From Nest to Sky: The Flight Development of Eagles
Those of us who live around here sometimes take the bald eagles for granted. We forget that they’re not common in all parts of the country. But we never tire of seeing them and their majestic white heads and enormous wingspans. This article delves into the fascinating process of how eagles, specifically bald eagles, learn to fly, from their first days as helpless hatchlings to their eventual independence.
The Eaglet's Early Life: From Hatching to Fledging
Eaglets come into the world totally helpless. They cannot hold their head up; their vision is limited; their legs are too weak to hold their weight. It can take days for them to completely hatch from the first pip to being totally free from the shell.
After hatching, food will be offered to the eaglet by the parent, who shreds meat off fish or whatever is available. Tiny pieces will be offered again and again as the eaglet struggles to hold his wobbly head still long enough to take the food. An eaglet has a crop - a storage area - below its chin. Food goes into the crop and is then digested as needed. When the crop is “full” you can see it bulging out. This crop is actually part of the esophagus where food is stored and softened.
The eaglets grow rapidly, they add about a half pound to a pound of body weight every week until they are about 9-10 weeks old, depending on if the eaglet is a male or female. At about three to six weeks, black juvenile feathers will begin to grow in. At about eight weeks, the appetites of the eaglets is at its greatest. The parents will hunt almost continuous to feed them.
Fledging: The First Flight and Beyond
Fledging is when the baby eagles, aka eaglets, learn to fly. It comes from the term "fledge," which refers to when the babies have acquired the feathers and wing muscles necessary for flight. Eaglets fly ‘in place’ over their nest until they feel strong and brave enough to fly for the first time at approximately 12 weeks of age. Winds stimulate the eaglets to exercise to the extent that, where winds are more consistent, they may exercise enough to fly by 10 weeks of age. In the absence of winds, their first flight may be delayed a week or more.
Read also: Finding Stillness
Eagles learn and hone their ability to fly several weeks before actually flying. They practice with short takeoffs and landings on and around the nest, gaining strength and improving their agility and landing ability. This is a crucial stage for the young birds as many try to fly prematurely, especially when startled or flushed from the tree by predators and people who get too close. When about 12 weeks old, the young eagles are ready to fledge. Eaglets hatch after 35 days of incubation and are in the nest for 10-13 weeks before they fledge or take their first flight.
Once the young eagles have fledged, they remain around the nest for 4 or 5 weeks, taking short flights while their primary feathers grow and strengthen. Their parents will still provide all of their food. After that, they’re on their own. Most likely the parents are off hunting for food for the eaglets.
The Learning Process: Imitation and Independence
Fledgling eaglets start their flight training by doing what they have, for months, watched their parents do - spreading their wings. Drafts of wind catch the outstretched wings and levitate the fledgling slightly from the nest, giving it a feel for flight. Once this imitative behavior leads to the eaglet’s first awkward, half-accidental flight, the parents begin a focused training process designed to teach their youngster, step by step, how to survive independently (assuming, of course, that the eaglet survives that first flight). During this training stage, the eagle goes to work with its parents, observing and imitating the motions of flight and the skills of hunting, and venturing further and further from the nest.
On average, they spend 4-12 weeks in the nesting territory post-fledging, the time during which they learn to hunt and fly. Young eagles from wild nests develop their hunting skills on their own, but spend considerable time after they fledge watching their parents, and undoubtedly learning by watching what the adults do. The drive to hunt, is innate.
The juvenile fledglings, with the exception of their color, look similar to their parents, but are nothing like them in behavior. The juveniles now have to learn to hunt, and they only what’s left of summer to learn. On average it takes about 4 - 12 weeks for young eagles to start hunting successfully. True, fully refined, specialized hunting skills, probably take years to develop.
Read also: Pittsburgh and Boston College Football: A Historical Overview
Nesting Habits and Parental Care
An eagle pair typically returns to the same territory and nest site each breeding season. This territory may cover an area greater than a square mile and include more than one nest. From year to year, the pair may abandon a nest for unknown reasons and build another, or it may continue adding to and renovating a single nest. In addition to large stands of trees, the nesting habitat must provide an abundant food source. Eagles defend their territory against potential nest predators like hawks, owls, gulls, crows, and ravens. They also protect the territory from other eagles, especially adult eagles.
Because eagles build sizable nests, a nest tree must be large and sturdy. Eagle nests are usually within view of a lake, river or large creek and a pair often chooses a dominant tree within the surrounding woodland. A good nest tree affords an easy glide upon leaving the nest and provides a favorable vantage point to scout for potential threats. Eagles build their nest in a branched crotch toward the top of the tree. The birds stack and interweave sticks and branches to create a bulky nest and line its center with soft material such as moss, grass, twigs and feathers.
If the nest tree remains intact through inclement weather, natural aging and other environmental influences, over the years a nest may reach enormous dimensions. A nest may exceed eight feet wide, 12 feet high and weigh more than two tons. Typical nests are five to six feet wide and more than three feet high. The bald eagle builds the largest nest of any North American bird. Sometimes nests are so large compared to their support tree they collapse the tree, especially when heavy snow or rains augment their weight. When trees decline in health sometimes they are no longer capable of supporting a large, heavy eagle nest.
Nest building and renovation may begin as early as November or December in areas where eagles remain local through winter. Some pairs need to rebuild their nest almost from scratch if winds and bad weather have damaged it. This nesting activity starts one to three months before the female lays eggs. An eagle nest is usually 40 to 100 feet above the ground.
Acts of courtship or renewing pair bonds can be as subtle as two eagles perching together on a branch or the pair rearranging twigs and dried grass at the nest. The eagle pair also vocalizes back and forth with a variety of shrill calls.
Read also: Historic Gridiron Clash
Incubating begins as the female lays the first egg. The clutch of one to three eggs is completed within three to six days. Both parents share in the task of incubating. However, females spend more time brooding than males. Once they begin incubating, the viewer usually sees only one eagle at a time, except when they make the incubation duty changeover at the nest. Each egg takes 35 days to hatch. An eagle sits very low in the nest when on eggs. Sometimes only its head and tail are visible. One eagle will bring food to the bird on duty and the two may switch roles. The incubating bird will carefully position itself by sitting chest first while wiggling side to side to accommodate the egg or eggs. A bird on eggs sits remarkably still for long periods. This is the period when nests are most vulnerable to abandonment. When eagles are flushed from their nest, their eggs may be exposed to cold or rainy weather and predators. Cold air can cause egg failure.
Young eagles break through and escape the egg-shell on their own. Eggs hatch one at a time with one to four days between hatchings. The adult on the nest calls to its mate for food when the foraging eagle is near. The birds communicate back and forth as the eagle bringing food approaches the nest. Both adults tear the fish apart and feed the young. As eggs hatch, behavior changes at the nest. An eagle sitting on hatchlings is more active than when on eggs.
Challenges and Threats to Young Eagles
Eaglets in the nest may die from falls, starvation or siblicide. About 50% of eaglets will not reach one year of age. Once fledged, many eagles die from impact injuries, starvation, disease, shooting, poisoning and electrocution. Lead poisoning is a common cause of injury and is most often fatal.
Eagles have faced difficulties for many reasons in the course of human history. In the 1950’s a chemical known as DDT (dichlorodiphenyltrichloroethane) was introduced as a widespread insecticide. In 1962, biologist Rachel Carson detailed the impacts of the indiscriminate spraying of DDT and other chemicals on wildlife, and birds in particular in her book Silent Spring. Subsequent research showed that DDT, (the chemical by-product of which was present in many aquatic environments, especially those near agricultural areas) interfered with the calcium processing of birds and resulted in deformed eggshells. In 1972, DDT was banned in the United States and the bald eagle was put on the newly created Endangered Species List.
Bald Eagle Facts
- Adult bald eagles have dark brown feathers on their body and wings, and white feathers on their head and tail.
- Juvenile bald eagles in their first year of life are dark brown over their body wings, head and tail. Their beak and eyes are dark. As they age, juveniles may show white feathers anywhere on the body, especially the breast and under the wings. At three to four years of age they begin to develop the white head and tail of the adult. Their beak and eyes lighten in color as they reach adulthood.
- In the wild, 70-80% of eagles die before they reach adulthood at five years of age. An eagle that makes it to adulthood might live 20-25 years.
- 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. These two foveae allow eagles to see straight ahead and to the side simultaneously. The fovea at 45 degrees is used to view things at long distances.
- Wingspan of an eagle depends on overall size. Eagles can achieve 30 mph using powerful wing-beats and even faster when diving after prey (stoop). Typical wingspan is between 6 and 7.5 feet.
- Fish is the primary food of bald eagles, but they will eat a variety of other animals and birds. Their prey items include waterfowl and small mammals like squirrels, prairie dogs, raccoons and rabbits. The average eagle needs between ½ and 1 pound of food each day. However, eagles do not need to eat daily.
- An eagle’s stomach is quite small, about the size of a walnut. However, eagles can eat up to 1/3 of their own body weight in food.
- Weight varies depending on latitude and gender. Generally, males weigh approximately 25% less than females from the same area. The average weight of a female bald eagle is 10-14 pounds; however there exists great variation depending on where an eagle is from. Southern bald eagles tend to be smaller than those in northern parts of their range. For example in Alaska, females might weigh up to 18 pounds, whereas eagles in Florida can weigh as little as 6-8 pounds.
- Bald eagles normally lay one to three eggs per year. There have been a few successful clutches of 4 eggs documented. The eggs hatch after about 35 days of incubation. The average Bald Eagle clutch size is just under 2 eggs / clutch (1.9).
- Bald eagles do not dive into the water but rather skim across the top catching fish near the surface. During salmon runs in Alaska, bald eagles have been observed standing on the shoreline and pouncing on salmon as the fish swims into shallow water.
tags: #how #do #eagles #learn #to #fly
