Bald eagles are majestic birds of prey that are the national symbol of the United States. However, despite their iconic status, bald eagles face a number of threats to their survival in the wild. Understanding the main causes of death for bald eagles can help inform conservation efforts to protect these birds.
Lead Poisoning
One of the biggest threats facing bald eagles is lead poisoning, which can cause severe neurological damage and death. Bald eagles primarily get lead poisoning from eating animals that have been shot with lead ammunition. When a hunter uses lead bullets or pellets to shoot deer, birds, or other prey, fragments of lead can get lodged in the carcass. An eagle scavenging on the remains will then ingest lead as it feeds.
Lead has severe neurological effects on eagles. It can cause them to become disoriented and unable to fly or hunt effectively. Eagles suffering from lead poisoning often starve to death because they become too weak to feed themselves. Lead can also induce seizures and make eagles more vulnerable to trauma such as collisions with cars or power lines.
Studies have found high rates of lead poisoning in eagle populations across North America. In Florida, nearly 20% of bald eagle deaths were linked to lead. In Oregon, about 10% of eagle deaths over an 18 year period were attributable to lead poisoning. Other research has reported between 10-15% of eagles suffering from poisoning in states like Iowa and Maine.
The good news is that lead ammunition bans can drastically reduce lead poisoning rates in eagles. In California, a statewide ban on using lead ammunition for hunting resulted in an over 80% decline in lead exposure among golden eagles. However, most states have yet to implement similar regulations, putting bald eagles at continued risk.
Collisions
Another major hazard for bald eagles comes from collisions with cars, power lines, trains, airplanes, wind turbines, and other objects. As large birds that inhabit areas near human infrastructure, eagles frequently come into contact with these structures.
One study found that between 25-35% of bald eagle deaths in states like Maryland and New York were attributable to some kind of collision. Eagles flying at low altitudes can easily be struck by passing cars. Power lines also pose a major hazard, as eagles often perch on them and can be electrocuted.
Wind turbines are an emerging threat for bald eagles. The rise of wind energy has led to more turbines being installed in areas inhabited by eagles. Eagles are sometimes struck by turbine blades while flying through wind facilities. More research is needed to find ways to minimize eagle collisions with turbines.
Airplanes pose collision risks too, especially near airports located close to the coasts and large inland lakes where eagles nest. Steps like removing animal carcasses from runway areas may reduce the chances of eagles getting struck by planes.
Territorial Fighting
Bald eagles are highly territorial, especially during nesting season. They frequently engage in battles over territory and mates that sometimes turn fatal. Territorial disputes are the most common cause of mortality for bald eagles in Florida, accounting for about 20% of deaths.
Eagles have large talons designed for hunting and capturing prey. When they fight over territories or mates, they will grapple each other with their talons and peck with their sharp beaks. This can result in severe puncture wounds and other traumatic injuries. If an eagle’s injuries are too severe, it may die from those wounds. Even minor injuries can become fatal if they impair an eagle’s ability to feed itself.
Younger eagles are more at risk from territorial disputes, as they may wander into areas occupied by nesting pairs. However, territorial fights can turn deadly even between established breeding pairs, especially if there is competition over prime nesting spots.
These conflicts are a natural part of bald eagle behavior and ecology. But habitat loss may force eagles into smaller breeding territories, increasing the potential for fatal conflicts.
Electrocution
Electrocution on power lines and electrical structures causes some bald eagle mortalities each year. Eagles’ large wingspans make them vulnerable to electrocution as they perch, roost, and build nests on power poles, transmission towers, and other equipment.
If an eagle’s wings simultaneously touch two energized parts or an energized line and grounded hardware, it will be electrocuted. This happens most often when eagles are landing on a structure, as their wings are spread wide and moving around to maintain balance.
Utilities have worked to reduce eagle electrocutions by installing protective devices on power poles, covering energized parts, and even running lines underground in some areas. Still, electrocutions continue to pose risks, especially to younger eagles as they learn to fly and land on new perches.
Targeted efforts to insulate dangerous power structures have shown results. In one Wyoming study, insulating just 10 structures decreased eagle electrocutions by 80% in the area. Similar projects in Arizona and Colorado have also significantly reduced eagle deaths.
Shootings
Though bald eagles are federally protected under the Bald Eagle Protection Act, some are still killed each year by shootings. There are a variety of motivations behind such shootings. Sometimes eagles are shot by hunters or farmers out of perceived threats to livestock. Other shootings are intentional acts of poaching.
From 2010 to 2021, over 70 bald eagles were confirmed to have been shot to death in the U.S. However, since many shooting deaths likely go undetected, the actual number is likely higher. Experts estimate that hundreds of eagles are shot each year across North America.
Shootings tend to spike during eagle nesting seasons, as the birds become more visible and territorial. A study of shooting deaths over 16 years in Michigan found February to April as peak months for shootings, coinciding with increased eagle activities.
Strict law enforcement and public education campaigns about eagle shootings can help reduce mortality rates. From 1987 to 2015, confirmed shooting deaths declined from 27 per year to less than 10 per year, partly thanks to outreach programs with hunters and farmers.
Effects of Climate Change
Climate change could become a growing factor in bald eagle deaths in the future. Rising temperatures, shifting weather patterns, and changing habitats are already impacting many wildlife species. Eagles may face new challenges and threats as climate change progresses.
Some potential hazards from climate change include:
- Loss of nesting trees in forests affected by drought, disease, pests, and wildfire.
- Declines in prey fish populations due to warming lakes and rivers.
- Greater severe weather mortality during extreme heat, storms, or cold snaps.
- Nest failures caused by increased rainfall and flooding during breeding season.
- Poor chick survival rates from heat stress during summer.
Bald eagle populations are also vulnerable to habitat shifts caused by climate change. For example, if ranges of new predators like coyotes expand northward, they may predate more eagle eggs and chicks. Milder winters could also expose eagles to new diseases year-round.
More research is still needed to precisely predict how climate change will impact bald eagle mortality. But conservationists warn the threats are real. Habitat protection efforts will need to take into account a changing climate to help bald eagles adapt.
Other Causes
While the above causes account for most bald eagle deaths, there are a few other miscellaneous factors to be aware of:
- Diseases – Eagle can suffer from avian cholera, West Nile virus, bird flu, and other infectious diseases spread by bacteria, parasites, or viruses. However, disease is responsible for less than 5% of eagle deaths on average.
- Predation – Healthy adult bald eagles have few natural predators. However, eagle eggs and chicks can be preyed upon by ravens, bears, bobcats, foxes, gulls, and owls.
- Poisons – Rodenticides and insecticides can poison eagles through tainted prey. However, rates of accidental poisoning deaths are fairly low.
- Trauma – Eagles can suffer fatal trauma from a variety of circumstances, such as nest collapses, falls while feeding, severe storms, or attacks by prey while hunting.
While these miscellaneous causes add to bald eagle mortality rates, they are responsible for less than 10% of deaths combined based on research studies.
Comparison of Causes
Looking at available research, we can compare and rank the main causes of bald eagle mortality across their range in North America. The approximate breakdown is as follows:
- Collisions – 25-35%
- Lead poisoning – 15-20%
- Electrocutions – 5-15%
- Shootings – 2-4%
- Territorial fights – 15-25%
- Other (disease, trauma, etc.) – less than 10%
The data shows collisions with cars and man-made structures is the single biggest cause of death for bald eagles. Lead poisoning from ammunition sources also kills a substantial number of eagles each year.
Targeted mitigation efforts can lower risks for eagles in many of these categories. For example, working with energy companies to insulate dangerous power structures has reduced electrocution deaths significantly. But new threats like wind turbines will require ongoing conservation strategies.
Geographic Differences
While these general causes account for most bald eagle deaths across North America, there are some regional differences in mortality rates:
Pacific Northwest
In the Pacific Northwest, electrocutions used to be the leading cause of death before power utilities implemented safeguards. Now lead poisoning, collisions, and shooting deaths occur most frequently in the region.
California
Lead poisoning is the primary killer of bald eagles in California. A ban on lead hunting ammunition has helped reduce mortality from lead exposure in recent years.
Northern States
In northern states like Minnesota and Maine, lead poisoning and collisions account for around 60% of bald eagle deaths combined. Harsh winters also occasionally directly kill eagles in the North.
Southeastern States
In the Southeast region, collisions and territorial fights are the top bald eagle mortality factors. The dense forest habitat provides many perch sites that become collision risks. Mild winters also facilitate year-round territorial disputes.
Southwestern States
Shootings and electrocutions are more common causes of death for southwestern bald eagles in states like Arizona and New Mexico. Rates of lead poisoning are lower in the Southwest.
Habitat characteristics, weather patterns, hunting cultures, and other local factors impact regional mortality trends. But across their range, collisions and lead poisoning remain among the most significant threats to bald eagles.
Factors in Eagle Mortality Rates
Several factors influence bald eagle death rates and causes across different regions:
Age
Younger bald eagles under 5 years old are at higher mortality risk from lead poisoning, electrocutions, predation, and starvation. Their inexperience makes them more prone to accidents and fatal miscalculations.
Territory Location
Eagles nesting near high-traffic roads, wind farms, power infrastructure, and airports are more likely to suffer collision deaths or electrocutions.
Prey Availability
Scarcity of fish and waterfowl can lead to starvation and risky feeding behaviors that put eagles in danger. Hungry eagles scavenge more, increasing odds of lead poisoning.
Weather Extremes
Severe storms, winter cold snaps, heat waves, flooding, and drought can all directly impact eagle mortality or cause ecological shifts affecting eagle survival.
Lead Exposure
Areas with higher hunting activity and use of lead ammunition lead to more lead poisoning deaths through eagles scavenging shot animals.
Anthropogenic Factors
Human infrastructure like roads, buildings, power lines, wind turbines, and electrical equipment underlies many major bald eagle mortality causes.
Understanding these risk factors can help guide targeted conservation programs to lower mortality rates in bald eagle populations.
Population Effects
Despite the threats they face, bald eagle populations remain healthy across most of their range. From a low point of under 500 nesting pairs in the lower 48 states in the 1950s, bald eagle numbers have rebounded to over 70,000 pairs today.
The negative population impacts of common mortality causes today are limited by a few key factors:
- Bald eagles have a relatively high reproductive rate, laying 1-3 eggs per year. Even factoring in nest failures, this robust reproduction replaces losses.
- The protections bald eagles received under the Endangered Species Act allowed populations to recover significantly. Take bans reduced shooting deaths.
- Conservation efforts focused on mitigating lead poisoning and electrocutions have lowered those mortality rates from peak historic levels.
- Better medical care and rehabilitation programs save many injured eagles each year that would have otherwise died.
However, the risks to bald eagles are ongoing. Continued habitat loss, climate change impacts, and new infrastructure threats require sustained management focus into the future.
Biologists also monitor eagle mortality closely for any signals the population is declining. If death rates increase substantially in a region over multiple years, it could indicate a new environmental hazard requiring rapid conservation action.
Conclusion
Collisions with vehicles and structures, lead ammunition poisoning, electrocutions, and territorial fighting are currently the primary causes of death for bald eagles in North America. Targeted mitigation strategies have succeeded in lowering risks from lead and electrocution. But new threats continue to emerge, requiring thoughtful management approaches rooted in scientific data and research.
Understanding the main mortality factors for bald eagles can inform both policy decisions and the action plans of conservation groups on the ground. With active protection of their habitat and reduction of anthropogenic hazards, these iconic raptors will continue as a symbol of wildlife vitality for generations to come.