Twelve winning essays from the YNA 2001 contest year
Bald eagles are an important part of the rich ecosystem of the Alaskan Chilkat Valley. In the late fall, over 3,000 bald eagles (Haliaeetus leucocephalus) gather here to feed on a run of chum salmon, making this the site of the largest bald eagle congregation in the world. For miles along the river, the eagles cover the snow-blanketed gravel bars and perch together in groups of up to 20 in the leafless cottonwood trees.
For the past three years, I have worked as a field assistant for a bald eagle ecology study in the Chilkat Valley. During this time, I have gained a greater understanding of the place I live through my many expeditions to observe eagles in their natural habitat. My experiences working for the eagle project inspired in me a love of field biology a swell as a fascination with bald eagles.
By early August last summer, as the field season came to a close, the young eagles from nests I had watched since before the eggs were laid had become active birds almost as big as their parents. I decided to conduct my own field research project to learn more about bald eagle activities surrounding the time of fledging, or first flight, not only because this is a little-studied and vital period of eagle development, but also because I wanted to be present for this final stage in the eaglets' lives before they left their nest areas and became independent birds.
I was interested to see if the exceedingly large abundance of food available to the resident population of 200-400 bald eagles affected eaglet behavior patterns during the fledging period. In addition, I saw the need to collect baseline data information about the fledging period in order to protect eaglets from potentially harmful human activities during this critical time.
The two nests I observed were in very different environments: one close to a river and estuary, the other close to the ocean. The Chilkoot River nest was at the top of a Sitka spruce tree on a small, forested ridge east of the Chilkoot River. I observed the nest from the road and hillside on the west side of the river, about 250 meters from the nest.
The Chilkoot River area is heavily used by brown bears, making it difficult for me to observe there alone. One evening as I was standing at the spotting scope, a young female brown bear with silver-tipped ears crossed the road less than 15 meters to my right, with apparent unconcern for my proximity. Largely as a result of the bear activity at the Chilkoot River, I logged nearly twice as many of my over 100 observation hours at Portage Cove.
The Portage Cove nest is located in a western hemlock on a hillside curving down to the shore of Portage Cove on the Lynn Canal. I observed this nest from the beach 200 meters from the nest, which is the only area from which the nest and surrounding trees are clearly visible.
Also visible a few miles across the channel are snow-tipped mountains shooting from sea level to 7,000 feet. When no eagles were present, I watched groups of ducks such as red-breasted mergansers, buffleheads and Barrow's goldeneyes bobbing in the water of the cove, or traced possible routes up the mountains.
On August 9, the first day of observations for my study, I set up the scope and tripod across the river from the Chilkoot River nest and prepared to begin three hours of continuous observation. Warm in the afternoon sun and with the rushing sound of the river filling my ears, I sat on the soft moss and pine needles and peered into the scope. Eaglets E1 and E2, both 11 weeks old, were perched in their nest on the skyline of the ridge. The eaglets had chocolate-brown feathers, dark brown eyes and bills, light white mottling on the underside of their wings, and yellow-gray ceres (the skin around the base of the beak). Older juvenile eagles are lighter in color and more heavily mottled, and adult eagles have distinctive white heads and tails, as well as yellow eyes and bills.
E1 preened, and then flapped its wings a few times. Although the Chilkoot River eaglets did not prove to follow the expected pattern of spending increasing percentages of time exercising their wings, or "flapping," as they grew older, the Portage Cove eaglets did spend increasing percentages of time on this behavior.
E1 flapped its wings again, more vigorously this time, and hopped into the air. As they approached fledging more closely, the eaglets in both nests observed began to jump into the air while flapping, rising as much as one meter above the nest. Eaglets also occasionally gripped the nest with their talons while flapping. Eaglets several times flapped so energetically that they almost fell from the nest, despite the tenacious grasp of their strong talons.
Finished flapping, E1 stood on one foot while clumsily scratching its bill with the other. Scratching vigorously, E1 over-balanced and quickly spread its wings so as to not fall off the nest. I laughed out loud.
Another behavior I observed during the pre-fledging period was aggression. One of these incidents of aggressive behavior occurred while the Portage Cove eaglets were eating in the same area of the nest. Several times, the eaglets stopped eating and lunged at each other with their beaks wide open, although they never actually made contact. This type of threat is used when the eaglets are close in size, in place of overt aggression. The eaglets were indeed close in size, and did not exhibit the sharp contrast in size (in favor of the female eagle) usually observed between eagles of different sexes.
A different day, E3 was perched on a branch above the nest. The branch was a favorite perch of the eaglets, and worn smooth from repeated use. E4 began to peck at E3's yellow feet and darkly mottled breast feathers, while E3 bowed its dark head. This head-down, stoop-shouldered stance has been identified as a defensive display. E4 continued to peck E3 until E3 hopped from the branch into the nest. E4 immediately hopped onto the vacated perch.
The Portage Cove eaglets were first seen perching on branches of the nest tree close to the nest on September 1 at 13 weeks old. The Portage Cove eaglets spent 2% of their time perching on branches of the nest tree close to the nest at 13 weeks old, and 32% of their time in this position at 14 weeks old. Both Portage Cove eaglets fledged at 14 weeks of age. This data supports conclusions of other scientists that pre-fledging eaglets spend increasing amounts of time perching on branches of the nest tree and trees close to the nest tree as the eaglets grow older. This behavior was precluded at the Chilkoot River nest by the lack of suitable perching branches close to the nest.
The actual first flight of an eaglet is often clumsy and awkward. Many eaglets have difficulty landing in trees after their fledging flight. On average, approximately half of eaglets are grounded after the first flight and eaglets that fledge early are more likely to end their first flight on the ground.
On September 13, E4 of the Portage Cove nest was 14 weeks old and had not yet been observed outside of the nest. I knew E4's sibling, E3, had fledged the day before, although I had not seen the actual first flight. It was calm, and raining lightly. Low clouds obscured the view of the mountains across the channel. Encased in rain gear, I had been observing the nest for over one hour.
E4 was perched on a small branch above its nest, and had been exercising its wings with vigorous flapping while clinging tightly to the branch. The eaglet stopped flapping and seemed to become part of the tree itself, motionlessly rooted to the branch. Suddenly, with no vocalization or other warning, E4 flew from its perch to the top of a tree 15 m from the nest tree. E4 landed on the new perch fairly smoothly, and perched there quietly. I wildly looked around for someone to talk to, to pour out my happiness at finally seeing a first flight, a behavior I had read about and longed to see for so long. I was alone, so all I could do was record the flight on my data sheet.
E4 was the last eaglet observed for this study to fledge. The Chilkoot River eaglets fledged at 13 and at 14 weeks old respectively, while both Portage Cove eaglets were 14 weeks old when they fledged. Fledging occurs from 8 to 14 weeks after hatching and most commonly at 10 to 12 weeks after hatching. It is interesting to note that if the actual hatching date at the Portage Cove nest was 2 days earlier than estimated, the fledging of E4 could have occurred while that eaglet was 15 weeks old, outside of the normal 8-14 weeks of age fledging range. E4's comparatively late fledging date may be the cause of the eaglet's unusually smooth first flight, or the flight I observed may not have been the actual first flight, although it was the first time I saw E4 outside of the nest.
There are several possible explanations for the comparatively late fledging dates recorded during this study. By staying in the nest longer than average, eaglets could benefit from increased size and strength at fledging. The abundance of fish late in the summer could make it possible for eaglets in the study area to remain in their nests while eaglets in areas with less abundant food must fledge earlier in order to forage farther from their nest areas. A strong pink salmon run occurred throughout the study period at the Chilkoot area, providing the Chilkoot River nest parent eagles with easy access to food for their eaglets. The Portage Cove area is also amply stocked with migrating anadromous fish and a resident fish population at this time of year. The rich marine environment in this area is evidenced by the increased productivity of marine eagle nests in the Chilkat Valley as compared to those nests in river or lake areas.
During the post-fledging period, eaglets begin to forage for food and often expand the distances they range from their natal nest site, gradually losing dependence on parent eagles and nest sites to become independent eagles.
I first observed a Chilkoot River eaglet catching food for itself on September 19, when E1 and E2 were 17 weeks old. It was overcast and cool, around 40 degrees. The river corridor was filled with the sounds of gulls feeding on the pink salmon running up the river. E2, perched on a granite boulder in a shallow section of the river, reached out one foot and grasped with its talons the silver back of a fish that was swimming past. Suddenly E2 was in the river, although I could not determine if the eagle had been pulled in by the fish or had intentionally waded into the water. E2 was submerged in the river, with only wings and head above the surface. Flapping its wings, E2 eventually pulled itself and the fish back onto the rock, and began to peck at the fish.
October 1, my 16th birthday, was cold, clear and windy. I drove the 17 miles from my house at Paradise Cove to Chilkoot Inlet happy in the warmth of the car, dreading the freezing winds that awaited me when I stepped outside.
The Chilkoot River poured into the Inlet, but the sound of the wind and the screeches of birds overwhelmed that of the water. The tide was low, and the gold-gray tidal flats of the Chilkoot estuary were covered with bald eagles, ravens, and hundreds of gulls feeding on the carcasses of pink salmon. Setting up the scope on the lee side of the car, I soon located the two fledgling eagles from the Chilkoot River nest among the crowd of adult eagles and juvenile eagles of various plumage classes.
E1 and E2, now 18 weeks old, were in zone 6 of 8 circular zones, each 200 meters wide, radiating from the Chilkoot River nest. I determined nest positions by using a global positioning system (GPS) device to plot several positions and a standard compass to take bearings from those positions to the nests. Using the triangulation technique, I plotted the nests at the intersection of the bearing lines. I estimated eaglet perch locations using a GPS and compass, then plotted the locations in one of the eight zones.
Within their 18th week of age, E1 and E2 were located in zones 3-8, or 600-1600 meters from the nest. These eaglets showed a strong increase in perch distance from the nest with increasing age, contrasting sharply with eaglets E3 and E4 of the Portage Cove nest: from 14 weeks after hatching, when both E3 and E4 fledged, to 19 weeks after hatching, when the study ended, E3 and E4 were exclusively located perching in zone 1, or not more than 200 meters from the Portage Cove nest.
The movements of the Chilkoot River eaglets were most likely influenced by the abundance of pink salmon carcasses located on the tidal flats over one kilometer from the nest and the large number of other adult and immature eagles observed feeding on the carcasses. Most eagles tend to attempt to steal rather than catch food, even when food is plentiful. The practice of stealing food is especially important for fledgling eagles that still lack experience and skill in hunting. Although the Chilkoot River eaglets were initially observed trying to catch and eat the pink salmon running up the river, the eaglets soon joined the majority of other eagles on the tidal flats and began to steal their food or forage on carrion.
The Portage Cove eaglets had no large concentration of food comparable to that of the Chilkoot pink salmon run in the area surrounding the nest. The supply of food in the area immediately surrounding the Portage Cove nest was most likely sufficient to allow the Portage Cove eaglets to remain in the nest area. A Portage Cove eaglet was several times observed soaring over areas more than 200 meters from the nest, but the eaglets were never observed perching in these areas.
Both eaglet sibling pairs were 19 weeks old on the dates I ended observations of the respective areas (October 9 at the Chilkoot River and October 15 at Portage Cove). Fledgling eaglets have been observed to remain in the natal area until 14-21 weeks old. However, eaglets in the Chilkat Valley may have very different post-fledging nest dependency periods than eaglets in other areas. Further studies should be conducted to answer this question, and to provide more data in order for significant conclusions to be reached regarding fledging behavioral patterns.
My time spent observing eagles for my study has brought me to a better understanding not only of the changes young eagles undergo during the time of fledging, but of the local environment as a whole. Through my expeditions to Portage Cove and Chilkoot River, I have truly become part of the Chilkat Valley. I have been outside early in the morning and after dark at night, in the rain, wind, and snow when most people are sheltered inside. I have spent lengthy hours observing eagles patiently, immersing myself in their activities until I forget myself.
I have lived in the Chilkat Valley all my life, but I am only beginning to discover the wonders of the wide array of other organisms that are part of the environment here. The environment of any one place is vastly complex. Perhaps the best way to study it is to choose one organism to focus on intensely. By studying that organism, you can begin to see the many ways it is related to and dependent on its environment. Through these eagles, I have come to better understand the place in which I live.
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This winning entry in the museum's Young Naturalist Awards 2001 takes a look at eaglet behavior and feeding patterns. Caitlin's narrative essay covers:
Supplement a study of biology with an activity drawn from this winning student essay.