Eagle Survival - Threats to Survival
DDT is an insecticide that has been widely used for pest control. The three letters come from the chemical name, dichloro-diphenyl-trichloroethane.
This grayish-white powder kills insects by affecting the nervous
system. DDT decays slowly and appears in the birds, fish, and animals
that eat plants.
Some DDT never reaches the pest or insect that it is supposed
to control. Small particles of DDT travel through the air and water. It
is then absorbed in the organs and tissues of birds and animals. DDT is
also taken indirectly when birds and fish eat organisms that contain
the pesticide. DDT then passes from one organism to another through the
food chain. A large fish can take in heavy amounts of DDT by eating
smaller fish that have the material in their body. If a bird then eats
the fish, the concentrated DDT is passed into the bird's body. This is
an illustration of a food chain.
DDT has been used on a large scale. DDT has killed useful
insects, as well as harmful ones. It may have also endangered other
animal life, including birds and fish. Ingestion of pesticides such as
DDT has disrupted the reproductive systems of female eagles, often
causing their eggs to be infertile. The amount of calcium in the
eggshells also becomes reduced, causing them to become thin and brittle
and , consequently, to break during incubation.
DDT was banned in the United States in 1973. However, since it
is used in Mexico, the effects of DDT are still being felt in the
United States. Wind carries DDT across the man-made borders.
To learn more about DDT and other chemical pesticides, read the landmark book by Rachel Carson (1962) entitled Silent Spring.
Leadshot Poisoning - Throughout history bald eagles have been
shot and killed by farmers, hunters, and profit-seekers. Though
shooting continues to be a major problem for the bald eagle, recently a
more subtle threat has surfaced. Lead poisoning has become one of the
primary causes of death for bald eagles. This poisoning occurs when the
bald eagle feeds off carrion (dead animals) that have been shot with
Through the process known as biomagnification or amplification
the lead ingested by the bald eagle from feeding on carrion remains in
the eagle's body and eventually accumulates to highly toxic levels.
This process of accumulating large quantities of poison in the body is
the same process that occurs with the chemical pesticide DDT which
nearly caused the extinction of the bald eagle in the early 1960s. As
with DDT, the bald eagle may not die immediately, but eventually
suffers from the lethal effects of the lead poisoning.
The problem became so serious that legislation has been passed
to prevent the use of lead shot. Particularly at risk are areas where
the bald eagle winters. These areas tend to be popular duck and
waterfowl hunting grounds.
For further information about the effects of lead poisoning on
bald eagles and other raptors, visit the Raptor Center at the
Univeristy of Minnesota.
Secondary Poisoning - In addition to leadshot poisoning, bald
eagles often die from eating carrion that have been deliberately
poisoned to attract undesirable predators. In a Spring, 1992 article
from the Region 6: Fish & Wildlife News entitled, "Death in the
Rockies," an account of the poisoning of wildlife such as bald eagles,
birds, foxes, skunks, and even domesticated dogs and cats was reported.
Sheep farmers have traditionally put out poison to kill
predators such as coyotes which are known to prey on sheep and lamb.
However, many predators, including the coyote and the bald eagle,
prefer to scavenge already dead prey and to save their energy when
possible. As a result, farmers who resort to illegally poisoning dead
animal carcasses have inadvertently killed other wildlife.
The poisons that are used are so strong that even a tiny bit
can continue to kill third, fourth, and even fifth generations of
non-targeted wildlife. In fact, reports of the death of children who
have handled dead birds, killed by ingesting wildlife that had died
from these deliberate poisonings, continue. This horrifying tradition
of poisoning kills not only bald eagles, but humans as well.
Electrocution is among the
top five causes of bald eagle deaths. A bald eagle perched upon a
high-voltage power pole may inadvertantly touch the power source and
the ground at the same time. When this happens, the bird is killed
instantly. Bald eagles may also fly directly into power lines that are
not visible in poor weather conditions.
Naturalists propose three suggestions for improving the safety
of current power lines for bald eagles: removing the top crossbar to
make them less attractive perch sites, installing a barrier to prevent
the eagle from touching the ground and power source at the same time,
and building artificial perches above the crossbar where electrocutions
As shocking as it may seem, poaching may still pose a danger for the bald eagle. Although federal law prohibits the shooting of eagles, as recently as 1989 between 20-60% of all eagles found dead had been shot. Since that time, educational programs seem to have created a greater awareness and support of our National Symbol.”
Throughout its history the bald eagle has suffered persecution
by farmers and landowners who suspect the bald eagle of preying on
their livestock. At one time, people were rewarded with bounties for
killing eagles and turning in their talons or body parts. This fear on
the part of ranchers and livestock owners lives on and many eagle
deaths still result from the actions of such irresponsible, uneducated
Some bald eagles are deliberately killed for their distinctive
feathers. People pay large sums of money to purchase authenic Indian
crafts and artifacts which require eagle feathers. These crafts are
sold in the Americas and Europe for high prices. For example, an Indian
war bonnet may fetch over $5,000 on the black market.
Often the juvenile bald eagle with its brown head is mistaken
for a hawk and killed. Much more often, eagle shootings are deliberate.
In addition to shootings, bald eagles are often found caught
in leg traps meant to capture other predators such as coyotes. Bald
eagles will opportunistically feed off the trap bait or even off an
animal already caught in the trap. The eagle becomes ensnared in a trap
when seizing bait or approaching a nearby trap. Some eagles lose a leg
or are otherwise crippled so that they become unable to fend for
themselves in the wild.
Today, even as the bald
eagle population slowly recovers as people work together to restore the
country's national symbol to its former status, new man-made threats
continue to arise. The factor which most significantly affects the
future of the bald eagle population is the destruction of its natural
habitat. As the human population grows, the bald eagle population
The most destructive human activity is the development of
waterfront property. Because eagles depend on shoreline habitats and
aquatic food sources, human development in these coveted areas poses
the greatest threat to the bald eagle's survival. In addition, the
cutting of "old growth forests" where bald eagles prefer to nest and
perch has conflicted with the interests of people seeking lumber for
housing and commercial products.
The sensitive issue of accomodating human needs and desires
while at the same time preserving our wildlife resources can destroy
communities. However, working together to come to reasonable agreements
regarding the protection of wildlife can also have the opposite effect.
At one time, only the bald eagle itself was protected by law. Today,
the eagle's "critical habitat" is also protected, but only on public
lands. Habitat management plans today call for groups of concerned
citizens, agencies, and organizations to work together to reach common
agreements regarding changes in the environment. Most habitat
management plans concerning the bald eagle involve protecting nesting
trees. Agreements which allow buffer zones around these sites have
permitted human activity while preserving bald eagle nests. The most
important weapon in the fight to save the bald eagle will be the
education of and communication with people.
At the top of its food chain, the
adult bald eagle has no natural predators. Unfortunately this does not
mean that the bald eagle as a species faces no dangers. In fact, the
mortality rate of eagle eggs, hatchlings, and first year juveniles is
A wide variety of factors can contribute to the infertility of
an eagle egg. A crow or raccoon may peck at the shell and crack it,
leaving it open to bacteria or dried membranes. The parent eagles may
be distracted and leave the egg unprotected from cold or heat. Since
the egg needs a steady temperature of 99.5 degrees Fahrenheit to
survive, an absence of less than a minute may cause death. Ingested
poisons from leadshot or other contaminants may render the egg
The juvenile eagle faces as many threats. In fact, the first
year of life of the bald eagle is its most dangerous. The young eagle
must learn to fly, hunt, and fend for itself in the wild. Without
parental assistance, young eagles often fall prey to the same fate as
older eagles: poisoning, shooting, electrocution, and even starvation.
In addition, man, knowingly or not, has become the bald
eagle's greatest threat. Through the destruction of the bald eagle's
natural habitat and the introduction of new chemicals into the
environment, man unwittingly has severely harmed the bald eagle's
chances of survival. Other irresponsible people deliberately kill or
maim bald eagles.
By learning about what the bald eagle needs to survive and
working together to prevent unnatural deaths, man can become the bald
eagle's greatest support rather than its most threatening enemy.
For a large bird with no
natural predators, the mortality or death rate of the bald eagle is
quite high. The chances of a juvenile bald eagle surviving its first
year of life today are less than 50%.
After that first year, the bald eagle stands a much better
chance of living to its full lifespan, which is anywhere from 30-40
years in the wild. The mortality rate of the bald eagle falls to about
25% after the first year of life. The hazards of the wild are clear
when one considers that a bald eagle raised in captivity may add an
additional ten years to its lifetime.
As strange as it may seem, the bald eagle's long lifespan
actually means that the greatest danger to the bald eagle is direct
mortality rather than reproductive failure. The near extinction of the
bald eagle due to the reproductive failure effects of the pesticide DDT
has been widely documented. But many do not realize that long-lived
bird populations suffer more from direct causes of death than from