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The Human
Effects on the Bald Eagle
Killing
Shooting
For centuries eagles have provided easy sport for white
settlers. In Florida John Audubon bragged about his marksmanship,
killing five eagles in a 24- hour period. He considered them good
eating, resembling the taste and texture of veal. Even today
shooting accounts for between 20-60% of the mortality each year.
Careless hunters cause some of the shootings, others are killed
for profit. Some parts of bald eagles are sold to produce
“authentic” Indian artifacts that are sold in Europe and the
United States.
Traps.
Often time’s eagles are caught in leg-hold traps meant for
fur bearing animals such as fox, coyote and raccoon. These traps
are baited with carrion and are particularly attractive to
scavenging eagles. Even if the bird survives the broken foot or
leg, it will typically destroy its wings by flapping frantically
to attempt to get away from the trap.
Poisoning
Poisoning is a relatively recent threat to bald eagles which
developed after man started using chemicals in the environment.
While the chemical age may have benefited mankind, it was
decimating the bald eagle population.
Poisons affect eagles in two ways. The first is that eagles can
ingest poisons that are placed in bait meant to trap other
animals. Secondly, they can assimilate poisons in the form of
pollutants or chemical contamination from their environment. The
first type of poisoning is usually immediate and dramatic, while
the second type can occur gradually, and the effects can be
subtle.
Poisoned bait has been used to kill nuisance animals for many
years, especially the coyote. The problem with poison bait is that
it indiscriminately kills many other scavengers, including bald
eagles. Some of the poisons used in the manner include thallium
sulfate, strychnine and cyanide. In the last two decades the use
of poison bait has been restricted, but problems with poisoning
will never disappear altogether.
DDT.
In 1939, the Audubon Society sponsored a major migration study
of bald eagles and chose a retired bank manager, Charles Broley,
to spearhead the project.
His studies
uncovered something far more sinister and important than new
information on migration patterns. Broley noticed that between
1939 and 1947 the number of productive nests declined
significantly in Florida, and that the few nests that produced
chicks averaged fewer nestlings. By 1947, 41% of Florida nests
failed to produce young. Three years later, the percentage of
unsuccessful nests had skyrocketed to 78%. Broley found only one
productive nest in Florida that year and it contained only a
single chick.
It was not just a Florida problem, as he also failed to discover
successful nests near his Ontario cabin. Clearly something was
very wrong. He theorized that DDT, an organochlorine pesticide,
already implicated in animal sterility, was the culprit.
DDT was used during World War II in such operations as delousing
prisoners of war and civilians, and for controlling insect
populations, which are vectors for human diseases such as malaria.
This pesticide
was heralded as a technological leap forward and was widely used
in the U.S. to control mosquitoes in marshes on both coasts. A
sign on the side of a truck spraying DDT over the crowds at the
Jones Beach State Park (New York) in 1945 proclaimed DDT as a
“powerful Insecticide Harmless to Humans”.
DDT is a chlorinated hydrocarbon. It, and its toxic breakdown
products, such as DDE, is fat-soluble, which allows it to
accumulate in fat tissues of any animal that may ingest it,
including humans. While relatively low in toxicity, it is
long-lived in the environment and is able to move up the food
chain in ever-increasing amount, bioaccumulation in the tissues of
predators high on the food chain. Bioaccumulation is the process
by which a toxin accumulates from low, very dispersed levels to
more concentrated levels as it works its way up the food chain
from invertebrates, to fish and eventually to eagles and other
animals occupying the top of the food chain (including humans).
Studies on
mergansers show it takes about 10 pounds of fish its toxins for
everyone pound of merganser. All of the DDE or DDT in the fish is
stored in the fatty tissues of birds for a ten-fold increase in
concentration. It takes about 10 pounds of merganser and the
associated DDT (equivalent to the amount of DDT in 100 pounds of
fish), to make one pound of eagle - another 10-fold increase.
Since this process continues throughout the life of the eagles,
DDT continues to accumulate at ever-higher levels.
“Sandy” Sprunt, research director of the National Audubon
Society, recovered eagle eggs and determined that there was a
direct correlation between the amount of DDT present in the eggs
and the probability of the eggs hatching.
At elevated
levels, DDT and DDE reduces the bird’s ability to metabolize
calcium from their food sources. Thin-shelled eggs resulted and
these often broke beneath the weight of the incubating adults
resulting in decline in reproduction noticed by Broley. Other
species, such as ospreys, brown pelicans and peregrine falcons,
all predators living at the top of their food chains, were also
affected in similar ways. In addition, some researchers have
suggested that some pesticides may mimic the effects of estrogen,
causing birth defects, infertility, and possibly even changes in
parenting behavior of eagles.
In 1962 Rachel Carson penned her famous book Silent Spring.
In this influential and eloquent book she described a spring
without birds - a world in which birds were disappearing due to
indiscriminate use of pesticides. Partly due to the mounting
evidence of the effect of DDT on wildlife, and the public outcry
generated by her book, Canada prohibited the use of DDT in 1970
with the U.S. following with its own ban in 1972.
While Carson’s popular book provided linkages between pesticides
and the bleak outlook for many of our popular birds and animals,
her voice was not the first to warn the world about the problems
associated with these toxins. In Birds Over America (1948)
Roger Tory Peterson gave one of the first inklings of trouble:
“Such
panaceas as DDT give us pause to ponder. However, some believe
that small doses of DDT may even be a boon to wildlife as a
substitute for marsh drainage in control of mosquitoes. But by
using this dangerous poison widely, before we know more about its
properties, we run the risk of turning our world into a biological
desert.”
The banning of DDT may have been more than just altruistic
behavior. Peter Mathiessen in Wildlife in America wrote:
“The recognition of man’s own vulnerability to a poisoned
environment was only one of several factors that expanded the
conservationists’ concerns from a small number of celebrated
birds and mammals to the whole range of living things…”
Regardless of why DDT was banned, bald eagles began to show
reproductive improvements almost immediately. In the Yellowstone
National Park ecosystem, the number of breeding pairs increased
from 30 to 50 in just 10 years after the ban. Only half of the
nests were successful prior to the DDT ban in Minnesota’s
Chippewa National Forest, but in the 1980s the number of
successful nests rose to 80%
Unfortunately, DDT is still popular in parts of the world where
its use is still legal. In Texas and New Mexico, for instance,
starlings have higher concentrations of DDT in their system than
they did just a few years ago. The source of the DDT is knot
known.
While the use
of DDT has been banned in the U.S., some pesticides still produced
and used in the U.S. contain large quantities of DDT as a
production by-product (and are therefore uncontrolled).
Lead poisoning.
Bald eagles feed on carrion and will take waterfowl that are
injured or diseased. A duck that has been injured during the
hunting season with lead shot will likely become a meal for an
eagle. In the process of consuming the duck the eagle is likely to
ingest the lead pellets from the body of a duck. This can lead to
lead poisoning.
Eagles can be
affected in a number of ways by lead poisoning. They can die
outright from ingesting too much. They can also consume enough to
become impaired, so much so that it is unable to pursue and
capture enough prey to stay alive. In this case the bird is likely
to die of starvation.
The third way
is the lead causes impaired physical responses, and the bird is
unable to avoid life-threatening situations, like moving out of
the way of a vehicle.
The U.S. Fish & Wildlife Service began a phased-in ban on the
use of lead shot beginning in 1987 after they were sued by the
National Wildlife Foundation for failure to protect the bald eagle
from the dangers of lead shot. In 1994 34% of the eagles admitted
to the University of Minnesota’s Raptor Center still had
symptoms of lead poisoning and over half had detectable traces in
their bodies.
Other poisons.
Even though the organochlorines, such as DDT, have been
banned, poisonings still occur with replacement pesticides. In
1994, an adult bald eagle was found near the southwestern
Wisconsin community of Grantsburg suffering from organophosphate
poisoning. The bird was rehabilitated and released near Fargo,
North Dakota, but was found dead of a second organophosphate
poisoning just two miles from the scene of the original poisoning
a year later. Still, organophosphates, which replaced the
organochlorines, are safer since they have shorter life spans and
therefore do not bioaccumulate at the same rate as the
organochlorines.
Carbofuran has also been implicated in the death of eagles.
Carbofuran is used to kill nematodes that destroy the roots of
some crops such as corn and rice and is implicated in the death of
thousands of birds (including eagles). Virginia banned the use of
the insecticide after the poisoning of an eagle and its chick.
Carbofuran was also implicated in the poisoning of 17 eagles in
northwestern Wisconsin in 1994.
Another hydrocarbon, PCB, a solvent used in some industrial
applications and in transformers, has been shown to cause birth
defects in newly hatched cormorants along the Great Lakes
coastline. Alarmingly, eagles living along Minnesota and
Wisconsin’s Lake Superior shoreline have 5-10 times the PCBs in
their systems as eagles nesting just a few miles inland and they
have lower nesting success than their inland-nesting cousins.
While the
number of eagle pairs has increased along the shore of Minnesota
and Wisconsin’s Lake Superior shoreline, these new pairs may be
forming from inland birds. As of yet, however, there isn’t a
conclusive study to indicate a direct link between reproductive
problems and high levels of PCBs in eagles.
The exact path that PCBs take in the food chain is unknown, since
the Lake Superior eagles do not feed much on lake trout, the
initial suspect. It may be that PCBs are bio-accumulating in
gulls, which scavenge dead lake trout that wash up on shore.
Eagles may then acquire the toxin whey they prey upon the gulls.
Habitat Interference
Obvious as well as subtle changes in the environment have
destroyed habitat so that less space is available for eagles to
exist.
Cutting of
nesting, roosting and perching trees has been going on since the
first white settlers arrived in North America, and still continues
today. While the eagle has been protected for many years it
wasn’t until 1978 that the eagles “critical habitat” was
afforded the same protection. This habitat is no longer open to
destruction.
Development of waterfront property is one of the most destructive
activities that humans have leveled at the bald eagle. Conflicts
with humans on shoreline habitat are intense, with the eagle being
the loser. Many eagles have been driven off of their nests because
of commercial and recreational activities too close to their
shorelines.
Clear-cutting of forests has decimated the tall, old trees that
eagles typically use for roosting, perching and nesting. The poor
harvesting practices also have caused run-offs into lakes, rivers
and streams that support the eagle. Additionally, high sediment
loads in the water create poor habitat for the fish that are
necessary for the eagle to feed on.
The construction of the lock and dams on the Mississippi
originally changed the prey species in the river to the detriment
of the eagle but now, the lock and dams are favorite spots for the
eagle to fish and to feed.
Habitat needs for an eagle include all of the components of the
environment that an eagle needs to survive and reproduce. This
means more than just forests and shorelines, it includes the
amount of prey available, the amount of isolation present as well
as many other components.
Disturbance
Human disturbance has a number of variables which make it
difficult to determine if one form is more detrimental than
another.
For instance
the type of activity and distance from the nest both influence
responses of adult birds. Also the stage at which nesting occurs,
the weather, and the duration of the disturbance may play a role
with other conditions. Once these variables are teamed with the
individual personalities and tolerance levels of a particular pair
of eagles it becomes difficult to determine the effect any one
form of disturbance may have on nesting success.
By far a
greater threat to the eagle is the man-made contaminants
introduced to the natural world. Eventually these pollutants move
through the food chain and concentrate in the base of the food
web, typically fish. When eagles feed on fish they are eating
concentrated amounts of whatever pollutants have been absorbed by
the fish.
Changes in Prey
A little-studied aspect of bald eagle problems is the vast
reduction of wildlife species that served as prey for the bald
eagle. Studies indicate that survival and increased reproductive
success is based on the abundance of food. As humans destroy and
deplete aquatic foods and other prey sources, the eagle has more
difficulty finding an adequate meal.
Because of
their diverse diet, as well as their scavenging feeding patterns,
bald eagles have been able to adapt to many changes in their prey
base - but this has also led to humans believing that the eagle is
competition for food sources, leading them to regard the eagle as
vermin.
Other Causes of Death
Eagles are also killed or injured by ingesting fishing lures
and hooks found imbedded in their prey. This is particularly
prevalent in the south along the seacoast. It is not difficult to
imagine a fish escaping its immediate danger by breaking the
fisherman’s line, only to die later from its injuries or
exhaustion and floating to the surface to be snagged by an eagle,
along with a hook or lure. (Eagles have also died or been injured
after ingesting lead fishing weights or becoming entangled in
discarded fishing line.)
Nearly one-fourth of bald eagle deaths are attributed to trauma,
such as collisions with vehicles, power lines and other
structures.
Gunshot
injuries account for 15% of deaths, and poisonings account for an
additional 16% deaths (including lead poisonings). Electrocution
takes fewer bald eagles than golden eagles, but 12% of known bald
eagle deaths still occur when eagles alight on power structures,
and with their large wingspan cover the distance between a
hot line and a ground.
While the conservation of bald eagles has succeeded, the bald
eagle should be looked at as a species relatively easy to save
when compared to many other endangered species. Eagles are large,
powerful birds, and like other charismatic mega-fauna, they are
relatively easy to like for most people. They also have a wide
historic range,and they are easy to identify (at least as
adults). The eagle is also the US National Bird which creates
strong affinity for many individuals, as well as groups such as
veterans.
The major threat before the 40s was shooting and habitat loss, but
this was successfully combated using education programs and legal
protections. After the mid 40s, the major threat eagles faced was
DDT. Once legislation prohibited its use in Canada and the United
States in the early 1970s, the bald eagle population climbed
rapidly from a low of about 500 nesting pairs in 1963 to over
4,000 pairs in the continental US by 1993.
Another
20-25,000 pairs exist in Alaska and Canada. |
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