The Bald Eagle's Near-Death Experience: Pollution's Deadly Legacy

The bald eagle, a majestic bird of prey and the national symbol of the United States, faced a significant threat of extinction due to various human-induced factors, including pollution. Once listed as endangered or threatened across the lower 48 states, the species experienced a remarkable recovery, with numbers soaring from a mere 417 nesting pairs in 1963 to over 70,000 by current estimates. While this conservation success story is commendable, pollution, specifically lead poisoning and toxic bacteria, has played a detrimental role in endangering the lives of these iconic birds.

Lead ammunition used by hunters

The bald eagle population in the United States has been on a steady road to recovery since the banning of the pesticide DDT in 1972. However, lead ammunition used by hunters continues to pose a significant threat to their survival.

Bald eagles often scavenge for food, feeding on carcasses of dead animals, including those shot by hunters. When hunters leave behind carcasses with lead bullet fragments, eagles consume the lead along with the meat. These bits of lead break down in the eagle's stomach and enter its bloodstream, causing lead poisoning. Lead poisoning can cause depression, weakness, organ failure, and anemia, and even death.

A study published in the Journal of Wildlife Management analyzed bald eagle count data and lead-poisoning veterinary reports from 1990 to 2018 across seven states in the northeastern United States. The study found that eating fragments of lead bullets reduced bald eagle population growth by 4 to 6 percent in this region. Another study by the New York State Department of Environmental Conservation (DEC) also found that lead-related mortality slows population growth, especially in states where lead bullets are still legal.

To address this issue, some states like California have banned lead bullets for hunting, and wildlife organizations provide hunters with free, non-lead ammunition. Other states have implemented rebate programs to encourage hunters to use non-lead ammunition and are focusing on educating hunters about the dangers of lead ammunition to raptors like bald eagles.

While there has been progress in reducing the use of lead ammunition, there is still opposition from hunting and gun rights organizations. Nonetheless, the efforts to increase awareness and provide alternatives to lead ammunition are crucial steps towards protecting bald eagles and ensuring their continued recovery.

Pesticides, dyes, gasoline, and plastic casings

In the case of bald eagle deaths, scientists detected the presence of bromine in toxic cyanobacteria. This led them to suspect that the cyanobacteria produced toxins due to bromine-based herbicides used to control the invasive weed *H. verticillata*. Lake managers applied these herbicides, which subsequently contaminated the lakes.

The introduction of bromine-based herbicides into the lakes disrupted the natural balance of the ecosystem. The herbicides coated the invasive weed *H. verticillata*, and when animals ingested this plant, the bromine-produced toxins accumulated in their tissues. This affected not just birds, but also fish, frogs, insects, and worms. As these affected animals became prey, the toxin moved up the food chain, eventually reaching bald eagles.

Bald eagles that consumed infected prey suffered from a neurological disease, vacuolar myelinopathy (VM), which caused significant physical impairment. The disease affected the birds' coordination while walking, swimming, and flying, leading to drowning, starvation, or injury. This disease and its devastating impact on bald eagle populations were a direct result of the contamination of lakes with bromine-based herbicides, which ultimately entered the food chain through affected plants and animals.

Toxins from cyanobacteria

In the mid-1990s, American bald eagles began dying in noticeable numbers from a neurological disease called avian vacuolar myelinopathy (AVM). AVM was first discovered in Arkansas in 1994, and over the following two decades, it claimed the lives of 160 eagles across the Southeast, including 80 from a single Georgia impoundment on the Savannah River, the J. Strom Thurmond Reservoir. It is believed that the actual death toll is significantly higher because the majority of the dead birds are never discovered.

The cause of death was determined to be a neurotoxin produced by a new species of cyanobacteria, Aetokthonos hydrillicola, which grows on the leaves of the invasive water plant Hydrilla verticillata in human-made lakes when bromide is present. This neurotoxin causes AVM, which results in brain lesions that impair motor skills and cause erratic movements, convulsions, and difficulty walking, swimming, or flying in afflicted animals.

Susan Wilde, a Warnell School of Forestry and Natural Resources aquatic science associate professor at the University of Georgia, discovered the cyanobacteria. According to Wilde, the invasive hydrilla and associated cyanobacteria are spreading to new lakes, and if this trend continues, it could significantly hinder the bald eagle's recovery and endanger other birds and aquatic wildlife.

Wilde's research revealed that in almost every site where bald eagles died, there was a significant invasion of hydrilla. She hypothesized that the eagles were consuming tainted prey: waterbirds called coots that eat the hydrilla, develop AVM, and then pass it on to the eagles that prey on them. Further investigation of the hydrilla revealed the presence of blue-green algae coating the leaves of the plants that the coots were eating, which was confirmed at every location where birds were dying from AVM.

To test the theory that cyanobacteria were producing the neurotoxin causing the disease in birds, researchers studied numerous lakes with hydrilla infestations. Some lakes had the new cyanobacterial colonies on the leaves, while others did not. By monitoring both types of lakes, the researchers found that only the lakes with Aetokthonos hydrillicola had birds suffering and dying from AVM.

Poisoning from gunshot and fishing weights

Bald eagles, the symbol of strength and freedom for Americans, were once near extinction due to various human activities. One of the leading causes of their population decline is poisoning from gunshot and fishing weights.

Bald eagles are at risk of lead poisoning from ingesting lead bullet fragments and lead fishing weights. Lead ammunition fragments and lead fishing tackle are left behind in the field, and eagles can ingest lead by consuming the remains of animals that have been shot or the small pieces of fishing gear that have been left behind. Lead fragments from ammunition spread far beyond the wound channel, and these fragments may exist in gut piles left in the field.

A recent study revealed that 47% of bald eagles and 46% of golden eagles showed signs of chronic lead poisoning, resulting from repeated lead exposure. Additionally, 33-35% of eagles suffered from acute lead poisoning due to exposure to high lead levels. These statistics were derived from samples taken across 38 states over eight years. The increasing lead levels in the blood of rehabilitation eagles indicate that lead toxicity is significantly impacting the health of bald eagle populations.

Lead poisoning has severe health consequences for bald eagles, including weakened immune systems, decreased reproductive success, and even death. The prognosis for eagles with acute poisoning is often poor, and even with lengthy and expensive treatment, the neurological side effects of lead may be permanent.

To address this issue, individuals can take several actions. Properly disposing of fishing lines and weights, choosing non-toxic gear, and participating in local cleanup efforts can help reduce eagles' exposure to lead. Additionally, residents can monitor local eagle populations and report sick or dead birds to the appropriate authorities. Hunters can also play a crucial role by transitioning to non-lead ammunition and tackle, such as copper-based alternatives, to reduce the risk of lead exposure in bald eagles.

Collision with vehicles and wind turbines

The bald eagle population in the United States was almost decimated by pollution, specifically due to the use of the insecticide DDT. By 1963, only 417 nesting pairs were found in the lower 48 states. DDT was banned in the United States in 1972, allowing recovery programs to be successful.

Bald eagles are vulnerable to collisions with vehicles. Due to their large wingspan of nearly 7 feet, eagles require a significant amount of space to take off and manoeuvre. When eagles feed on roadkill, they may be at risk of collision if they need to suddenly fly away from oncoming traffic. Eagle-vehicle collisions are responsible for about 11% of banded raptor deaths from the 1920s to the 2000s, and such incidents have become more common in recent years. Conservation officers and biologists work to rescue and rehabilitate eagles that have been struck by vehicles, but the birds often have a slim chance of recovery due to severe injuries.

In addition to vehicle collisions, wind turbines also pose a significant threat to bald eagles. The placement of wind turbines in areas where eagles congregate, such as near waterways or shorelines, can lead to a high number of bird fatalities. The size and design of wind turbines, with towering heights and long blades, create a large area of impact as the blades rotate. The Altamont Wind Resource Area in California, for example, has seen the deaths of more than 2,000 Golden Eagles due to wind turbines.

To mitigate the impact on bald eagles, it has been suggested that wind farms should not be located within a certain distance of shorelines. The Fish & Wildlife Service recommends a minimum distance of 3 miles, while the American Bird Conservancy suggests 5 miles. Some studies have also proposed painting one blade of wind turbines black, which could reduce bird deaths by up to 70%. However, this idea has faced resistance from engineers due to technical concerns and the complexity of implementing such measures.

The combination of vehicle collisions and wind turbine strikes continues to pose a threat to the recovery of bald eagle populations, highlighting the need for careful planning and the implementation of protective measures to ensure the safety of these majestic birds.

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