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Nature’s aviators

The life of an eagle

Posted 11/4/20

The background

Today, it’s not uncommon to venture near a river or waterway and see a bald eagle—frequently, you can spot more than one. They’re usually seen soaring overhead or …

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explore the great outdoors

Nature’s aviators

The life of an eagle


The background

Today, it’s not uncommon to venture near a river or waterway and see a bald eagle—frequently, you can spot more than one. They’re usually seen soaring overhead or perched in a tree by the water, waiting for a fish to swim by. There are many breeding pairs in the region. In the winter, many eagles migrate from the North to take advantage of open water foraging spots. During a Veterans Day ceremony in Port Jervis, NY last year, an adult eagle soared over the ceremony as if to salute all the people who served and those who didn’t come back home.

Eagles were not always easily found in the region, however. As recent as the early 1990s, there were no nests on the Upper Delaware, in fact, there were a mere five nesting territories combined for both NY and PA as of the late 1970s to early 1980s. These nests were unable to produce young because the eggs could not stand the weight of an incubating adult due to eggshells that were too thin. This was caused by a pesticide known as DDT, a very persistent compound that had detrimental effects on wildlife. Regulatory actions regarding DDT took place during the 1950s and 1960s, and the EPA issued a cancellation order for all uses of DDT in 1972.

The damage was done in this and many other regions in the U.S. DDT is a chemical that can accumulate in the fatty tissue of animals. Through the process of bio-accumulation eagles ingested DDT through predation of fish in waterways where DDT had accumulated via runoff and other means. With eagles, DDT interfered with females producing eggs with strong shells. After the total ban on DDT, levels of DDT slowly but gradually fell to levels low enough to be deemed to be safe for eagle habitat. However, with so few nests in the region unable to produce young, the bald eagle was at real risk of being extirpated from widespread regions in the Northeastern U.S. and elsewhere.

In 1976, when NYSDEC started reintroducing just under 200 eagles over the span of a decade, there were no active bald eagle breeding territories in the region. Pennsylvania started their own reintroduction program a few years later, and both states were closely monitoring suitable habitats for new breeding territories as the transplanted young eagles became adults and able to breed. In this region, one of the prime areas monitored was the Upper Delaware Scenic and Recreational River, a 73-mile stretch between Sparrow Bush and Hancock, NY designated under the Scenic and Wild Rivers Act. NY and PA, as well as the NPS surveyed the Upper Delaware for many years without finding any breeding territories. In 1993, the situation changed; a single pair appeared and occupied an area along the river. They did not produce young for the first year, but successfully fledged young the following year. In the springtime, there are more than 20 occupied nests in the Upper Delaware corridor.

At the beginning

Bald eagles start their courtship flights over the winter, with both members of the pair locking talons and spiraling towards the ground; they unlock their talons and pull out of the dive, sometimes pretty close to the ground. They mate during late winter (not in the air, as rumor has it, but perched on a branch). During this time, eagles may be seen bringing dried grass to the nest to act as a bed for the soon-to-come eggs. The dry grass also helps the incubating adults further insulate the eggs against the colder days of late winter and early spring.

The actual eggs appear around the first or second week of March in the region, with some pairs starting incubation during the last few days of February. The incubation period for bald eagle eggs is about 35 days, and both the male and female will exchange incubation duty. Both adults pluck some feathers from their breast; this is their “brood patch.” This enables an incubating adult to more efficiently incubate eggs by ensuring skin to egg contact. (An eagle’s body temperature is 102 degrees.) An “egg cup,” a small depression, is hollowed out in the nest lining material. This aids in incubation, especially during colder weather. Nests average two eggs, sometimes with as few as one and as many as three young. There have been reports of four, but that is rare.

The youngsters

When the eggs hatch, they hatch in the order that they were laid. A female eagle usually lays an egg every other day, so in a nest with three young, there may be several days between youngest and oldest sibling. A newly hatched bald eagle weighs in at a couple of ounces and is about four to five inches long. These new eaglets are covered in natal down, which is white in color.

As the eaglets approach two weeks of age, their white natal down is replaced by a gray secondary, or mesoptile down. This is also referred to as thermal down, and it has better insulating properties than the white down. The eaglets are developing the ability to self-thermoregulate, so they don’t need to be brooded as much. The eaglets are more mobile during this time; some observers get their first look at the young as they are large enough to peer over the nest wall.

From two to about five weeks, young eagles enter a phase of accelerated growth. This growth spurt can add about 150 grams of weight gain per day for the average eaglet. The adults are busy foraging for the young at this time as this growth rate requires a lot of food. Three or four fish a day may be delivered to the nest during this time. The adults have another ongoing task during this time. They may be seen either carrying sticks to the nest, or adjusting what is there. As the young are getting more active, they can get too close to the side of the nest and fall out or be pushed out by another sibling. The addition of sticks to form a kind of “fence” may prevent the loss of an eaglet.

Getting ready to fly

At around four weeks of age, patches of contour feathers start to become evident growing in patches amongst the grey down. These patches are small at first, but rapidly fill in to a point where the eaglet’s back is mostly covered with contour feathers. Just after this time is when the growth rate slows, by the time the young is six weeks old, it has reached 90% of its terminal weight (the weight at fledging, 10 to 12 weeks of age). Energy used for body mass increase is now being used for feather development.

At six to eight weeks old, young eagles start to look like a dark brown version of their parents. The belly feathers are the last to fill in as the flight feathers continue to grow. The young eagles are capable of feeding themselves for the most part. An adult may drop food in the nest and then fly off to let the young sort out which sibling gets to eat. When food isn’t being delivered by the adults, the young spend a lot of time sleeping. They flop down on their belly and nap. It’s probably safer for the young to sleep lying down at this point because the once spacious nest is now very crowded with up to three young and an adult or two. It’s more difficult for a young to get pushed out of the nest or slip off the side.

Get ready, get set, GO!

As eagles get to be nine to 10 weeks of age, the flapping of wings becomes more evident. They do this in a lazy manner when they are younger, but at this time, they are much more serious about it. They need to be flight-ready in a couple more weeks when they attempt to successfully fledge. An average eagle nest is about 90 feet off the ground, and it’s a long drop if a mistake is made. When these young start their wing flapping, the siblings in the nest either duck or head off to the other side. This wing exercise builds muscle strength and confidence, both of which will be needed on the day. As these young gain skill, their feet leave the nest, and they seem to hover several feet above the nest for a few seconds. These young frequently go out on branches and wing exercise; this further builds up confidence.

Any time after 10 to 11 weeks of age is a likely time for a young eagle to fledge. A few may get injured or not make it, but the majority will successfully fledge, maybe landing in an awkward spot. Their landings rapidly improve with experience. With the successful fledge, young eagles now start learning survival skills it will need to survive, how to hunt, how to steal food and how to defend itself. The adults aid in this process in the first few weeks after fledging, but by the last weeks of summer, the fledglings are now on their own.

Becoming an adult

As the summer wears on, fledgling eagles range further and further from the nest. As winter approaches, they may stay in the immediate area or head hundreds of miles away. Immature eagles go through the next four years wandering and foraging, though many do return to the region during summer months. They are brown but pick up a mottled appearance with successive molts. Aging of immature bald eagles can be done by knowing the various color phases of an eagle’s plumage. When an eagle becomes around four years old, it looks like an adult with some mottling; the white head may have black streaks, the bill is yellow with some dark streaks, and white streaks may be present in the brown. This is the mark of a subadult. The next molt after this stage will be the classic adult phase we all recognize; this occurs at about between four and five years of age.

A happy ending

A bald eagle is sexually mature and capable of breeding when it reaches adulthood, and when eagles seek a mate and nesting territory, it typically finds a territory within 50 to 100 miles of its natal nest; banded young banded during NYSDEC nest surveys have been found several years later as part of a breeding pair just a few miles up and down river.

An eagle in the region may live 25 to 30 years on the average; a breeding adult may produce 20 or more broods of young in its lifetime. That number may go a little higher. In 2015, an eagle was recovered in upstate NY after being killed by a car, and it had a leg band. The band was checked, and it turned out that this male eagle was one of the original eagles transported from Wisconsin to New York in 1977. This eagle was 38 years old when it met its demise, which is now the longest-living eagle recorded in the United States. He could have been breeding for 34 years and likely fathered a lot of eaglets.

So is the end of a story of what first looked like a certain loss turned into a success through conservation efforts of many agencies, organizations, and individuals. For myself, I was honered to be a part of the effort, especially with Pete Nye and others with the NYSDEC. I had to wonder when I saw that eagle fly over that one Veterans Day over the ceremony in Port Jervis; Was that one I tracked, or is it in my field notebook somewhere? The hope of myself and many other people is that we thoughtfully plan development that may occur in eagle habitat, for habitat loss is a big threat at the present time.

eagles, life cycle, growing, Delaware


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