.
Bald eagles are philopatric animals, meaning they are attached to the place where they were born and raised. they tend to return to their original habitat to breed. About 2,500 eagles migrate to the south eastern states from the northern states and Canada just to breed. Eagles who retain a nesting territory and partner are more productive than those that don’t.
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