This essay will discuss many different key factors
that played into the population decline in the bald eagle species and what measures have been
taken by humans to restore their population. It will also describe how those factors came
to be and what measures were and are still being used to insure that the birds will not have
to face the same threats as before. It will also discuss several different human groups and
their research done in this field. Researching and testing of many kinds have been performed
on the eagle species and this paper will examine and explain a few of the methods that were used.
Bald Eagles: Many Struggles for Survival
Academic affiliation: Oklahoma State University
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Bald eagles are beautiful and magnificent creatures
that are very easily identified by their distinctive characteristics: "The plumage of an adult
bald eagle is brown with a white head and tail...their legs are feathered half way down the
tarsus, and the beak, feet, and eyes are bright yellow... Bald eagles have massive tarsi,
short and powerful grasping toes, and long talons...the wing span of an eagle can reach seven
and a half feet" (Endangered species 1). Unfortunately, these birds suffered in many ways due
to their beauty and magnificence. Bald eagles were hunted and killed by humans for the purpose
of making money for their feet and feathers. "A bounty of fifty cents" (Name 77) was put out in
1917 for each eagle captured and killed. The birds were also hunted by human kind because some
people believed that the birds were attacking and killing larger mammals, which the people owned.
Bald eagles main meal is fish and they also eat the bodies of animals that are already dead.
"Added to the local prejudice against the bird, which has been inflamed by exaggerated tales
of damage done, and to the pleasure that many people take in killing something, especially some
large and conspicuously beautiful creature, it has resulted in an enormous destruction of them"
(Name 77). Thousands of eagles died because of the bounty that was put out for their deaths.
The population of bald eagles decreased rapidly due
to the various forms of human disruption. "It's estimated that 50,000 breeding pairs
occupied the lower 48 states in pre-colonial times. By the 1960s, only 400 pairs occupied
this area" (North Dakota's Endangered and Threatened Species: Bald Eagle 1). One form of
disruption on the birds due to the people was the use of certain chemicals and pesticides.
These chemicals have many affects on the air, water, and environment. The polluted water
and air began to affect many different species. The polluted water began in infest the
fish species with the chemicals. Fish are the birds' main source of food, so the eagles
also became infested with the harmful chemicals. DDT is a well-known chemical worldwide.
It is blamed for the huge decrease in the eagle population. This chemical was widely used
throughout the U.S by many people for various reasons. Humans were unaware of the horrible
affects that the chemical had on the economy: "DDT in a female's body disturbs the
shell-making process, causing her to produce very weak shells or no shells at all"
(Endangered Species 2). This greatly affected the eagle population because the birds
were unable to reproduce. The shells, if present, were too thin and brittle for the
adults to hatch.
Another form of disruption that harmed the bird
population was the development which took away the tall trees in which they lived. Bald
eagles prefer to build their nests in the tops of very tall trees. Development requires
that these very same tall trees be cut down for the use of telephone poles, houses, lumber,
paper, etc. Humans finally realized that they were cutting down the homes of the bald eagles
and began to replant trees in hope that the birds would survive. The American Forests
"planted 19,950 pines and oaks to reduce runoff, expand habitat and benefit balds at
Eastern Neck National Wildlife Refuge along the Chesapeake Bay on Maryland's Eastern
Shore" (Enloe 33).
Lead poisoning was another way that bald eagles began
to decline in population due to the disturbance of humans. Ruth Wusk discusses this issue in
"The Bald Eagle: Nebraska's Winter Visitors": "Lead poisoning from the ingestion of spent lead
shot has been identified as a serious mortality factor in waterfowl" (4). The fish would become
contaminated, crippled, or die from the lead. The bald eagle would then eat the contaminated
waterfowl, "ingesting lead pellets imbedded in the waterfowl's flesh or gizzard," causing the
eagle to become ill and die (The Bald Eagle 4).
The bald eagle species have made a miraculous
comeback in population size. Strangely enough, this comeback is because humans have figured
out ways to protect and save the bald eagles species from extinction. Humans finally realized
that the chemical DDT was fatal to the bald eagle species. The use of the chemical DDT was
then banned from use in the United States in 1972. Laws were also passed which protected the
birds from other harmful chemicals and also from being killed by humans. "In 1973, the American
bald eagle became one of the first species listed as endangered under the new Endangered Species
Act" (Enloe 34). This particular act played a huge role in the recovery of the birds'
population. It not only protected the birds from being killed but also protected their habitat
The number of active nests among the bald eagles
in 1998 has increased as much as 10 times the amount that were present in 1963 according to the
information that Jim Kraus provides in "bald eagle recovery in the lower 48 states":
Federal statues such as the Bald and Golden Eagle Protection Act and the Migratory Bird Treaty
Act will continue to protect eagles form direct take... Other statues, including the Lacey Act,
the National Environmental Protection Act, the Clean Water Act (CWA), and the Federal Insecticide,
Fungicide, and Rodenticide Act, will continue to provide protection from illegal trade, transport,
and contaminants. (E27)
Each state will continue to perform studies and tests and monitor the birds' reproduction and the
chemicals used in the state to ensure that the eagles do not experience the decline that happened
in the early part of the 1900s. Many studies have been performed by a number of individuals and
groups in order to try to learn more about the species and find other ways of helping their
population recover. Cheryl R. Dykstra and her team performed many studies to try to learn more
about the energy needed for bald eagle nestlings. The studies were done along the Wisconsin
shoreline of Lake Superior and in northern Wisconsin. The studies were done during the winter
season and breeding season. Doubly labeled water measurements were taken as a way to determine
To determine energy requirements of nestlings uses doubly-labeled water (DLW) to measure nestling
energy expenditure, in a technique which has been validated for bald eagle nestlings in the
laboratory. In order for the DLW measurements to present typical eaglet expenditures for any
region, the nestling studied should be in average-size broods located in representative habitat
for that region. (175)
This technique was done in several different places in order to provide accurate results. The
nestlings were also weighed and measured to provide additional information for the testing.
Blood and plasma samples were also taken from the brachial vein of each bird tested. Studies
were also performed to determine how many eggs were laid and hatched in these areas. Energy
within the nestlings were also monitored and used in research.
Andrew Hansen also performed tests on bald eagles
to try to help their population recover. Tests were performed to define the differences
between active and inactive nesting. Field studies were done to determine breeding status.
Active nests, breeding area, breeding density, and successful nests were included in the
studies. Eleven different habitat characteristics were used to determine breeding patterns:
Two of the habitat variables described the breeding area: timber type (old-growth, second-growth,
or old growth tree left in logged area); and degree of human activity...the remaining nine
variables described the nest tree: relative age (immature, mature, or decadent); height class
(canopy height, super-canopy height); condition (live, normal; live, broken or dead top; dead);
species (black cottonwood, Sitka spruce); height: diameter at breast height; distance to water;
base elevation above water; and nest height. (1388)
Artificial feeding experiments were also performed.
After some major floods removed all the eagles' natural food sources, artificial food was placed
in the same area for the birds. Results from the several different kinds of studies which were
performed in various locations at different times, show that food abundance and habitat quality
play a major role in the reproduction of the bald eagles.
Marco Restani and his team performed tests to
determine "Numerical and Functional responses of migrant bald eagles exploiting a seasonally
concentrated food source" (Restani 563). Numerical and Functional responses of bald eagles
were examined and determined by observations of the birds during various feeding times in
several ways. Bald eagles often migrate in large numbers, to locations in which have an
abundant supply of food. These studies were conducted in Montana along the Missouri River
from mid-October to mid-December. Numerical responses were studied and determined by a
four-person team in a motorboat. "To determine the numerical response, a four-person team
conducted surveys by motorboat once a week from late September through December 1991-1995 to
estimate the number of eagles congregating at Hauser Reservoir" (Restani 563). Functional
responses were studied and determined by a team in a parked vehicle. "To determine the
functional response of scavenging eagles, we recorded the foraging behavior of bald eagles
from a vehicle parked 20150 rn from the primary salmon spawning and eagle foraging sites"
(Restani 563). Consumption and handling rates were similar between sub-adults and adults;
however, attacking rates appeared higher among the sub-adult eagles.
Bald eagles have gone from a population of
over 50,000 breeding pairs down to as low as 400 breeding pairs due to several different
factors. With the help of humans, the bird species have now increased greatly in
population size. In fact, the bald eagle species have made such a great recovery that
they may be taken off the endangered species list altogether. The many different programs
that humans have created for the species are still being used today to provide that the
species will not ever have to suffer the same injustices that they once faced.
Dykstra, Cheryl R., et al. "Daily energy expenditure of nestling Bald Eagles in
northwestern Wisconsin." The Condor 103.1 (2001): 175-79.
Enloe, Charles. "Once again, a land of eagles." American Forests 108.3 (2002): 32-37.
Endangered Species: Bald Eagle. Ed. Ron Fellows. Bureau Of Land
Management Bakersfield Field Office. 13 Feb. 2003
Hansen, Andrew. "Regulation of Bald Eagle Reproductive Rates in Southeast Alaska."
Ecology 68.5 (1987): 1387-1392.
Kraus, Jim. "Bald Eagle recovery in the lower 48 states." Endangered Species Update
16.6 (1999): E26-E27.
Name, Van W. G. "Notes and Comments: Threatened Extinction of the Bald Eagle." 2.1
North Dakota's Endangered and Threatened Species: Bald Eagle. Dec 30, 2002.
Northern Prairie Wildlife Research Center. 13 Feb 2003
Restani, Marco, et al. "Numerical and functional responses of migrant bald eagles
exploiting a seasonally concentrated food source." The Condor 102.3 (2002): 561-68.
The Bald Eagle: Nebraska's Winter Visitors. Ed. Ruth Wusk. Nebraska Games and
Parks Commission Wildlife Resources. 18 Feb. 2003
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